Soil carbohydrates play a vital role in maintaining the health of soil and ensuring that ecosystems function properly. The method used in this study for measuring carbohydrate content in soil is quick, effective and easy. The investigation involved using three techniques to extract 45 soil samples of different soil texture; hot water, cool water and classical methods. The extracted samples were then analyzed for carbohydrates using two techniques; the widely accepted Phenol Sulfuric Acid method and the newly developed Sulfuric Acid UV method. The results revealed that the hot water extraction method consistently yielded higher levels of carbohydrates compared to the other methods. Interestingly the newly developed Sulfuric Acid UV method consistently produced similar results to those obtained from the well established Phenol Sulfuric Acid method. These results suggest that the novel Sulfuric Acid UV method, which relies on absorption in the ultraviolet region to analyze soil carbohydrate content is indeed a viable approach. This has allowed scientists and researchers to have an alternative means of determining soil carbohydrate levels.

@article{albalasmeh_novel_2024,
  title = {A novel approach for determining soil carbohydrates using {UV} spectrophotometry},
  issn = {1658077X},
  doi = {10.1016/j.jssas.2024.01.004},
  language = {en},
  urldate = {2024-02-09},
  journal = {Journal of the Saudi Society of Agricultural Sciences},
  author = {Albalasmeh, Ammar A. and Mohawesh, Osama and Gharaibeh, Mamoun A. and Ghezzehei, Teamrat A.},
  month = feb,
  year = {2024},
  pages = {S1658077X24000109},
  sort-word = {method}
}

Soil water infiltration is a critical process in the soil water cycle and agricultural practices, especially when wastewater is used for irrigation. Although research has been conducted to evaluate the changes in the physical and chemical characteristics of soils irrigated by treated wastewater, a quantitative analysis of the effects produced on the infiltration process is still lacking. The objective of this study is to address this issue. Field experiments previously conducted on three adjacent field plots characterized by the same clayey soil but subjected to three different irrigation treatments have been used. The three irrigation conditions were: non-irrigated (natural conditions) plot, irrigated plot with treated wastewater for two years, and irrigated plot with treated wastewater for five years. Infiltration measurements performed by the Hood infiltrometer have been used to estimate soil hydraulic properties useful to calibrate a simplified infiltration model widely used under ponding conditions, that were existing during the irrigation stage. Our simulations highlight the relevant effect of wastewater usage as an irrigation source in reducing cumulative infiltration and increasing overland flow as a result of modified hydraulic properties of soils characterized by a lower capacity of water drainage. These outcomes can provide important insights for the optimization of irrigation techniques in arid areas where the use of wastewater is often required due to the chronic shortage of freshwater.

@article{p2020-Albalasmeh-et-al,
  title = {Using Wastewater in Irrigation: The Effects on Infiltration Process in a Clayey Soil},
  author = {Albalasmeh, Ammar A. and Gharaibeh, Mamoun A. and Alghzawi, Ma’in Z. and Morbidelli, Renato and Saltalippi, Carla and Ghezzehei, Teamrat A. and Flammini, Alessia.},
  doi = {10.3390/w12040968},
  pdf = {https://www.mdpi.com/2073-4441/12/4/968/pdf},
  sort-word = {CO2 flux, Soil respiration, Soil Carbon, aggregation, modeling,biogeoscience},
  journal = {Water},
  status = {published},
  volume = {12},
  number = {4},
  pages = {968},
  year = {2020}
}

Background and Aims: Wetting-drying cycles are important environmental processes known to enhance aggregation. However, very little attention has been given to drying as a process that transports mucilage to inter-particle contacts where it is deposited and serves as binding glue. The objective of this study was to formulate and test conceptual and mathematical models that describe the role of drying in soil aggregation through transportation and deposition of binding agents. \{Methods: We used an ESEM to visualize aggregate formation of pair of glass beads. To test our model, we subjected three different sizes of sand to multiple wetting-drying cycles of PGA solution as a mimic of root exudates to form artificial aggregates. Water stable aggregate was determined using wet sieving apparatus. \{Results: A model to predict aggregate stability in presence of organic matter was developed, where aggregate stability depends on soil texture as well as the strength, density and mass fraction of organic matter, which was confirmed experimentally. The ESEM images emphasize the role of wetting-drying cycles on soil aggregate formation. \{Conclusions: Our experimental results confirmed the mathematical model predictions as well as the ESEM images on the role of drying in soil aggregation as an agent for transport and deposition of binding agents.

@article{p2013-Albalasmeh-Ghezzehei,
  author = {Albalasmeh, Ammar A. and Ghezzehei, Teamrat A.},
  date-modified = {2018-05-27 21:12:15 +0000},
  doi = {10.1007/s11104-013-1910-y},
  journal = {Plant and Soil},
  status = {published},
  keywords = {Rhizosheath, Aggregate formation/stabilization, Root exudate, Rhizosphere water content, Polygalacturonic Acid (PGA)},
  month = oct,
  number = {1-2},
  pages = {739--751},
  publisher = {Springer Nature},
  researchgate = {https://www.researchgate.net/publication/255176344_A_new_method_for_rapid_determination_of_carbohydrate_and_total_carbon_concentrations_using_UV_spectrophotometry},
  title = {Interplay between soil drying and root exudation in rhizosheath development},
  volume = {374},
  year = {2013},
  bdsk-url-1 = {https://doi.org/10.1007%2Fs11104-013-1910-y},
  bdsk-url-2 = {http://dx.doi.org/10.1007/s11104-013-1910-y},
  bdsk-url-3 = {https://doi.org/10.1007/s11104-013-1910-y}
}

A new UV spectrophotometry based method for determining the concentration and carbon content of carbohydrate solution was developed. This method depends on the inherent UV absorption potential of hydrolysis byproducts of carbohydrates formed by reaction with concentrated sulfuric acid (furfural derivatives). The proposed method is a major improvement over the widely used Phenol-Sulfuric Acid method developed by DuBois, Gilles, Hamilton, Rebers, and Smith (1956). In the old method, furfural is allowed to develop color by reaction with phenol and its concentration is detected by visible light absorption. Here we present a method that eliminates the coloration step and avoids the health and environmental hazards associated with phenol use. In addition, avoidance of this step was shown to improve measurement accuracy while significantly reducing waiting time prior to light absorption reading. The carbohydrates for which concentrations and carbon content can be reliably estimated with this new rapid Sulfuric Acid-UV technique include: monosaccharides, disaccharides and polysaccharides with very high molecular weight.

@article{p2013-Albalasmeh-Berhe-Ghezzehei,
  author = {Albalasmeh, Ammar A. and Berhe, Asmeret Asefaw and Ghezzehei, Teamrat A.},
  date-modified = {2018-05-31 13:28:00 +0000},
  doi = {10.1016/j.carbpol.2013.04.072},
  journal = {Carbohydrate Polymers},
  status = {published},
  number = {2},
  pages = {253-61},
  researchgate = {https://www.researchgate.net/publication/255176344_A_new_method_for_rapid_determination_of_carbohydrate_and_total_carbon_concentrations_using_UV_spectrophotometry},
  sort-word = {method},
  title = {A new method for rapid determination of carbohydrate and total carbon concentrations using UV spectrophotometry},
  volume = {97},
  year = {2013},
  bdsk-url-1 = {https://doi.org/10.1016/j.carbpol.2013.04.072}
}

Soil structure degradation by fire is usually attributed to qualitative and quantitative change of organic and inorganic binding agents, especially in high severity burns (> 300 A degrees C) that last for prolonged periods (> 1 hour). In contrast, controlled burns are typically managed to be low in intensity and severity. Such burns are considered benign to soil structural stability because organic matter and inorganic binding agents (e.g., gypsum) are relatively stable at such low temperatures. Recent observations at a controlled burn site in the eastern Great Basin (Nevada) showed soil aggregate breakdown found in shrub canopies where soil temperatures briefly exceeded 300 A degrees C as well as interspaces between shrubs, where the temperatures were likely lower than beneath shrubs because of less surface biomass. These alterations cannot be explained in terms of thermal alteration of binding agents. This study was designed to test whether pressure created by rapidly vaporized pore water can cause aggregate breakdown. We subjected three different sizes of aggregates (0.25-1, 1-2 and 2-4 mm) of soils derived from the eastern Great Basin burn site as well as from a forest and urban garden in California to rapid and slow (3 A degrees C/min) heating rates. These treatments were conducted at 5 peak temperatures (75, 100, 125, 150 and 175 A degrees C). Post-burn water stability of the aggregates showed that rapid heating rate caused more pronounced degradation of aggregate stability than slow heating. Moreover, the heating-rate dependent structural degradation increased with peak temperature. For the majority of the aggregates, the effect also increased with initial water content. In all the soils tested, there was no preferential loss of organic matter in the rapid-heating treatment that can explain the observed enhanced breakdown of aggregates. Our observations indicate that soil structural degradation under low-intensity fire occurs as a result of mechanical stresses extorted by rapidly escaping steam from soil pores under rapid heating rate.

@article{p2013-Albalasmeh-et-al,
  author = {Albalasmeh, A. A. and Berli, M. and Shafer, D. S. and Ghezzehei, T. A.},
  date-modified = {2018-05-27 21:09:37 +0000},
  doi = {10.1007/s11104-012-1408-z},
  journal = {Plant and Soil},
  status = {published},
  number = {1-2},
  pages = {335-344},
  researchgate = {https://www.researchgate.net/publication/236455238_Degradation_of_moist_soil_aggregates_by_rapid_temperature_rise_under_low_intensity_fire},
  title = {Degradation of moist soil aggregates by rapid temperature rise under low intensity fire},
  volume = {362},
  year = {2013},
  bdsk-url-1 = {https://doi.org/10.1007/s11104-012-1408-z}
}

We studied the long-term impact of contrasting tillage and cover cropping systems on soil structure and hydraulic properties. Complete water retention and conductivity curves for the top (0–5 cm) and subsurface (20–25 cm) soils were characterized and contrasted. Dynamic water storage and retention were evaluated using numerical simulations in HYDRUS-2D software. Compared with standard-till (ST) and no-cover-crop (NO) systems, soils under no-till (NT) and cover cropping (CC) systems showed improved soil structure in terms of pore size distribution (PSD). Changes in hydraulic conductivity (K) under these systems led to an increased infiltration rate and water retention. However, NT and CC plots had lower water content at field capacity (33 kPa suction) and lower plant-available water (PAW) compared with ST and NO plots. Numerical simulations, however, showed that NT and CC plots have higher water storage (albeit marginal in magnitude) and water availability following irrigation. Because the numerical simulations considered retention and conductivity functions simultaneously and dynamically through time, they allow the capture of hydraulic states that are arguably more relevant to crops. The study concludes that the long-term practices of NT and CC systems were beneficial in terms of changes to the PSD. NT and CC systems also marginally improved soil water conductivity and storage at the plot scale.

@article{p2022-Araya-et-al-a,
  title = {Long-Term Impact of Cover Crop and Reduced Disturbance Tillage on Soil Pore Size and Soil Water Storage},
  author = {Araya, S. N. and Mitchell, J. P. and W., Hopmeans J. and Ghezzehei, T. A.},
  journal = {SOIL},
  volume = {8},
  pages = {177–198},
  status = {published},
  doi = {10.5194/soil-8-177-2022},
  pdf = {https://soil.copernicus.org/articles/8/177/2022/soil-8-177-2022.pdf},
  year = {2022}
}

This study investigates the ability of machine learning models to retrieve the surface soil moisture of a grassland area from multispectral remote sensing carried out using an unoccupied aircraft system (UAS). In addition to multispectral images, we use terrain attributes derived from a digital elevation model and hydrological variables of precipitation and potential evapotranspiration as covariates to predict surface soil moisture. We tested four different machine learning algorithms and interrogated the models to rank the importance of different variables and to understand their relationship with surface soil moisture. All the machine learning algorithms we tested were able to predict soil moisture with good accuracy. The boosted regression tree algorithm was marginally the best, with a mean absolute error of 3.8 % volumetric moisture content. Variable importance analysis revealed that the four most important variables were precipitation, reflectance in the red wavelengths, potential evapotranspiration, and topographic position indices (TPI). Our results demonstrate that the dynamics of soil water status across heterogeneous terrain may be adequately described and predicted by UAS remote sensing and machine learning. Our modeling approach and the variable importance and relationships we have assessed in this study should be useful for management and environmental modeling tasks where spatially explicit soil moisture information is important.

@article{p2019-Araya-et-al-b,
  author = {Araya, S. N. and Fryjoff-Hung, A. and Anderson, A. and Viers, J. H. and Ghezzehei, T. A.},
  journal = {Hydrology and Earth System Sciences},
  volume = {25},
  pages = {2739–2758},
  status = {published},
  title = {Advances in soil moisture retrieval from multispectral remote sensing using unoccupied aircraft systems and machine learning techniques.},
  doi = {10.5194/hess-25-2739-2021},
  mendeley = {https://www.mendeley.com/catalogue/c94110d7-78aa-3804-b08d-409e2acc5e2b/},
  year = {2021}
}

Saturated hydraulic conductivity (Ks) is a fundamental soil property that regulates the fate of water in soils. Its measurement, however, is cumbersome and instead pedotransfer functions (PTFs) are routinely used to estimate it. Despite much progress over the years, the performance of current generic PTFs estimating Ks remains poor. Using machine learning, high‐performance computing, and a large database of over 18,000 soils, we developed new PTFs to predict Ks. We compared the performances of four machine learning algorithms and different predictor sets. We evaluated the relative importance of soil properties in explaining Ks. PTF models based on boosted regression tree algorithm produced the best models with root‐mean‐squared log‐transformed error in ranges of 0.4 to 0.3 (log10(cm/day)). The 10th percentile particle diameter (d10) was found to be the most important predictor followed by clay content, bulk density (ρb), and organic carbon content (C). The sensitivity of Ks to soil structure was investigated using ρb and C as proxies for soil structure. An inverse relationship was observed between ρb and Ks, with the highest sensitivity at around 1.8 g/cm3 for most textural classes. Soil C showed a complex relationship with Ks with an overall positive relation for fine‐textured and midtextured soils but an inverse relation for coarse‐textured soils. This study sought to maximize the extraction of information from a large database to develop generic machine learning‐based PTFs for estimating Ks. Models developed here have been made publicly available and can be readily used to predict Ks.

@article{p2019-Araya-Ghezzehei,
  author = {Araya, Samuel N. and Ghezzehei, Teamrat A.},
  journal = {Water Resources Research},
  status = {published},
  doi = {10.1029/2018WR024357},
  mendeley = {https://www.mendeley.com/catalogue/1d6f2e16-3c52-32b5-ae9a-70d1a5d1adff/},
  data = {10.6071/M3T95H},
  volume = {55},
  pages = {5715-5737},
  month = jul,
  sort-word = {modeling},
  pdf = {https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2018WR024357},
  keywords = {soil hydraulic conductivity, machine learning, pedotransfer function, soil structure, bulk density, organic carbon},
  title = {Using Machine Learning for Prediction of Saturated Hydraulic Conductivity and Its Sensitivity to Soil Structural Perturbations},
  year = {2019}
}

Peatlands are garnering much attention for their greenhouse gas feedback potential in a warming climate. As of yet, the coupled biogeochemical and hydrological processes that control the amount and timing of soil organic matter (SOM) mineralization and, ultimately, whether peatlands will be sinks or sources of atmospheric CO 2 are not fully understood. Soil structure is a key feature of soils that mediates the coupling between biogeochemical and hydrological processes. However, we know very little about how soil structure responds when soils are exposed to wettingedrying cycles outside their normal range. In order to better understand how high elevation peatlands will respond to increasingly dry years, we incubated soils from high elevation meadows in the Sierra Nevada at 5 different water potentials and measured the CO 2 flux for over one year. We found that the cumulative carbon mineralization had a U-shaped pattern, with the greatest mineralization at the wettest (À0.1 bar) and driest (À4 bar) water potentials, across all hydrologic regions of the meadow. We propose a conceptual model that reproduces a similar pattern by incorporating the concept of dual porosity medium, with two distinct pore-size populations representing inter-and intra-aggregate porosity. Availability of water and oxygen to the two pore-size populations depends on the soil’s equilibrium water potential. The model and the data suggest that the decomposition rates of intra-aggregate SOM may increase due to prolonged drought events that lead to accelerated release of C from previously untapped pool.

@article{p2015-Arnold-Ghezzehei-Berhe,
  author = {Arnold, Chelsea and Berhe, Asmeret Asefaw and Ghezzehei, Teamrat A.},
  doi = {10.1016/j.soilbio.2014.10.029},
  journal = {Soil Biology and Biochemistry},
  status = {published},
  keywords = {CO2 flux, Soil respiration, Hydrology, Meadows, Climate extremes, Soil structure},
  month = sep,
  pages = {28-37},
  researchgate = {https://www.researchgate.net/publication/268630795_Decomposition_of_distinct_organic_matter_pools_is_regulated_by_moisture_status_in_structured_wetland_soils},
  sort-word = {aggregation, biogeoscience},
  title = {Decomposition of distinct organic matter pools is regulated by moisture status in structured wetland soils},
  volume = {81},
  year = {2015}
}

A new method was developed to measure soil consolidation by capillary suction in organic soils. This method differs from previous methods of measuring soil consolidation in that no external load is utilized and only the forces generated via capillary suction consolidate the soil matrix. This limits the degree of consolidation that can occur, but gives a more realistic ecological perspective on the response of organic soils to desiccation in the field. This new method combines the principles behind a traditional triaxial cell (for measurements of volume change), a pressure plate apparatus, (to facilitate drainage by capillary suction), and the permeameter, (to measure saturated hydraulic conductivity), and allows for simultaneous desaturation of the soil while monitoring desiccation induced volume change in the soil. This method also enables detection of historic limit of dryness. The historic limit of dryness is a novel concept that is unique to soils that have never experienced drying since their formation. It is fundamentally equivalent to the pre-compression stress of externally loaded soils. This method is particularly important for forecasting structural and hydrologic changes that may occur in soils that were formed in very wet regimes (e.g., wet meadows at the foot of persistent snow packs and permafrost peats) as they respond to a changing climate.

@article{p2015-Arnold-Ghezzehei,
  author = {Arnold, Chelsea and Ghezzehei, Teamrat A.},
  data = {10.6084/m9.figshare.1243716.v1},
  doi = {10.1002/2014WR015745},
  journal = {Water Resources Research},
  status = {published},
  keywords = {Ecosystems, Seasons, Soil respiration, Productivity, Spring, Winter, Carbon dioxide, Ecosystem functioning},
  month = sep,
  number = {1},
  pages = {e106058},
  pdf = {https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1002/2014WR015745},
  researchgate = {https://www.researchgate.net/publication/267875649_A_method_for_characterizing_desiccation-induced_consolidation_and_permeability_loss_of_organic_soils},
  sort-word = {method, mechanics},
  title = {A method for characterizing desiccation-induced consolidation and permeability loss of organic soils},
  volume = {51},
  year = {2015}
}

By the end of the 20th century, the onset of spring in the Sierra Nevada mountain range of California has been occurring on average three weeks earlier than historic records. Superimposed on this trend is an increase in the presence of highly anomalous “extreme years, where spring arrives either significantly late or early. The timing of the onset of continuous snowpack coupled to the date at which the snowmelt season is initiated play an important role in the development and sustainability of mountain ecosystems. In this study, we assess the impact of extreme winter precipitation variation on aboveground net primary productivity and soil respiration over three years (2011 to 2013). We found that the duration of snow cover, particularly the timing of the onset of a continuous snowpack and presence of early spring frost events contributed to a dramatic change in ecosystem processes. We found an average 100% increase in soil respiration in 2012 and 2103, compared to 2011, and an average 39% decline in aboveground net primary productivity observed over the same time period. The overall growing season length increased by 57 days in 2012 and 61 days in 2013. These results demonstrate the dependency of these keystone ecosystems on a stable climate and indicate that even small changes in climate can potentially alter their resiliency.

@article{p2014-Arnold-Ghezzehei-Berhe,
  author = {Arnold, Chelsea and Ghezzehei, Teamrat A. and Berhe, Asmeret Asefaw},
  data = {10.6084/m9.figshare.1113179.v19},
  date-modified = {2018-05-27 21:16:54 +0000},
  doi = {10.1371/journal.pone.0106058},
  journal = {PLoS ONE},
  status = {published},
  keywords = {Ecosystems, Seasons, Soil respiration, Productivity, Spring, Winter, Carbon dioxide, Ecosystem functioning},
  month = sep,
  number = {9},
  pages = {e106058},
  researchgate = {https://www.researchgate.net/publication/265509769_Early_Spring_Severe_Frost_Events_and_Drought_Induce_Rapid_Carbon_Loss_in_High_Elevation_Meadows},
  sort-word = {biogeoscience},
  title = {Early Spring, Severe Frost Events, and Drought Induce Rapid Carbon Loss in High Elevation Meadows},
  volume = {9},
  year = {2014}
}

Hydrodynamic dispersion is responsible for spreading of dissolved mass within a single phase in porous media. It typically arises because of variability in local flow velocities. Because the pattern of spreading by dispersion is similar to Fickian diffusion, dispersion has been traditionally modeled as a pseudo-diffusive process that depends on the concentration gradient. However, there is no physical basis for this dependence of dispersion on concentration gradient. This unphysical formulation of dispersive flux has led to a number of major shortcomings including (a) lack of a self-consistent, mechanistic, and independent approach for predicting dispersion coefficient; and (b) dependence of the dispersion coefficient on transport distance. In this paper we show that the shape of dispersive spreading can be described using a model based on a variably sized bundle of capillaries and purely advective transport. The model suggests that dispersion can be described in terms of the variance of the pore size distribution only. Breakthrough curves of the proposed model can be exactly matched with the traditional diffusive-type dispersion model. By utilizing this equivalence, we derived relationships between the traditional dispersivity coefficient, pore size variance, and transport distance. The plausibility of the proposed expressions was tested using three illustrative examples that compare aspects of the proposed model with measurements obtained from the literature.

@article{p2013-Arriaza-Ghezzehei,
  author = {Arriaza, Juan Lopez and Ghezzehei, Teamrat A.},
  date-modified = {2018-05-27 21:00:59 +0000},
  doi = {10.1615/JPorMedia.v16.i1.20},
  journal = {Journal of Porous Media},
  status = {published},
  keywords = {porous media, transport, stochastic methods},
  number = {1},
  pages = {11--19},
  researchgate = {https://www.researchgate.net/publication/240613686_Explaining_Longitudinal_Hydrodynamic_Dispersion_Using_Variance_of_Pore_Size_Distribution},
  title = {Explaining longitudinal hydrodynamic dispersion using variance of pore size distribution},
  volume = {16},
  year = {2013},
  bdsk-url-1 = {https://doi.org/10.1615%2Fjpormedia.v16.i1.20},
  bdsk-url-2 = {http://dx.doi.org/10.1615/jpormedia.v16.i1.20},
  bdsk-url-3 = {https://doi.org/10.1615/jpormedia.v16.i1.20},
  bdsk-url-4 = {https://doi.org/10.1615/JPorMedia.v16.i1.20}
}

Abstract The constitutive relations of the Richardson-Richards equation encode the macroscopic properties of soil water retention and conductivity. These soil hydraulic functions are commonly represented by models with a handful of parameters. The limited degrees of freedom of such soil hydraulic models constrain our ability to extract soil hydraulic properties from soil moisture data via inverse modeling. We present a new free-form approach to learning the constitutive relations using physically constrained neural networks. We implemented the inverse modeling framework in a differentiable modeling framework, JAX, to ensure scalability and extensibility. For efficient gradient computations, we implemented implicit differentiation through a nonlinear solver for the Richardson-Richards equation. We tested the framework against synthetic noisy data and demonstrated its robustness against varying magnitudes of noise and degrees of freedom of the neural networks. We applied the framework to soil moisture data from an upward infiltration experiment and demonstrated that the neural network-based approach was better fitted to the experimental data than a parametric model and that the framework can learn the constitutive relations.

@article{p2024_bandai-b,
  author = {Bandai, Toshiyuki and Ghezzehei, Teamrat A. and Jiang, Peishi and Kidger, Patrick and Chen, Xingyuan and Steefel, Carl I.},
  title = {Learning Constitutive Relations From Soil Moisture Data via Physically Constrained Neural Networks},
  journal = {Water Resources Research},
  volume = {60},
  number = {7},
  pages = {e2024WR037318},
  keywords = {inverse modeling, soil hydraulic functions, physics-informed machine learning, neural networks, soil moisture},
  doi = {10.1029/2024WR037318},
  pdf = {https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2024WR037318},
  note = {e2024WR037318 2024WR037318},
  year = {2024}
}

To minimize uncertainty related to soil processes in extreme events, we need accurate soil hydraulic properties across the entire range of soil water content. However, conventional methods are time-consuming and limited to specific ranges. To estimate soil hydraulic properties throughout the entire range, we conducted inverse modeling using upward infiltration experiments, where a shortwave infrared imaging camera was used to obtain high-resolution soil moisture data in space and time. Because the commonly used van Genuchten–Mualemmodel is unsuitable for describing soil hydraulic properties for dry conditions, we tested an alternative model, the Peters-Durner-Iden model, which considers both capillary and film water. The inverse modeling successfully estimated soil hydraulic properties for sandy loam and loam soils, and we demonstrated that the Peters-Durner-Iden model captured soil moisture dynamics better than the van Genuchten–Mualemmodel for dry conditions. However, both models could not adequately describe the soil moisture data for the other soils. The direct observation of the water flow via shortwave infrared images clarified that the reduced success was because of violating the one-dimensional flow assumption for coarse-textured soils and the micro-heterogeneity in soil hydraulic properties for soils with fine silt and clay materials.

@article{p2024-Bandai-et-al,
  title = {Estimating soil hydraulic properties from oven-dry to full saturation using shortwave infrared imaging and inverse modeling},
  journal = {Journal of Hydrology},
  volume = {635},
  pages = {131132},
  year = {2024},
  issn = {0022-1694},
  doi = {https://doi.org/10.1016/j.jhydrol.2024.131132},
  pdf = {https://pdf.sciencedirectassets.com/271842/1-s2.0-S0022169424X00050/1-s2.0-S0022169424005274/main.pdf},
  author = {Bandai, Toshiyuki and Sadeghi, Morteza and Babaeian, Ebrahim and Jones, Scott B. and Tuller, Markus and Ghezzehei, Teamrat A.},
  keywords = {Inverse modeling, Shortwave infrared imaging, Soil moisture, Soil hydraulic functions}
}

Modeling water flow in unsaturated soils is vital for describing various hydrological and ecological phenomena. Soil water dynamics is described by well-established physical laws (Richardson–Richards equation – RRE). Solving the RRE is difficult due to the inherent nonlinearity of the processes, and various numerical methods have been proposed to solve the issue. However, applying the methods to practical situations is very challenging because they require well-defined initial and boundary conditions. Recent advances in machine learning and the growing availability of soil moisture data provide new opportunities for addressing the lingering challenges. Specifically, physics-informed machine learning allows both the known physics and data-driven modeling to be taken advantage of. Here, we present a physics-informed neural network (PINN) method that approximates the solution to the RRE using neural networks while concurrently matching available soil moisture data. Although the ability of PINNs to solve partial differential equations, including the RRE, has been demonstrated previously, its potential applications and limitations are not fully known. This study conducted a comprehensive analysis of PINNs and carefully tested the accuracy of the solutions by comparing them with analytical solutions and accepted traditional numerical solutions. We demonstrated that the solutions by PINNs with adaptive activation functions are comparable with those by traditional methods. Furthermore, while a single neural network (NN) is adequate to represent a homogeneous soil, we showed that soil moisture dynamics in layered soils with discontinuous hydraulic conductivities are correctly simulated by PINNs with domain decomposition (using separate NNs for each unique layer). A key advantage of PINNs is the absence of the strict requirement for precisely prescribed initial and boundary conditions. In addition, unlike traditional numerical methods, PINNs provide an inverse solution without repeatedly solving the forward problem. We demonstrated the application of these advantages by successfully simulating infiltration and redistribution constrained by sparse soil moisture measurements. As a free by-product, we gain knowledge of the water flux over the entire flow domain, including the unspecified upper and bottom boundary conditions. Nevertheless, there remain challenges that require further development. Chiefly, PINNs are sensitive to the initialization of NNs and are significantly slower than traditional numerical methods.

@article{C_hess-26-4469-2022,
  author = {Bandai, T. and Ghezzehei, T. A.},
  title = {Forward and inverse modeling of water flow in unsaturated soils with discontinuous hydraulic conductivities using physics-informed neural networks with domain decomposition},
  journal = {Hydrology and Earth System Sciences},
  volume = {26},
  year = {2022},
  number = {16},
  pages = {4469--4495},
  pdf = {https://hess.copernicus.org/articles/26/4469/2022/hess-26-4469-2022.pdf},
  data = {https://doi.org/10.5281/zenodo.6030635},
  doi = {10.5194/hess-26-4469-2022}
}

Water retention curve (WRC) and hydraulic conductivity function (HCF) are essential information to model the movement of water in the soil using the Richardson-Richards equation (RRE). Although laboratory measurement methods of WRC and HCF have been well established, the lab-based WRC and HCF can not be used to model soil moisture dynamics in the field because of the scale mismatch. Therefore, it is necessary to derive the inverse solution of the RRE and estimate WRC and HCF from field measurement data. We are proposing a physics-informed neural networks (PINNs) framework to obtain the inverse solution of the RRE and estimate WRC and HCF from only volumetric water content measurements. The PINNs was constructed using three feedforward neural networks, two of which were constrained to be monotonic functions to reflect the monotonicity of WRC and HCF. The PINNs was trained using noisy synthetic volumetric water content data derived from the simulation of soil moisture dynamics for three soils with distinct textures. The PINNs could reconstruct the true soil moisture dynamics from the noisy data. As for WRC, the PINN could not precisely determine the WRCs. However, it was shown that the PINNs could estimate the HCFs from only the noisy volumetric water content data without specifying initial and boundary conditions and assuming any information about the HCF (e.g., saturated hydraulic conductivity). Additionally, we showed that the PINNs framework could be used to estimate soil water flux density with a broader range of estimation than the currently available methods.

@article{P2020-Bandai,
  author = {Bandai, Toshiyuki and Ghezzehei, Teamrat},
  title = {Physics-informed neural networks with monotocnicity constraints for Richardson-Richards equation--Estimation of constitutive relationships and soil water flux density from volumetric water content measurements.},
  journal = {Water Resources Research},
  volume = {57},
  number = {2},
  pages = {e2020WR027642},
  year = {2021},
  doi = {10.1029/2020WR027642},
  data = {https://github.com/ToshiyukiBandai/PINNs_RRE},
  pdf = {https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2020WR027642},
  keywords = {inverse method, machine learning , partial differential equation,physics‐informed neural networks,soil moisture,soil water flux density},
  status = {published},
  mendeley = {https://www.mendeley.com/catalogue/792322df-e0b2-3cc2-8826-2c8730d232f4/}
}

Recent research has shown that the native shrub, Guiera senegalensis J.F. Gmel, interplanted with crops dramatically increases crop yield in the Sahel. However, little is known about the crop development when grown alongside shrubs. The objectives were to determine the effect of shrub presence on crop development under varying fertilizer rates in northern Senegal. Peanut (\em Arachis hypogaea L.) and pearl millet (\em Pennisetum glaucum (L.)) response in the presence or absence of shrubs was investigated from 2013 to 2016 under low to adequate rainfall. The experiment had a split-plot factorial design with presence or absence shrubs (+shrub, −shrub) as the main plot (elevated densities of 1200 to 1500 shrubs ha−1) and fertilizer rate (0, 0.5, 1 or 1.5 times the recommended N–P–K rate) as the subplot factors. Major developmental phases, leaf counts and plant height were determined. Shrub presence made crops grow up to four times taller and reach 50% flowering when −shrub plots did not reach 50% in 2013 and 2015 (millet). Changes to crop development, even in low rainfall years, indicate that shrub intercropping buffers against crop water stress. Thus, under drought conditions, shrub-based agroforestry can contribute to reduce fertilizer necessity and to mitigate against climate change.

@article{p2021-Bayala-et-al,
  author = {Bayala, Roger and Diedhiou, Ibrahima and Bogie, Nathaniel A. and Bright, Matthew B. H. and Badiane, Y. Ndour and Ghezzehei, Teamrat A. and Dick, Richard P.},
  journal = {Journal of Agronomy and Crop Science},
  volume = {208},
  pages = {158-167},
  status = {published},
  keywords = {crop/stress physiology, drought stress, site specific analysis},
  doi = {10.1111/jac.12568},
  mendeley = {https://www.mendeley.com/catalogue/bc0ee8f8-5d8d-3bfe-b11d-bea72134e194/},
  title = {Intercropping with Guiera senegalensis in a semi-arid area to mitigate early-season abiotic stress in A. hypogea and P. glaucum},
  year = {2022}
}

Aims\{To investigate the physiological responses of groundnut (Arachis hypogea) and pearl millet (Penisetum glaucum) that were intercropped with the native evergreen woody shrubs Piliostigma reticulatum (D.C.) Hochst and Guiera senegalensis J.F. Gmel compared to control crops throughout two growing seasons at two sites with contrasting climate and soil types in Senegal.Methods\\Shrubs grown in groundnut and millet fields at higher than native density were coppiced annually with aboveground biomass returned to the soil and no additional fertilizer. Crop leaf area index (LAI), handheld normalized difference vegetation index (NDVI), leaf water potential, and soil moisture and temperature were monitored in 2012–2013.Results\\At the drier site, the presence of shrubs reduced soil temperature at 5 cm depth by up to 5 °C during early crop growth. Shrub presence increased LAI by up to 266%, NDVI by up to 217% and increased groundnut leaf water potential throughout the day at the wetter site. Shrub effects on crop physiology were stronger overall at the drier site.Conclusions\\These results improve the understanding of how this unique agroforestry system alters the growing environment and the physiological response of associated crops throughout the season.

@article{p2019-Bogie-et-al-c,
  author = {Bogie, N.A. and Bayala, R. and Diedhiou, I. and Dick, R. and Ghezzehei, T.A.},
  doi = {10.1007/s11104-018-3882-4},
  journal = {Plant and Soil},
  status = {published},
  volume = {1-2},
  pages = {143–159},
  month = jan,
  sort-word = {agroecology},
  mendeley = {https://www.mendeley.com/catalogue/621c9e9c-8372-3721-b62a-c5169e03047a/},
  pdf = {https://link.springer.com/content/pdf/10.1007/s11104-018-3882-4.pdf},
  title = {Intercropping With Two Native Woody Shrubs Improves Water Status and Development of Interplanted Groundnut and Pearl Millet in the Sahel},
  year = {2019}
}

Scarcity of plant available water is a major challenge for rainfed agriculture throughout the Sahel. At two long-term experiments in Central and Southern Senegal, optimized intercropping with native woody shrubs, Piliostigma reticulatum (DC.) Hochst or Guiera senegalensis J.F. Gmel, (elevated densities and annual coppiced biomass returned to soils) have shown significant improvement of soil-plant-water relations, nutrient availability, and crop yields. The objective was to investigate soil physical properties to develop a mechanistic understanding for the observed improvement of water dynamics due to optimized shrub intercropping. The field experiments had a split-plot factorial design with shrubs as the main factor and fertilizer rate (0, 0.5, 1.0. 1.5 times the recommended addition of N-P-K fertilizer) as the subplot factor. This experiment was carried out at the sites of Keur Matar Arame (Keur Matar) with G. senegalensis and Nioro du Rip (Nioro) with P. reticulatum. Water retention characteristic, unsaturated hydraulic conductivity, surface evaporation, and surface infiltration were measured in the zero fertilizer treatment. At Keur Matar samples were collected from crop + shrub plots near (<0.5 m) the shrub canopy (CSn), crop + shrub plots far (>1 m) from the canopy (CSf) and in crop only plots (CO). At Nioro samples were taken in CSn, CSf, CO, and also from bare soil with no crops or shrubs growing (BS). Infiltration in CO plots compared to CSn plots was 75% and 28% higher at Keur Matar and Nioro, respectively. At Keur Matar water retention was significantly higher at wilting point (−1.5 MPa) in the CSn treatment than in the CSf treatment with values of 0.030 and 0.016 m3 m−3, respectively. At Nioro there was no significant difference in wilting point water content between treatments. These results indicate that shrubs slow down soil water as it infiltrates in the sandy soils and that the large additions of shrub biomass over a ten year period has had a small but significant effect on water retention at wilting point. This study highlights the role that shrub presence and biomass additions play in altering centimeter-scale soil properties.

@article{p2018-Bogie-et-al,
  author = {Bogie, N and Bayala, R. and Diedhiou, I. and Dick, R.P. and Ghezzehei, T. A.},
  doi = {10.1016/j.still.2018.05.010},
  status = {published},
  journal = {Soil and Tillage Research},
  keywords = {Soil structure, Sahel, Agroforestry},
  month = may,
  pages = {153--163},
  sort-word = {agroecology},
  title = {Alteration of soil physical properties and processes after ten years of intercropping with native shrubs in the Sahel},
  volume = {182},
  year = {2018},
  bdsk-url-1 = {https://doi.org/10.1016/j.still.2018.05.010}
}

Hydraulic redistribution (HR) by woody vegetation has been proposed as a potential water source for crops in intercropped systems. The native woody shrub, Guiera senegalensis J.F. Gmel, grows in the fields of farmers across the African Sahel and has shown profound yield benefits to associated pearl millet (Pennisetum glaucum) crops, especially in drought years. We tested whether this benefit resulted from the shrubs performing hydraulic redistribution (HR) with pearl millet using some of this HR water. During an experimentally imposed drought, an enriched deuterium (2H) water tracer applied to 1 m deep roots of G. senegalensis shrubs was detected (2H ≥ +300‰) in aboveground stems of intercropped millet within 12–96 h of tracer introduction. The only viable path for the 2H-enriched H2O into millet was via HR by the shrubs, which confirmed active HR during the time when growing millet was under severe drought stress. Millet biomass production when intercropped with shrubs was over 900% greater than crops grown without shrubs present. Improvement of growing conditions previously found near shrubs cannot fully account for the benefit to associated millet under extreme drought stress without considering the positive impact of the transfer of HR water. This finding illuminates HR and water transfer as an important mechanism in a successful agroforestry system in a region where food security is a serious issue.

@article{p2018-Bogie-et-al-b,
  author = {Bogie, NA and Bayala, R and Diedhiou, I and Conklin, MH and Fogel, M and Dick, R and Ghezzehei, TA},
  doi = {10.3389/fenvs.2018.00098},
  status = {published},
  journal = {Frontiers in Environmental Science: Agroecology \& Land Use Systems},
  pages = {98},
  pdf = {http://www.readcube.com/articles/10.3389/fenvs.2018.00098},
  sort-word = {agroecology},
  title = {Hydraulic Redistribution by Native Sahelian Shrubs: Bioirrigation to Resist In-Season Drought},
  volume = {6},
  year = {2018},
  bdsk-url-1 = {https://doi.org/10.3389/fenvs.2018.00098}
}

Organic amendments, such as compost and biochar, mitigate the environmental burdens associated with wasting organic resources and close nutrient loops by capturing, transforming, and resupplying nutrients to soils. While compost or biochar application to soil can enhance an agroecosystem’s capacity to store carbon and produce food, there have been few field studies investigating the agroecological impacts of amending soil with biochar co-compost, produced through the composting of nitrogen-rich organic material, such as manure, with carbon-rich biochar. Here, we examine the impact of biochar co-compost on soil properties and processes by conducting a field study in which we compare the environmental and agronomic impacts associated with the amendment of either dairy manure co-composted with biochar, dairy manure compost, or biochar to soils in a winter wheat cropping system. Organic amendments were applied at equivalent C rates (8 Mg C ha−1). We found that all three treatments significantly increased soil water holding capacity and total plant biomass relative to the no-amendment control. Soils amended with biochar or biochar co-compost resulted in significantly less greenhouse gas emissions than the compost or control soils. Biochar co-compost also resulted in a significant reduction in nutrient leaching relative to the application of biochar alone or compost alone. Our results suggest that biochar co-composting could optimize organic resource recycling for climate change mitigation and agricultural productivity while minimizing nutrient losses from agroecosystems.

@article{GAO2023,
  title = {Biochar co-compost improves nitrogen retention and reduces carbon emissions in a winter wheat cropping system},
  journal = {Global Change Biology Bioenergy},
  volume = {00},
  doi = {10.1111/gcbb.13028},
  pages = {1-16},
  year = {2023},
  author = {Gao, Si and Harrison, Brendan and Thao, Touyee and Gonzales, Melinda and An, Di and Ghezzehei, Teamrat and Diaz, Gerardo and Ryals, Rebecca}
}

Direct characterization of hysteresis in water retention (WR) is typically cumbersome and time consuming. Thus, such data are scarce, and even when available are typically ignored in unsaturated flow modeling. One reason for disregard of this ubiquitous and significant feature of WR is lack of a universally applicable index for the degree of hysteresis that allows a priori assessment of its effect on flow. In this note we show that the mismatch between the hydraulic capacity functions of the primary drainage and imbibition curves can serve as generalized index for degree of hysteresis (H). Moreover, we showed that hysteresis indices of a broad range of soils are linearly related with the natural-logarithm of the van Genuchten n parameter (r^2 = 0.73). This model allows predicting the degree of hysteresis and the missing hysteresis branch using only wetting or drying water retention data. The robustness of the proposed index was illustrated by comparing it with error in simulated moisture redistribution in a horizontal column that arises from ignoring hysteresis.

@article{p2011-Gebrenegus-Ghezzehei,
  author = {Gebrenegus, Thomas and Ghezzehei, Teamrat A.},
  date-modified = {2018-05-30 21:20:08 +0000},
  doi = {10.2136/sssaj2011.0082N},
  journal = {Soil Science Society of America Journal},
  status = {published},
  number = {6},
  pages = {2122-2127},
  researchgate = {https://www.researchgate.net/publication/274221936_An_Index_for_Degree_of_Hysteresis_in_Water_Retention},
  sort-word = {method},
  title = {An Index for Degree of Hysteresis in Water Retention},
  volume = {75},
  year = {2011},
  bdsk-url-1 = {https://doi.org/10.2136/sssaj2011.0082N}
}

The shrink-swell behavior of active clays in response to changes in physicochemical conditions creates great challenges for construction of geotechnical barriers for hazardous waste isolation, and is of significant importance for management of agricultural and natural resources. Initiation and evolution of desiccation cracks in active clays are strongly dependent on physicochemical initial and boundary conditions. To investigate effects of bentonite content (20, 40, 60%), pore fluid chemistry (0.05 and 0.5M NaCl) and drying rates (40 and 60\,^∘C) on cracking behavior, well-controlled dehydration experiments were conducted and X-ray Computed Tomography (CT) was applied to visualize and quantify geometrical features of evolving crack networks. A stochastic model based on the Fokker-Plank equation was adopted to describe the evolution of crack aperture distributions (CAD) and to assess the impact of physicochemical factors on cracking behavior. Analyses of crack porosity and crack specific surface area showed that both clay content and temperature had larger impact on cracking than pore fluid concentration. More cracks formed at high bentonite contents (40 and 60%) and at high drying rate (60\,^∘C). The drift, diffusion and source terms derived from stochastic analysis indicated that evaporative demand had greater influence on the dynamics of the CAD than solution chemistry.

@article{p2011-Gebrenegus-Ghezzehei-Tuller,
  author = {Gebrenegus, T. and Ghezzehei, T. A. and Tuller, M.},
  date-modified = {2018-05-27 20:29:25 +0000},
  doi = {10.1016/j.jconhyd.2011.07.004},
  journal = {Journal of Contaminant Hydrology},
  status = {published},
  number = {1-2},
  pages = {100-112},
  researchgate = {https://www.researchgate.net/publication/51629103_Physicochemical_controls_on_initiation_and_evolution_of_desiccation_cracks_in_sand-bentonite_mixtures_X-ray_CT_imaging_and_stochastic_modeling},
  title = {Physicochemical controls on initiation and evolution of desiccation cracks in sand-bentonite mixtures: X-ray CT imaging and stochastic modeling},
  volume = {126},
  year = {2011},
  bdsk-url-1 = {https://doi.org/10.1016/j.jconhyd.2011.07.004}
}

Soil aggregate degradation during medium and high severity fires is often identified as the main mechanism that leads to loss of soil organic matter due to fire. Low severity fires, however, are considered not to cause aggregate degradation assuming that temperatures <250º C, as occurring during low-severity burns, have only limited effects on the stability of the soil organic binding agents. Recent studies suggest that low severity burns may cause soil aggregate degradation due to rapid vaporization of soil pore water that can induce pressure on the soil aggregates beyond their yield stress. Such pressure-driven degradation of soil aggregates may expose physically protected organic carbon to decomposition. Our study investigated the effect of a low-severity fire on soil organic matter, water extractable organic C and N as well as respiration for two initial soil moisture conditions undergoing three "heating regimes" using aggregates from a California forest and Nevada shrubland soil. We found that initially moist soil aggregates that were rapidly heated up degraded the most, showing increased cumulative carbon mineralization when compared to aggregates that were not heated, aggregates that were dry before being heated , and initially moist soil aggregates that were slowly heated. Our results suggest that exposure of previously physically protected organic carbon within the soil aggregates to oxidative conditions was the most likely cause of increased rates of decomposition of organic matter after low-intensity burns. Additionally, we show that for a shrubland soil, aggregates with relatively low organic carbon content, low severity burns increased cumulative carbon mineralization. We hypothesized that this was due to decomposition of cytoplasmic material from lysed microbes. Our results suggest that low severity burns can accelerate decomposition of soil organic carbon protected in soil aggregates.

@article{p2018-Jian-et-al,
  author = {Jian, Mathew and Berhe, Asmeret Asefaw and Berli, Markus and Ghezzehei, Teamrat A.},
  date-added = {2018-05-27 04:59:20 +0000},
  date-modified = {2018-06-27 20:51:02 +0000},
  doi = {10.3389/fenvs.2018.00066},
  journal = {Frontiers in Environmental Sciences-Soil Processes},
  status = {published},
  month = jun,
  sort-word = {biogeoscience, structure},
  title = {Vulnerability of physically protected soil organic carbon to loss under low severity fires},
  year = {2018},
  bdsk-url-1 = {https://doi.org/10.3389/fenvs.2018.00066}
}

Low-severity wildfires and prescribed burns have been steadily increasing for over three decades, currently accounting for more than half of total burned area in the Southwestern United States. Most observations immediately after low-severity burns report little adverse impacts on soil properties and processes. In a few studies, however, significant deterioration of soil structure has been observed several months after such fires. Here we show that rapid vaporization of pore water during low-severity burns raises pneumatic gas pressure inside large aggregates (20-30 mm) to damaging levels; on the order of aggregate tensile strength and high-enough to cause visco-plastic deformation. However, the impact on soil structure was not immediately perceptible. This suggests that other natural forces, such as wetting-drying and thermal cycles, are required to disrupt the weakened aggregates. Thus, adverse consequences of the suggested mechanism on soil processes and services (e.g., infiltration, erodibility, and organic matter protection) are likely overlooked.

@article{p2018-Jian-Berli-Ghezzehei,
  author = {Jian, Mathew and Berli, Markus and Ghezzehei, Teamrat A.},
  data = {10.6084/m9.figshare.6349469.v1},
  date-added = {2018-05-27 06:01:51 +0000},
  date-modified = {2018-11-14 14:17:28 -0800},
  doi = {10.1029/2018GL078053},
  journal = {Geophysical Research Letters},
  status = {published},
  month = may,
  number = {5553-5561},
  sort-word = {soil structure, aggregation},
  title = {Soil Structural Degradation during Low-severity Burns},
  volume = {45},
  year = {2018},
  bdsk-url-1 = {https://doi.org/10.1029/2018GL078053}
}

Determination of saturated hydraulic conductivity, Ks, and the van Genuchten water retention curve θ(h) parameters is crucial in evaluating unsaturated soil water flow. The aim of this work is to present a method to estimate Ks, α and n from numerical analysis of an upward infiltration process at saturation (Cap0), with (Cap0 + h) and without (Cap0) an overpressure step (h) at the end of the wetting phase, followed by an evaporation process (Evap). The HYDRUS model as well as a brute-force search method were used for theoretical loam soil parameter estimation. The uniqueness and the accuracy of solutions from the response surfaces, Ks–n, α–n and Ks–α, were evaluated for different scenarios. Numerical experiments showed that only the Cap0 + Evap and Cap0 + h + Evap scenarios were univocally able to estimate the hydraulic properties. The method gave reliable results in sand, loam and clay-loam soils.

@article{p2017-Pena-Sancho-et-al,
  author = {Pe{\~{n}}a-Sancho, C. and Ghezzehei, T.A. and Latorre, B. and Gonz{\'{a}}lez-Cebollada, C. and Moret-Fern{\'{a}}ndez, D.},
  date-modified = {2018-11-14 14:03:19 -0800},
  doi = {10.1080/02626667.2017.1343476},
  journal = {Hydrological Sciences Journal},
  status = {published},
  keywords = {soil hydraulic properties, inverse methods, HYDRUS},
  month = jul,
  number = {10},
  pages = {1683--1693},
  sort-word = {method},
  title = {Upward infiltration{extendash}evaporation method to estimate soil hydraulic properties},
  volume = {62},
  year = {2017},
  bdsk-url-1 = {https://doi.org/10.1080%2F02626667.2017.1343476},
  bdsk-url-2 = {http://dx.doi.org/10.1080/02626667.2017.1343476},
  bdsk-url-3 = {https://doi.org/10.1080/02626667.2017.1343476}
}

We conducted two 64-day incubation experiments to assess how locally produced almond-shell biochar influences soil physical and hydraulic properties. Biochar was created using slow pyrolysis at different temperatures (350 °C or 700 °C), separated into different particle sizes (<250 μm or 1–2 mm), and applied at 10 ton/ha or 60 ton/ha to a coarse-textured soil. While our analysis shows that biochar yielded greater cation exchange capacity (CEC) and specific surface area (SSA) with increasing pyrolysis temperature and finer particle size, its contributions to improving soil hydraulic properties were marginal. In the first experiment, the addition of biochar at high rates slightly improved water stable aggregate (WSA) (3.8%–5.3% increase) but has no effect on saturated hydraulic conductivity (Ksat). Soil respiration measured throughout the experiment were not significantly different among treatments. In the second experiment, the addition of biochar increased soil infiltration rate at the initial stage (8.18E–4 cm/s), but this effect diminished over time. WSA was lower for biochar amended soil and lowest at high application rates (5%–21% reduction). Cumulative carbon dioxide (CO2) flux varied between biochar particle sizes and rates. Additionally, a significant difference between the two experiments was also observed, with cumulative CO2 (38%–56% greater) and WSA (11%–40%) being inversely correlated. Our findings suggest that almond-shell derived biochar has a limited impact on arable loamy sand soil properties, specifically for water retention under short-term conditions.

@article{Thao31122025,
  author = {Thao, Touyee and Lopez, Vivian D. and Gonzales, Melinda and Berhe, Asmeret A. and Diaz, Gerardo and Ghezzehei, Teamrat A.},
  title = {Impact of almond shell biochar properties and application rate on soil physical and hydraulic characteristics},
  journal = {Sustainable Environment},
  volume = {11},
  number = {1},
  pages = {2485688},
  year = {2025},
  publisher = {Taylor \& Francis},
  doi = {10.1080/27658511.2025.2485688},
  pdf = {https://doi.org/10.1080/27658511.2025.2485688}
}

The growing water scarcity jeopardizes crop production for global food security, a problem poised to worsen under climate change–induced drought. Amending soils with locally derived biochar from pyrolyzed agricultural residues may enhance soil moisture retention and resilience, in addition to climate change mitigation. However, prior studies on the hydrologic benefits of biochar focused on optimal moisture, not water-limited conditions where biochar’s large wettable surface area could aid plants and microbes. We hypothesized that biochars differing in feedstocks would positively augment soil moisture and respiration, with overall impacts most beneficial under drier conditions. Using water vapor sorption isotherms, we used film theory to estimate the specific surface area (SSA) of biochars. We then modeled and tested the moisture retention of a coarse-textured soil amended with biochar. Additionally, a 109-day lab incubation experiment was also conducted to examine biochar effects on respiration across a moisture range spanning optimal to wilting point. Among seven tested biochars, almond shell biochar significantly increased soil moisture and yield the second highest SSA. Despite drying treatments, the amended soil maintained higher respiration than the control, indicating enhanced biological activity. The results demonstrate biochars counter drying effects in coarse soils through physical and biological mechanisms linked to increased sorptive capacity. Our findings contribute to the development of sustainable water and waste management strategies tailored to the needs of California Central Valley, where the potential for biochar application is substantial. Above all, our research fills a crucial gap by providing context-specific insights that can inform the effective utilization of locally produced biochars in the face of increasing water scarcity and excess biomass challenges.

@article{p2024-Taho-et-al,
  title = {Biochar Impacts on Soil Moisture Retention and Respiration in a Coarse-Textured Soil under Dry Conditions},
  number = {(Early View)},
  year = {2024},
  language = {en},
  journal = {Soil Sci. Soc. Am. J.},
  doi = {10.1002/saj2.20746},
  pdf = {https://acsess.onlinelibrary.wiley.com/doi/epdf/10.1002/saj2.20746},
  author = {Thao, Touyee and Harrison, Brendan and Gonzalez, Melinda and Ryals, Rebecca and Dahlquist‐Willard, Ruth and Diaz, Gerardo C. and Ghezzehei, Teamrat A.},
  pages = {1-13}
}

Finding feasible solutions for sustainable food production is challenging. Here we try to understand the balance between crop productivity and ecological stewardship using agroecological-based soil management strategies. We evaluated the potential of different organic materials such as dairy manure compost and different biochar manure co-composts, derived locally from agricultural wastes, to enhance soil ecosystem services. We assessed their potential impact on soil moisture and nutrient retention, greenhouse gas emissions, and crop productivity using data collected from an outdoor tomato column study. Results from the experiment showed potential of biochar co-composts to positively affect soil health by lessening loss of essential nutrients such as NO3−-N and NH4+-N, sustained tomato yield, and uphold crop water use efficiency. However, yield response to soil organic amendment is constrained by external factors such as irrigation strategies, with treatments under deficit irrigation greatly impacted. Overall, we observed a positive effect of adding biochar manure co-composts to soil, although best management practices are needed to optimize crop productivity and avoid unintentional consequences.

@article{p2023-Taho-et-alb,
  title = {The effects of different biochar‐dairy manure co‐composts on soil moisture and nutrients retention, greenhouse gas emissions, and tomato productivity: {Observations} from a soil column experiment},
  volume = {6},
  issn = {2639-6696, 2639-6696},
  shorttitle = {The effects of different biochar‐dairy manure co‐composts on soil moisture and nutrients retention, greenhouse gas emissions, and tomato productivity},
  doi = {10.1002/agg2.20408},
  language = {en},
  number = {3},
  journal = {Agrosystems, Geosciences \& Environment},
  author = {Thao, Touyee and Harrison, Brendan P. and Gao, Si and Ryals, Rebecca and Dahlquist‐Willard, Ruth and Diaz, Gerardo C. and Ghezzehei, Teamrat A.},
  year = {2023},
  pages = {e20408}
}

Application of biochar amendments in agricultural systems has received much attention in recent years. In this study, we assess the 5-year impacts of biochar application on soil water and plant interactions for an irrigated fresh market tomato (Solanum lycopersicum) and a rainfed pasture (Poaceae) cropping system. In particular, we focus on three varieties of locally produced biochar from agricultural waste materials—almond shell, walnut shell, and almond pruning residues that are pyrolyzed using a mobile pyrolysis unit. We used the soil hydrological model HYDRUS-1D to explicitly track seasonal and annual soil water fluxes through changes in water retention, drainage, evaporation, and plant water uptake under biochar application. Modeling results show that the application of biochar at 5% increased soil water availability within the top 20 cm for a rainfed system, irrespective of biochar amendment type. This is clearly indicative of higher plant water uptake and greater water use efficiency (WUE) under biochar application. In contrast, a similar biochar amendment for the irrigated system did not affect WUE, instead reducing seasonal soil evaporation loss and thereby reducing irrigation demand. In both cropping systems, year-to-year variability in precipitation significantly impacted the total amount of water saved under biochar application with certain amendments retaining more water than others. Given that biochar application increased water retention irrespective of cropping systems, we further used a simple approach to determine yield trade-off, if any, between control and biochar treatments. Our economic balance clearly demonstrates that the water saved by amending soil with biochar does not offset the yield disparity if compensated with carbon credits and therefore, application of biochar should be actively considered for both its direct and indirect benefits to potential greenhouse gas mitigation (e.g., diverting orchard waste from open burning), water savings, and soil health.

@article{p2023-Taho-et-al,
  author = {Thao, Touyee and Arora, Bhavna and Ghezzehei, Teamrat A.},
  journal = {Vadose Zone Journal},
  pages = {e20266},
  title = {Impact of biochar amendments on soil water and plant uptake dynamics under different cropping systems},
  year = {2023},
  doi = {10.1002/vzj2.20266},
  pdf = {https://acsess.onlinelibrary.wiley.com/doi/epdf/10.1002/vzj2.20266}
}

Most plants derive their water and nutrient needs from soils, where the resources are often scarce, patchy, and ephemeral. In natural environments, it is not uncommon for plant roots to encounter mismatched patches of water-rich and nutrient-rich regions. Such an uneven distribution of resources necessitates plants to rely on strategies that allow them to explore and acquire nutrients from relatively dry patches. We conducted a laboratory study to provide a mechanistic understanding of the biophysical factors that enable this adaptation. We grew plants in split-root pots that permitted precisely controlled spatial distributions of resources. The results demonstrated that spatial mismatch of water and nutrient availability does not cost plant productivity compared to matched distributions. Specifically, we showed that nutrient uptake is not reduced by overall soil dryness, provided that the whole plant has access to sufficient water elsewhere in the root zone. Essential strategies include extensive root proliferation towards nutrient-rich dry soil patches that allows rapid nutrient capture from brief pulses. Using high-frequency water potential measurements, we also observed nocturnal water release by roots that inhabit dry and nutrient-rich soil patches. Soil water potential gradient is the primary driver of this transfer of water from wet to dry soil parts of the root zone, which is commonly known as hydraulic redistribution (HR). The occurrence of HR prevents the soil drying from approaching the permanent wilting point, and thus supports root functions and enhance nutrient availability. Our results indicate that roots facilitate HR by increasing root-hair density and length and deposition of organic coatings that alter water retention. Therefore, we conclude that biologically-controlled root adaptation involves multiple strategies that compensate for nutrient acquisition under mismatched resource distributions. Based on our findings, we proposed a nature-inspired nutrient management strategy for significantly curtailing water pollution from intensive agricultural systems.

@article{P2020-Yan,
  title = {Root uptake under mismatched distributions of water and nutrients in the root zone},
  author = {Yan, Jing and Bogie, Nathaniel A. and Ghezzehei, Teamrat A.},
  journal = {Biogeosciences},
  volume = {17},
  pages = {6377–6392},
  status = {published},
  doi = {10.5194/bg-17-6377-2020},
  data = {doi:10.6071/M39M2T},
  pdf = {https://bg.copernicus.org/articles/17/6377/2020/bg-17-6377-2020.pdf},
  mendeley = {https://www.mendeley.com/catalogue/f48d8444-28ae-390c-a09d-07f73a0aadb6/},
  year = {2020}
}

Soil science is at a critical juncture in defining its disciplinary identity. This paper critically examines a recent proposal to define the field primarily through its societal contributions, arguing that such an approach risks constraining soil science’s scientific identity. By analyzing historical perspectives and drawing parallels with other scientific disciplines, we demonstrate that transformative solutions often emerge from fundamental research. We propose a definition that positions soil science as a natural science studying the complex planetary surfaces, encompassing both living and nonliving systems, and maintaining intellectual freedom while remaining responsive to environmental challenges.

@article{Ghezzehei2025a,
  author = {Ghezzehei, Teamrat A. and Berhe, Asmeret A.},
  title = {{Commentary}: Defining soil science: Balancing fundamental research and societal needs},
  journal = {Soil Science Society of America Journal},
  volume = {89},
  pages = {e70059},
  year = {2025},
  publisher = {Taylor \& Francis},
  doi = {10.1002/saj2.70059},
  pdf = {https://acsess.onlinelibrary.wiley.com/doi/epdf/10.1002/saj2.70059}
}

Aggregation is a vital characteristic of soil structure that affects its physical and biogeochemical properties. Aggregation results from the cohesion of primary minerals with organic or inorganic constituents. It depends on the dynamic balance between binding and fragmentation. There are two classes of aggregation. Mechanical aggregates are formed instantly by external forces and are often unstable. Hierarchical aggregates result from slow binding and are stable. Characterization of aggregation includes size, shape, stability, configuration, and their arrangement within soil. The nature of hierarchical aggregation is inferred from size separation of aggregates. Aggregate stability indicates their ability to persist under disruptive forces.

@incollection{GHEZZEHEI2023,
  title = {Aggregation},
  booktitle = {Encyclopedia of Soils in the Environment},
  volume = {5: Soil Physics},
  edition = {2nd},
  publisher = {Elsevier},
  editor = {Goss, Michael J. and Oliver, Margaret},
  year = {2023},
  isbn = {978-0-12-409548-9},
  doi = {10.1016/B978-0-12-822974-3.00277-9},
  author = {Ghezzehei, Teamrat A.},
  keywords = {Aeration, Aggregate, Cementation, Clod, Imaging, Organic matter, Rhizosheath, Rhizosphere, Root, Sequestration, Soil health, Tillage, X-racy CT},
  sort-word = {aggregation}
}

Soil water status is one of the most important environmental factors that control microbial activity and rate of soil organic matter decomposition (SOM). Its effect can be partitioned into effect of water energy status (water potential) on cellular activity, effect of water volume on cellular motility and aqueous diffusion of substrate and nutrients, as well as effect of air content and gas-diffusion pathways on concentration of dissolved oxygen. However, moisture functions widely used in SOM decomposition models are often based on empirical functions rather than robust physical foundations that account for these disparate impacts of soil water. The contributions of soil water content and water potential vary from soil to soil according to the soil water characteristic (SWC), which in turn is strongly dependent on soil texture and structure.The overall goal of this study is to introducea physically based modelling framework of aerobic microbial respiration that incorporates the role of SWC under arbitrary soil moisture status.The model was tested by compariing it with published datasets of SOM decomposition under laboratory conditions.

@article{p2019-Ghezzehei-et-al,
  author = {Ghezzehei, Teamrat A. and Sulman, Benjamin and Arnold, Chelsea L. and Bogie, Nathaniel A. and Berhe, Asmeret Asefaw},
  doi = {10.5194/bg-16-1187-2019},
  data = {10.6084/m9.figshare.7749332},
  pdf = {https://bg.copernicus.org/articles/16/1187/2019/bg-16-1187-2019.pdf},
  sort-word = {CO2 flux, Soil respiration, Soil Carbon, aggregation, modeling,biogeoscience},
  journal = {Biogeosciences},
  status = {published},
  volume = {16},
  pages = {1187-1209},
  month = mar,
  title = {On the role of soil water retention characteristic on aerobic microbial respiration},
  mendeley = {https://www.mendeley.com/catalogue/4b76b8fa-36d3-3812-bd2f-086f3faaab4f/},
  year = {2019}
}

Plant roots alter soil properties at an expensive physiological cost by releasing large quantities of organic carbon (rhizodeposition). The role of rhizodeposits in enhancing beneficial microbial activity and bio-geochemical nutrient mobilization is widely appreciated. But the role of rhizodeposits in water uptake has started gaining modest attention only recently. In this study we present a single root model, which demonstrates the possibility for rhizodeposits to create built-in water potential gradient. The conceptual basis for this model rests on three premises: (a) rhizodeposits are distributed in declining profile with distance from the root surface, (b) considerable fraction of rhizodeposits are strongly adhered to soil particles, and (c) rhizodeposits have the ability to retain water. Thus, variable concentration of affixed rhizodeposits results in a gradient of water potential without commensurate decline in water content with proximity to root surface. To corroborate premises (b) and (c), we conducted experiments using synthetic analog of rhizodeposits (Polygalacturonic Acid, PGA) and glass-bead and sand media. Envi-ronmental scanning electron microscopy was utilized to show affixation of PGA on glass beads during drying as well as pore-scale enhanced water retention. Macroscopic enhancement of water retention was characterized by dew-point potentiametry. We simulated water uptake by a root at constant potential transpiration rates representing high atmospheric demand and considered three distinct spatial distri-bution patterns of rhizodeposits as well as a control (without rhizodeposition). The model simulations indicate that the benefit of such variable distribution of exudates is more pronounced when (a) the poten-tial water uptake rate is high or (b) the rhizodeposits are constrained to a narrow volume of rhizosphere soil.

@article{p2015-Ghezzehei-Albalasmeh,
  author = {Ghezzehei, Teamrat A. and Albalasmeh, Ammar A.},
  doi = {10.1016/j.ecolmodel.2014.10.028},
  journal = {Ecological Modeling},
  status = {published},
  keywords = {Rhizosphere, Roots, Exudates, Water-uptake},
  month = feb,
  pages = {53-63},
  researchgate = {https://www.researchgate.net/publication/271327580_Spatial_distribution_of_rhizodeposits_provides_built-in_water_potential_gradient_in_the_rhizosphere},
  sort-word = {rhizosphere, modeling},
  title = {Spatial distribution of rhizodeposits provides built-in water potential gradient in the rhizosphere},
  volume = {298},
  year = {2015}
}

Recently, the potential for biochar use to recapture excess nutrients from dairy wastewater has been a focus of a growing number of studies. It is suggested that biochar produced from locally available excess biomass can be important in reducing release of excess nutrient elements from agricultural runoff, improving soil productivity, and long-term carbon (C) sequestration. Here we present a review of a new approach that is showing promise for the use of biochar for nutrient capture. Using batch sorption experiments, it has been shown that biochar can adsorb up to 20–43% of ammonium and 19–65% of the phosphate in flushed dairy manure in 24 h. These results suggest a potential of biochar for recovering essential nutrients from dairy wastewater and improving soil fertility if the enriched biochar is returned to soil. Based on the sorption capacity of 2.86 and 0.23 mg ammonium and phosphate, respectively, per gram of biochar and 10–50% utilization of available excess biomass, in the state of California (US) alone, 11 440 to 57 200 tonnes of ammonium-N and 920–4600 tonnes of phosphate can be captured from dairy waste each year while at the same time disposing up to 8–40 million tons of excess biomass.

@article{p2014-Ghezzehei-Sarkhot-Berhe,
  author = {Ghezzehei, T. A. and Sarkhot, D. V. and Berhe, A. A.},
  date-modified = {2018-05-27 21:16:04 +0000},
  doi = {10.5194/se-5-953-2014},
  journal = {Solid Earth},
  status = {published},
  month = apr,
  number = {1},
  pages = {1101--1125},
  researchgate = {https://www.researchgate.net/publication/265407626_Biochar_can_be_used_to_capture_essential_nutrients_from_dairy_wastewater_and_improve_soil_physico-chemical_properties},
  sort-word = {environmental quality},
  title = {Biochar can be used to recapture essential nutrients from dairy wastewater and improve soil quality},
  volume = {5},
  year = {2014}
}

Subsurface geochemical and biological transformations often influence fluid flow by altering the pore space morphology and related hydrologic properties such as porosity and permeability. In most coupled-processes models changes in porosity are inferred from geochemical and biological process models using mass-balance. The corresponding evolution of permeability is estimated using (semi-) empirical porosity–permeability functions such as the Kozeny–Carman equation or power-law functions. These equations typically do not account for the heterogeneous spatial distribution and morphological irregularities of the geochemical precipitates and biomass. As a result, predictions of permeability evolution are generally unsatisfactory. In this communication, we demonstrate the significance of pore-scale precipitate distribution on porosity–permeability relations using high resolution simulations of fluid flow through a single pore interspersed with crystals. Based on these simulations, we propose a modification to the Kozeny–Carman model that accounts for the shape of the deposits. Limited comparison with published experimental data suggests the plausibility of the proposed conceptual model.

@article{p2012-Ghezzehei,
  author = {Ghezzehei, T. A.},
  date-modified = {2018-05-27 20:41:59 +0000},
  doi = {10.1016/j.advwatres.2011.12.015},
  journal = {Advances in Water Resources},
  status = {published},
  keywords = {Porosity, Permeability, Clogging, Coupled processes, Mineral precipitation},
  pages = {1-6},
  researchgate = {https://www.researchgate.net/publication/249314234_Linking_sub-pore_scale_heterogeneity_of_biological_and_geochemical_deposits_with_changes_in_permeability},
  title = {Linking sub-pore scale heterogeneity of biological and geochemical deposits with changes in permeability},
  volume = {39},
  year = {2012},
  bdsk-url-1 = {https://doi.org/10.1016/j.advwatres.2011.12.015}
}

@incollection{2012-Ghezzehei-b,
  author = {Ghezzehei, TA},
  booktitle = {Handbook of Soil Sciences},
  status = {published},
  editor = {Huang, P.M. and Li, Y. and Sumner, M.E.},
  publisher = {CRC Press, Boca Raton, Fla.,},
  researchgate = {https://www.researchgate.net/publication/285160921_Soil_Structure},
  sort-word = {aggregation, review},
  title = {Soil Structure},
  volume = {1. Properties and Processes},
  year = {2012}
}

Methane hydrate present in permafrost and sub oceanic sediments has been identified as a potentially large energy source. Producing natural gas from hydrate results in the hydrate dissociating into gas and water, which then become distributed in the pore space according to gravitational, viscous, and capillary forces. Capillary pressure is a function of the medium (wettability, geometry) and the saturations of all phases (e.g. gas, hydrate, water) in the pore space. The presence of hydrate alters the geometry of the pore space, changing the capillary pressure-saturation relationship from the hydrate-free condition. Understanding the capillary pressure-saturation relationship of hydrate-bearing media is important for modeling the flow of gas and water through that medium, and predicting natural gas production from hydrate-bearing reservoirs. We have developed a method for measuring the capillary pressure-saturation relationship in methane hydrate-bearing sand, and our measurements and modeling aid in understanding the behavior of the gas and water in hydrate-bearing sediment. Our experiments involve hydrate formation in unsaturated sand, saturating the sample, followed by step-wise drainage from full water saturation to residual water saturation while measuring the pressure difference between the water and gas phases. During drainage, a number of intermediate static equilibrium conditions were established during which flow was discontinued. The static equilibrium observations provide water saturation vs. capillary pressure relations. For selected samples, drainage was followed by step-wise imbibition to full saturation.

@inproceedings{2010-Ghezzehei-Kneafsey,
  author = {Ghezzehei, Teamrat Afewerki and Kneafsey, Timothy J.},
  booktitle = {Offshore Technology Conference},
  status = {published},
  date-modified = {2018-05-27 20:28:21 +0000},
  doi = {10.4043/20550-MS},
  pages = {OTC-20550-MS},
  publisher = {Offshore Technology Conference},
  researchgate = {https://www.researchgate.net/publication/241786055_Capillary_Pressure_and_Relative_Permeability_of_Methane_Hydrate_Bearing_Sediments},
  title = {Measurements of the Capillary Pressure-Saturation Relationship of Methane Hydrate Bearing Sediments},
  year = {2010},
  bdsk-url-1 = {https://doi.org/10.4043%2F20550-ms},
  bdsk-url-2 = {http://dx.doi.org/10.4043/20550-ms},
  bdsk-url-3 = {https://doi.org/10.4043/20550-ms},
  bdsk-url-4 = {https://doi.org/10.4043/20550-MS}
}

Application of time domain reflectometry (TDR) in soil hydrology often involves the conversion of TDR‐measured dielectric permittivity to water content using universal calibration equations (empirical or physically based). Deviations of soil‐specific calibrations from the universal calibrations have been noted and are usually attributed to peculiar composition of soil constituents, such as high content of clay and/or organic matter. Although it is recognized that soil disturbance by TDR waveguides may have impact on measurement errors, to our knowledge, there has not been any quantification of this effect. In this paper, we introduce a method that estimates this error by combining two models: one that describes soil compaction around cylindrical objects and another that translates change in bulk density to evolution of soil water retention characteristics. Our analysis indicates that the compaction pattern depends on the mechanical properties of the soil at the time of installation. The relative error in water content measurement depends on the compaction pattern as well as the water content and water retention properties of the soil. Illustrative calculations based on measured soil mechanical and hydrologic properties from the literature indicate that the measurement errors of using a standard three‐prong TDR waveguide could be up to 10%. We also show that the error scales linearly with the ratio of rod radius to the interradius spacing.

@article{p2008-Ghezzehei,
  author = {Ghezzehei, Teamrat A.},
  date-modified = {2018-05-31 13:28:50 +0000},
  doi = {10.1029/2007WR006502},
  journal = {Water Resources Research},
  status = {published},
  keywords = {TDR, water content, compaction, measurement error},
  number = {8},
  pdf = {https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2007WR006502},
  sort-word = {method},
  title = {Errors in determination of soil water content using time domain reflectometry caused by soil compaction around waveguides},
  volume = {44},
  year = {2008},
  bdsk-url-1 = {https://doi.org/10.1029/2007WR006502}
}

@article{p2008-Ghezzehei-b,
  author = {Ghezzehei, Teamrat A.},
  date-modified = {2018-05-27 20:01:54 +0000},
  journal = {Soil Science Society of America Journal},
  status = {published},
  number = {1},
  pages = {277},
  publisher = {Soil Science Society of America},
  title = {Book Review: Clay Swelling and Colloid Stability},
  volume = {72},
  sort-word = {review},
  year = {2008},
  bdsk-url-1 = {https://doi.org/10.2136%2Fsssaj2007.0024br},
  bdsk-url-2 = {http://dx.doi.org/10.2136/sssaj2007.0024br},
  bdsk-url-3 = {https://doi.org/10.2136/sssaj2007.0024br}
}

The Gardner and van Genuchten models of relativepermeability are widely used in analytical and numerical solutions toflow problems. However, the applicability of the Gardner model to realproblems is usually limited, because empirical relative permeability datato calibrate the model are not routinely available. In contrast, vanGenuchten parameters can be estimated using more routinely availablematric potential and saturation data. However, the van Genuchten model isnot amenable to analytical solutions. In this paper, we introducegeneralized conversion formulae that reconcile these two models. Ingeneral, we find that the Gardner parameter alpha G is related to the vanGenuchten parameters alpha vG and n by alpha G/alpha vG  ; 1.3 n. Thisconversion rule will allow direct recasting of Gardner-based analyticalsolutions in the van Genuchten parameter space. The validity of theproposed formulae was tested by comparing the predicted relativepermeability of various porous media with measured values.

@article{p2007-Ghezzehei-Kneafsey-Su,
  author = {Ghezzehei, Teamrat A. and Kneafsey, Timothy J. and Su, Grace W.},
  date-modified = {2018-05-27 20:24:43 +0000},
  doi = {10.1029/2006WR005339},
  journal = {Water Resources Research},
  status = {published},
  month = oct,
  number = {10},
  pdf = {https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2006WR005339},
  researchgate = {https://www.researchgate.net/publication/236496415_Correspondence_of_the_Gardner_and_van_Genuchten-Mualem_relative_permeability_function_parameters},
  title = {Correspondence of the Gardner and van Genuchten-Mualem relative permeability function parameters},
  volume = {43},
  year = {2007},
  bdsk-url-1 = {https://doi.org/10.1029%2F2006wr005339},
  bdsk-url-2 = {http://dx.doi.org/10.1029/2006wr005339},
  bdsk-url-3 = {https://doi.org/10.1029/2006wr005339},
  bdsk-url-4 = {https://doi.org/10.1029/2006WR005339}
}

The Nopal I mine in Pena Blanca, Chihuahua, Mexico, contains a uranium ore deposit within fractured tuff. Previous mining activities exposed a level ground surface 8 m above an excavated mining adit. In this paper, we report results of ongoing research to understand and model percolation through the fractured tuff and seepage into a mined adit both of which are important processes for the performance of the proposed nuclear waste repository at Yucca Mountain. Travel of water plumes was modeled using one-dimensional numerical and analytical approaches. Most of the hydrologic properly estimates were calculated from mean fracture apertures and fracture density. Based on the modeling results, we presented constraints for the arrival time and temporal pattern of seepage at the adit.

@inproceedings{2006-Ghezzehei-et-al,
  author = {Ghezzehei, T.A. and Dobson, P.F. and Rodriguez, J.A. and Cook, P.J.},
  booktitle = {Proceedings of the 11th International High Level Radioactive Waste Management Conference, IHLRWM},
  status = {published},
  date-modified = {2018-05-27 19:55:55 +0000},
  institution = {US-DOE-OCRWM},
  month = jun,
  pdf = {https://www.osti.gov/servlets/purl/893843},
  publisher = {American Nuclear Society},
  title = {Infiltration and Seepage Through Fractured Welded Tuff},
  year = {2006},
  bdsk-url-1 = {https://doi.org/10.2172%2F893843},
  bdsk-url-2 = {http://dx.doi.org/10.2172/893843},
  bdsk-url-3 = {https://doi.org/10.2172/893843}
}

Flow diversion around subsurface cavities in unsaturated fractured media is important to numerous environmental and engineering applications. This paper provides analytical solutions to partial and complete flow diversion around cavities intersected by fractures under steady state conditions. It is focused on a typical trifracture junction located upstream from a cavity surface. Fractures are modeled as two‐dimensional porous media with an exponential relationship between the capillary pressure and unsaturated hydraulic conductivity. The solutions show that the vertical distance between the fracture end and the nearest junction (Z) and the slope of the unsaturated hydraulic conductivity (α) are by far the most important determinants of flow diversion. In fact, the product of Z and α enters the threshold flux and liquid entry flux equations as a dimensionless sorptive length (s). This relationship between Z and α is shown to have important implications for uncertainty and scalability of calibrated model parameters. The solutions given in this paper are expected to be directly applicable to cavities on the order of the fracture spacing.

@article{p2005-Ghezzehei-b,
  author = {Ghezzehei, Teamrat A.},
  date-modified = {2018-05-27 20:15:11 +0000},
  doi = {10.1029/2004WR003860},
  journal = {Water Resources Research},
  status = {published},
  month = nov,
  number = {11},
  pdf = {https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2004WR003860},
  researchgate = {https://www.researchgate.net/publication/228661472_Flow_diversion_around_cavities_in_fractured_media},
  title = {Flow diversion around cavities in fractured media},
  volume = {41},
  year = {2005}
}

In recent years, significant advances have been made in our understanding of the complex flow processes in individual fractures, aided by flow visualization experiments and conceptual modeling efforts. These advances have led to the recognition of several flow regimes in unsaturated individual fractures subjected to different initial and boundary conditions. For an idealized smooth fracture surface the most important regimes are film flow, rivulet flow, and sliding of droplets. The existence of such significantly dissimilar flow regimes has been a major hindrance in the development of self-consistent conceptual models of flow for single fracture surfaces that encompass all the flow regimes. The objective of this study is to delineate the existence of the different flow regimes in individual fracture surfaces. For steady state flow conditions, we developed physical constraints on the different flow regimes that satisfy minimum energy configurations, which enabled us to segregate the wide range of fracture flux (volumetric flow rate per fracture width) into several flow regimes. These are, in increasing order of flow rate, flow of adsorbed films, flow of sliding drops, rivulet flow, stable film flow, and unstable (turbulent) film flow. The scope of this study is limited to wide-aperture smooth fractures with the flow on the opposing sides of fracture being independent.

@article{p2005-Ghezzehei,
  author = {Ghezzehei, Teamrat A.},
  date-modified = {2018-05-27 20:14:34 +0000},
  doi = {10.1029/2004WR003164},
  journal = {Water Resources Research},
  status = {published},
  month = nov,
  number = {11},
  pages = {W11503},
  pdf = {https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2004WR003164},
  researchgate = {https://www.researchgate.net/publication/228979913_Constraints_for_flow_regimes_on_smooth_fracture_surfaces},
  title = {Constraints for flow regimes on smooth fracture surfaces},
  volume = {40},
  year = {2005}
}

Flow processes in unsaturated fractures considerably differ from flow in rock matrix because of the dominance of gravitational forces, accentuated by variations in fracture geometry. This gives rise to liquid fragmentation, fingering, and intermittent flow regimes that are not amenable to standard continuum representation. We develop an alternative modeling framework to describe the onset of liquid fragmentation and subsequent flow behavior of discrete liquid clusters. The transition from a slowly growing anchored liquid element to a finger‐forming mobile liquid element is estimated from the force balance between retarding capillary forces dominated by contact angle hysteresis and suspended liquid weight. A model for liquid fragmentation within the fracture plane (smooth and parallel walled fractures) for given a steady input flux and aperture size is developed and tested. Predictions of sizes and detachment intervals of liquid elements are in good agreement with experimental results. The results show that the mass of detached liquid element is only weakly related to flow rate but increases with fracture aperture size. Periodic discharge similar to that experimentally observed is a result of the interplay between capillary, viscous, and gravitational forces. We show that the presence of even a few irregularities in a fracture plane may induce complicated flux patterns downstream. Similar erratic fluxes are observed in studies involving gravity‐driven unsaturated flow.

@article{p2005-Ghezzehei-Or,
  author = {Ghezzehei, TA and Or, D},
  date-modified = {2018-05-27 20:11:40 +0000},
  doi = {10.1029/2004WR003834},
  journal = {Water Resources Research},
  status = {published},
  keywords = {fracture, intermittent, episodic, finger, dripping, vadose zone},
  number = {12},
  pages = {W12406},
  researchgate = {https://www.researchgate.net/publication/37451025_Liquid_fragmentation_and_intermittent_flow_regimes_in_unsaturated_fractured_media},
  title = {Liquid fragmentation and intermittent flow regimes in unsaturated fractured media},
  volume = {41},
  year = {2005}
}

Cavities excavated in unsaturated geological formations are important to activities such as nuclear waste disposal and mining. Such cavities provide a unique setting for simultaneous occurrence of seepage and evaporation. Previously, inverse numerical modeling of field liquid-release tests and associated seepage into cavities were used to provide seepage-related large-scale formation properties, ignoring the impact of evaporation. The applicability of such models was limited to the narrow range of ventilation conditions under which the models were calibrated. The objective of this study was to alleviate this limitation by incorporating evaporation into the seepage models. We modeled evaporation as an isothermal vapor diffusion process. The semiphysical model accounts for the relative humidity (RH), temperature, and ventilation conditions of the cavities. The evaporation boundary layer thickness (BLT) over which diffusion occurs was estimated by calibration against free-water evaporation data collected inside the experimental cavities. The estimated values of BLT were 5 to 7 mm for the open underground drifts and 20 mm for niches closed off by bulkheads. Compared with previous models that neglected the effect of evaporation, this new approach showed significant improvement in capturing seepage fluctuations into open cavities of low RH. At high relative-humidity values (>85%), the effect of evaporation on seepage was very small.

@article{p2004-Ghezzehei-et-al,
  author = {Ghezzehei, T. A. and Trautz, R. C. and Finsterle, S. and Cook, P. J. and Ahlers, C. F.},
  date-modified = {2018-05-27 20:09:29 +0000},
  doi = {10.2136/vzj2004.0806},
  journal = {Vadose Zone Journal},
  status = {published},
  month = aug,
  number = {3},
  pages = {806--818},
  researchgate = {https://www.researchgate.net/publication/236570656_Modeling_coupled_evaporation_and_seepage_in_ventilated_tunnels},
  sort-word = {nuclear waste},
  title = {Modeling Coupled Evaporation and Seepage in Ventilated Cavities},
  volume = {3},
  year = {2004}
}

This study deals with deformation of small pores in wet soils of relatively high bulk density such as in the final settlement phase of tilled or disturbed soils. Pore deformation was modeled by volume reduction of spherical voids embedded in a homogenous soil matrix. External constant stress and overburden were considered as steady stresses because the change in interaggregate contact stress under overburden is slow compared to the associated strain rate. In contrast, stress due to passage of farm implements was considered as transient because the rate of change of interaggregate stress is comparable with the strain rate. Rheological behavior of the soil matrix under steady and transient stresses was obtained from independent rheological measurements. Experimental data from the literature were used to illustrate the model. Model predictions of relative density compared favorably with experimental data for constant stress application as well as for constant strain rate experiments. Results showed that the rate of densification decreased as the relative density approached unity (complete pore closure) and the relative stress required for driving densification increased exponentially with increasing relative density.

@article{p2003-Ghezzehei-Or-b,
  author = {Ghezzehei, Teamrat A. and Or, Dani},
  date-modified = {2018-05-27 19:55:55 +0000},
  journal = {Water Resources Research},
  status = {published},
  month = mar,
  number = {3},
  sort-word = {aggregation},
  title = {Stress-induced volume reduction of isolated pores in wet soil},
  doi = {10.1029/2001wr001137},
  volume = {39},
  year = {2003}
}

Evaporation from the surface of a porous medium is a complex process, governed by interplay between (1) coupled liquid and vapor flow in the porous medium, and (2) relative humidity, temperature, and aerodynamic conditions in the surrounding air. In order to avoid the computational expense of explicitly simulating liquid, gas, and heat flow in the porous medium (and the possible further expense of simulating the flow of water vapor in the atmosphere), evaporative potentials can be treated in a simplified manner within a model where liquid is the only active phase. In the case of limited air mixing, evaporation can be approximated as a diffusion process with a linear vapor-concentration gradient. We have incorporated a simplified scheme into the EOS9 module of iTOUGH2 to represent evaporation as isothermal Fickian diffusion. This is notable because the EOS9 module solves a single equation describing saturated and unsaturated flow, i.e., phase transitions and vapor flow are not explicitly simulated. The new approach was applied to three simple problems and the results were compared to those obtained with analytical solutions or the EOS4 module, which explicitly considers advective and diffusive vapor flow. Where vapor flow within the porous medium can be neglected, this new scheme represents significant improvement over the computational expense of explicitly simulating liquid, gas, and heat flow, while providing an adequate reproduction of the overall hydrologic system. The scheme is set up to allow parallel flow of liquid and vapor, so that evaporation from an actively seeping face can be simulated. In addition, dynamic relative humidity boundary conditions can be simulated using standard iTOUGH2 features.

@inproceedings{2003-Ghezzehei-Trautz-Finsterle,
  author = {Ghezzehei, T. A.and R. A. Trautz and Finsterle, S.},
  booktitle = {International TOUGH Symposium Proceedings},
  status = {published},
  organization = {Lawrence Berkeley National Laboratory, Berkeley, California, May 12-14.},
  pdf = {http://tough.lbl.gov/assets/files/02/documentation/proceedings/2003-GhezzeheiTrautzFinsterle.pdf},
  sort-word = {nuclear waste, evaporation},
  title = {Evaluating the effectiveness of liquid diversion around an underground opening when evaporation is non- negligible},
  year = {2003}
}

The loose and fragmented soil structure that results from tillage operations provides favorable physical conditions for plant growth. This desirable state is structurally unstable and deteriorates with time because of overburden, external stresses, and capillary forces. The objective of this study was to model these structural changes by coupling soil intrinsic rheological properties with geometry and arrangement of aggregates represented as monosized spheres. Calculations of interaggregate stresses and strains, and associated changes in density and porosity, were performed for a rhombohedral unit cell. Soil rheological properties determined by application of steady shear stress were used for calculations of strains under steady interaggregate stresses. The models developed herein correspond to the initial stage of deformation when discrete aggregates exist. At strains exceeding 0.12 the interaggregate voids are isolated and the current model no longer applies and an alternative approach is presented elsewhere. Unit cell calculations were up scaled to an aggregate-bed scale by considering a one-dimensional stack of unit cells, which allows only vertical stress transmission. The stress acting at an interaggregate contact is fully accommodated (dissipated) by viscous flow when it exceeds the yield stress (strength) of the aggregates. The stress is fully transmitted to subsequent unit cells when it is less than the yield stress. Plausibility of the models was demonstrated by illustrative examples that highlight the different features of the models. The results were in qualitative agreement with observations from the literature for deformation of either loose structure, and for highly dense cases close to maximal bulk density.

@article{p2003-Ghezzehei-Or,
  author = {Ghezzehei, Teamrat A. and Or, Dani},
  date-modified = {2018-05-27 19:55:55 +0000},
  journal = {Soil Science Society of America Journal},
  status = {published},
  number = {1},
  pages = {12},
  researchgate = {https://www.researchgate.net/publication/37450858_Pore-Space_Dynamics_in_a_Soil_Aggregate_Bed_under_a_Static_External_Load},
  title = {Pore-Space Dynamics in a Soil Aggregate Bed under a Static External Load},
  volume = {67},
  year = {2003}
}

Tilled agricultural soils are in a constant state of change induced by variations in soil strength due to wetting and drying and compaction by farm implements. Changes in soil structure affect many hydraulic and transport properties; hence their quantification is critical for accu- rate hydrological and environmental modeling. This study highlights the role of soil rheology in determining time-dependent stress–strain relationships that are essential for prediction and analysis of structural changes in soils. The primary objectives of this study were (i) to extend a previously proposed aggregate-pair model to prediction of compaction under external steady or transient stresses and (ii) to provide experimentally determined rheological information for the above models. Rheological properties of soils and clay minerals were measured with a rotational rheometer with parallel-plate sensors. These measurements, under controlled steady shear stress application, have shown that wet soils have viscoplastic behavior with well-defined yield stress and nearly constant plastic viscosity. In contrast, rapid transient loading (e.g., passage of a tractor) is often too short for complete viscous dissipation of applied stress, resulting in an elastic (recoverable) component of deformation (viscoelastic behavior). Measured viscoelastic properties were expressed by complex viscosity and shear modulus whose components denote viscous energy dissipa- tion, and energy storage (elastic). Results show that for low water contents and fast loading (tractor speed), the elastic component of deformation increases, whereas with higher water contents, viscosity and shear modulus decrease. Steady and oscillatory stress application to an aggregate pair model illustrates potential use of rheological properties towards obtaining predictions of strains in soils.

@article{p2001-Ghezzehei-Or,
  author = {Ghezzehei, Teamrat A. and Or, Dani},
  date-modified = {2018-05-30 21:37:12 +0000},
  journal = {Soil Science Society of America Journal},
  status = {published},
  number = {3},
  pages = {624},
  pdf = {https://www.researchgate.net/publication/37450913_Rheological_Properties_of_Wet_Soils_and_Clays_under_Steady_and_Oscillatory_Stresses},
  sort-word = {aggregation, mechanics},
  title = {Rheological Properties of Wet Soils and Clays under Steady and Oscillatory Stresses},
  doi = {10.2136/sssaj2001.653624x},
  volume = {65},
  year = {2001}
}

The desired soil structure following tillage of agricultural soils is often unstable and susceptible to coalescence of aggregates and reduction of interaggregate porosity due to wetting and drying cycles. This process of aggregate rejoining was modeled by equating the rate of work done by liquid‐vapor menisci, to the rate of energy dissipation due to viscous deformation of a pair of spherical aggregates. The nonlinearity of wet soil viscous flow behavior was accounted for by introducing a Bingham rheological model. A natural outcome of the analysis was the formulation of a mathematical condition for the onset and termination of coalescence based on soil strength at specified water content. The condition states that sufficient energy in excess of soil strength (yield stress) must be available for coalescence to proceed. The rate of aggregate coalescence is proportional to available energy and is inversely related to the coefficient of plastic viscosity. Transport of wet soil to the periphery of the interaggregate contact by viscous flow leads to smoothing of the neck, resulting in pore closure, on the one hand, and restricting the minimum matric potential that can be achieved, on the other. The interplay between rheology and geometry prevent coalescence from proceeding indefinitely. Independently determined soil rheological properties were used to illustrate the use of the model. Coalescence under constant water content and during wetting‐drying cycles was calculated. Comparison of data from experiments on one‐dimensional, aggregate bed settlement has shown reasonable agreement with the model predictions.

@article{p2000-Ghezzehei-Or,
  author = {Ghezzehei, T. A. and Or, D.},
  date-modified = {2018-05-30 21:21:45 +0000},
  journal = {Water Resources Research},
  status = {published},
  month = feb,
  number = {2},
  pages = {367-379},
  researchgate = {https://www.researchgate.net/publication/37450893_Dynamics_of_soil_aggregate_coalescence_governed_by_capillary_and_rheological_processes},
  sort-word = {soil structure, aggregation},
  title = {Dynamics of soil aggregate coalescence governed by capillary and rheological processes},
  volume = {36},
  year = {2000},
  doi = {10.1029/1999WR900316}
}