Recent models of the unsaturated hydraulic conductivity curve (UHCC) are the sum of separate UHCCs for domains of capillary water, film water, and water vapor. A new theoretical framework for aggregating domain conductivities to a bulk soil UHCC reveals that this requires parallel domains. The same theory also generates arithmetic, harmonic, and geometric averages of the liquid-water conductivities, which can be arithmetically averaged with the vapor conductivity. However, current models for capillary and film conductivities are intrinsic, i.e., valid within their respective domain. The vapor conductivity is a bulk conductivity, i.e., it gives the conductivity of the gaseous domain as it manifests itself in the soil. Conversion relationships use the domain volume fractions as approximations of the as-yet unknown weighting factors to convert between intrinsic and bulk conductivities. This facilitates consistent averaging of domain conductivities. The fitted curves for the capillary and film water depend on the averaging (or adding) method. Hence, they are not strictly characteristic of their respective domains. Truly intrinsic domain conductivity functions may not exist, or are perhaps merely tools to arrive at a good fit of the UHCC of the bulk soil. Given these complications, a simpler junction model that joins a capillary and a film limb at a junction point offers an attractive alternative. 

How to cite: de Rooij, G. H.: The unsaturated soil hydraulic conductivity as a sum, an average, or a junction of domain conductivities, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2947, https://doi.org/10.5194/egusphere-egu24-2947, 2024.

Land surface models need information on soil hydraulic properties (SHP) that are often estimated using pedotransfer functions (PTFs). Due to a lack of calibration data, PTFs must be applied that were trained for regions and land use types outside the region of interest. In this study, we quantify the transferability of PTFs to new regions as function of mathematical complexity and number of covariates. For that purpose, we trained new PTFs for forest soils based on a dataset of 25 soil profiles from climatically moderate regions of Switzerland. In a second step, we tested the new and some existing PTFs in a drier and hotter Swiss region (Valais). Tests of transferability showed that increasing the mathematical complexity (from a linear model to a highly non-linear random forest model) was always beneficial for the predictive power in new regions. Increasing the number of covariates revealed a trade-off between improving the accuracy of the predicted soil water retention curve and reducing accuracy of the soil hydraulic conductivity. Interestingly, the use of environmental predictors (climate data) hampers transferability the most due to large climatic contrasts between the calibration and validation regions. These results suggest that transferability works better for PTFs using fewer predictors. We recommend the use of non-linear PTFs based on soil texture, soil density, and organic carbon content for optimal prediction accuracy in regions without training data. This work highlights that the models with the highest number of predictors are not optimal for achieving transferability and that reducing the number of predictors can be beneficial.

How to cite: Schoch, J., Nussbaum, M., Walthert, L., Carminati, A., and Lehmann, P.: Does model complexity of a pedotransfer function for soil hydraulic properties hamper its transferability?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6425, https://doi.org/10.5194/egusphere-egu24-6425, 2024.

Hydraulic Conductivity and Water Retention Functions of Porous Rock and Glacial Till Soil: Quasi-Steady Centrifuge versus Evaporation Methods

Maria Clementina Caputo¹, Lorenzo De Carlo¹, Antonietta Celeste Turturro¹, Horst Herbert Gerke²

¹CNR National Research Council, IRSA Water Research Institute, via Francesco De Blasio 5, 70132 Bari, Italy

²Research Area 1 “Landscape Functioning”, Leibniz-Centre for Agricultural Landscape Research (ZALF), Eberswalder Straße 84, D-15374 Müncheberg, Germany

Experimental laboratory measurements of the hydraulic conductivity and the water retention functions have a crucial role in describing the solid matrix-water dynamics. However, the direct determination of the hydraulic conductivity, K, as a function of the pressure head, h, is still difficult.  It is often estimated indirectly from the water retention curve, which relates the water content, θ, to h, or obtained by using pedotransfer functions or by field data of  pumping tests.

In this study the unsaturated hydraulic conductivity values of carbonate porous rocks and soil clods were measured by means of evaporation, Quasi-Steady Centrifuge (QSC) and double-membrane steady-through flow methods. Water retention curves were obtained by using the evaporation, QSC, suction table, Mercury Intrusion Porosimetry (MIP) and pressure chambers methods. Samples belonging to two rock lithotypes collected in southern Italy and to two soil clods coming from northeastern Germany were tested. The data were fitted to the unimodal and bimodal functions of van Genuchten and the Peters-Durner-Iden models by using the LABROS SoilView Analysis software. The bimodal functions better described the experimental data of both the studied rocks and soils.

The soil compaction during the centrifuge runs performed by applying the QSC method, corroborated by changed values of bulk density, porosity, tortuosity, and pore connectivity after the runs, confirms that this method is not suitable to non-rigid media.

The results confirm that the QSC method allows measuring the unsaturated hydraulic conductivity values for rock samples. The larger range of experimental hydraulic conductivity values helps to improve the fitting and obtain the more accurate of the hydraulic conductivity function to better describe the unsaturated rock-soil-water dynamics.

How to cite: Caputo, M. C.: Hydraulic Conductivity and Water Retention Functions of Porous Rock and Glacial Till Soil: Quasi-Steady Centrifuge versus Evaporation Methods, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20473, https://doi.org/10.5194/egusphere-egu24-20473, 2024.

Films, rivulets, snapping rivulets, sliding drops, slugs—many flow modes besides filled-tube, Poiseuille-type flow occur in macropores. Some of these fit reasonably into Darcian formulations and the analog of laminar viscous flow in water-filled tubes. But others do not. These exceptions may be the main reason for failures to predict the speed and travel distance of preferential flow.

A useful first step for an improved model of macropore flow is the classification of diverse flow modes into categories based on their intrapore boundary conditions. Within a flowing macropore, the gas-liquid and liquid-solid interfaces, with the effects of interfacial constraints such as surface tension and contact angles, determine the geometry of the flowing liquid phase and its controlling frictional influences. A classification scheme with four categories can account for the various flow modes that have been observed in lab and field experiments. This categorization helps to distinguish which flow modes are amenable to Darcian or Poiseuille-type representation and which are not. Some of the exceptions are approachable with wave or film-flow concepts as in several recently-developed models. Yet there are other flow modes that do not fit well in any of these models, and in some cases these may be the most important means of rapid and long-distance transport. Other sorts of physical processes may provide suitable analogs for these, for example free-fall concepts like initial acceleration, speed-dependent frictional forces, and terminal velocity. In any case, the diversity of macropore flow modes needs to be considered in the development of markedly improved models of preferential flow.

How to cite: Nimmo, J. R.: Diverse modes of macropore flow—How to include them in predictive models?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4130, https://doi.org/10.5194/egusphere-egu24-4130, 2024.

The SWAP model employs a finite difference numerical solution of the Richards equation, including root water uptake, to simulate the movement and predict the state of soil water and associated quantities in the vadose zone. The relation between hydraulic conductivity K, pressure head h, and water content θ can be described by parameters of the Van Genuchten-Mualem (VGM) relations, where the quality of these parameters determines the quality of the model output. We developed a stochastic procedure to evaluate the outputs of the SWAP model according to the uncertainty and correlations in the VGM parameters. Specific software was developed to (1) fit VGM parameters to observed retention and conductivity data to obtain values, standard errors, and correlations of transformed parameters (software HPFit); (2) generate p stochastic realizations of the VGM parameters using Cholesky decomposition (software StochHyProp), and (3) run the SWAP model with each of the p parameter realizations for specific scenarios, extracting simulation results (software RunSWAP), e.g., the simulated water balance components evaporation, transpiration, bottom flux, and runoff. The results from the last step yield the respective frequency distributions of the output values. Examples will show that the most commonly performed prediction using the average VGM parameter values does not always agree with the median of the stochastic realizations. The developed procedure allows the quantitative analysis of the uncertainty of SWAP model output, adding to the interpretation of the predictive power of hydrological models like SWAP.

How to cite: de Jong van Lier, Q.: Using the SWAP model for the stochastic analysis of hydraulic parameter uncertainty propagation in soil water balance components, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3939, https://doi.org/10.5194/egusphere-egu24-3939, 2024.

Incorporating Victorian brown coal (VBC) into the soil as a reliable amendment can markedly alter the hydraulic properties of the soil. A pivotal phenomenon influencing soil hydraulic parameters, particularly the soil permeability coefficient, is the extent of cracking and shrinkage observed during the drying process and consequent moisture loss. This study investigates the impact of incorporating VBC into clay and its influence on the two-dimensional cracking and shrinkage characteristics of the mixture. Various mixtures of brown coal from Latrobe valley in Victoria, Australia and clay, ranging from 2% to 20% brown coal content, were prepared and subjected to liquid limit and plastic limit tests. The samples were then readied for cracking and shrinkage assessments under liquid limit moisture conditions as an initial moisture content, featuring a sample diameter of 150 mm and a thickness of 10 mm. Results from the liquid limit tests demonstrated a decreasing trend in the liquid limit of the mixture with increasing brown coal content, registering values of 38.3%, 37.4%, 36.5%, 34.9%, and 32.9% for 0%, 2%, 5%, 10%, and 20% brown coal mixtures, respectively. Plastic limit tests indicated a 1.7% reduction, decreasing from 20.6% to 18%.9, with the addition of 20% brown coal. Furthermore, cracking and shrinkage tests revealed a substantial reduction in the cracking index (cracking intensity factor, CIF) of the mixture upon the addition of brown coal, reaching zero for mixtures containing 5%, 10%, and 20% brown coal after exposure to a 45℃ temperature for 30 hours. Additionally, the shrinkage index (shrinkage intensity factor, SIF) decreased from 15.4% for the clay soil sample to 14.9%, 14.6%, 13.9%, and 13.1% for the 2%, 5%, 10%, and 20% brown coal mixtures, respectively. This underscores the positive influence of brown coal on mitigating soil cracking and shrinkage, emphasizing its significance in soil science research.

How to cite: Baghbani, N., Bucka, F., and Baumgartl, T.: Soil management using lignite to improve soil cracking properties and performance, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-22326, https://doi.org/10.5194/egusphere-egu24-22326, 2024.

Despite its limited vertical extent, the thin soil layer provides essential functions for climate and ecosystems globally. For accurate large scale process description, land use models compute the water distribution in soils based on spatial domains with a width-to-thickness ratio of about 1000:1: a geometry as thin as a sheet of paper. Most models simulate the water flow in these ‘soil sheets’ by solving the Richardson-Richards equation in 1D, neglecting smaller scale structures and lateral flow, and implicitly making strong assumptions on the relations between water content, matric potential, hydraulic conductivity, and water flux. To quantify the accuracy of this conceptualization, we compare drainage simulations of wet soils for the 1D column simplification with the full 2D-and 3D geometry using the correct sheet-like size ratio. The role of different climates, soil types, and heterogeneities at smaller scale is analyzed. These simulations based on the full geometry provide guidelines for (i) the applicability of Richardson-Richards equation in land surface models and (ii) the development of appropriate averaging schemes of soil hydraulic properties in the 1D scenario.

How to cite: Lehmann, P., Duddek, P., and Carminati, A.: Water flow across the skin of the Earth – how badly are we doing?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7858, https://doi.org/10.5194/egusphere-egu24-7858, 2024.

Existing water balance assessments may lack precision due to overlooking the spatial variations in factors such as soil and topography while transferring the results from point modelling to a wider area. In cooperation with Bayer AG Crop Science Division and Hamburg University, within the DREAM (Digital Run-off Exposure Assessment and Management) project, a model is being developed which is temporally dynamic as well as spatially differentiated to provide a more nuanced and location-specific understanding of quantitative water dynamics. It is based on high resolution grid data and features a multi-layered soil water model, the goal of which is to depict volumes of water in different soil layers. It is to be employed in an agricultural context and serve as a toolbox of possible runoff reduction measures for plant protection products.

Since risk management is a highly localized undertaking, the model operates at a field- or sub-field-level with a spatial resolution of up to one meter. The temporal resolution of simulation steps is variable; from one hour to a day. The necessary input data – that being a digital terrain model, information about the vegetation as well as soil and weather data – create conditions specific to the site.

It is embedded in the open source geoinformation system (GIS) SAGA. The modular approach allows for flexible development and changes on short notice. The model includes an algorithm that determines soil water movement, incorporating the groundwater layer as the model’s lower boundary. To achieve this, an expanded bucket model for soil water movement, based on the works of Glugla, is used. Should the infiltration capacity of the soil - calculated via the Green-Ampt-Method - be surpassed, runoff occurs. In this case, the model possesses the ability to depict runoff and its flow paths through the terrain, along with the respective volumes and flow velocity based on Gauckler-Manning-Strickler.

While the current focus lies on the movement of water, the model is designed for gradual expansion and improvement, allowing for future considerations such as the tracking of solutes moving into larger depths or even into groundwater.

How to cite: Graaf, F., Bock, M., Conrad, O., and Sur, R.: Design of a spatially differentiated water balance modelling tool, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21584, https://doi.org/10.5194/egusphere-egu24-21584, 2024.

Pollutant transfers in the critical zone is an issue for decades both because of complex physico-chemical interactions in the porous media and because of the emergence of new materials/molecules rejected in the environment for which rules are not ready. The study presented here is part of a research project which aimed to predict transfers and residence time of pollutants in the critical zone including the Unsaturated Zone (UZ), and in aquifers on the CEA Cadarache site (France). This site benefits from a large instrumentation for decades to survey both the water dynamic and quality in the aquifers below the industrial facilities. One of the remaining challenges is to study the distributed recharge in the UZ. In situ measurements of saturated hydraulic conductivity Ks are often time-consuming, but also costly to implement at a catchment scale. To overcome this difficulty, an approach using Pedotransfer Functions (PTFs) is possible in order to spatialize this parameter of the UZ (Nasta et al., 2021; Weihermüller et al., 2021). The main objective of the study is to evaluate a spatialization strategy of Ks values using PTFs calibrated from an intensive in situ measurement campaign.

A total of 48 measurement points were selected on the study site, covering an area of around 900 hectares. The points were chosen to represent the different types of geological formations at the outcrop as well as the different types of land cover on the site. For all those locations, in situ hydraulic conductivity measurements were carried out with a disc infiltrometer, using the multi-potential method (Vandervaere, 1995), together with physico-chemical analyses of the surface soils. The results obtained show that for most of the measurement points, a fairly clear break in the slope of the exponential function K(h) appears for potentials h around -30 / -20 mm. The estimate of the value of Ks is chosen as being the value of K(h) obtained for the last value of potential h = - 5 mm, considering that saturation has been reached. On site, Ks varies from 20 to 410 mm/h.

Several PTFs for estimating Ks were selected (Rawls & Brakensiek, 1985, Wösten et al., 1999, Weynants et al., 2009, Szabó et al., 2021, Rosetta (Schaap et al., 2001; Zhang & Schaap, 2017)). The study will help us to identify some geological or land cover drivers for Ks ranges and to select which PTFs are able to represent such a variability.

How to cite: Andriatahiana, S. N., Sinabarigui, I., Courtois, N., Vandervaere, J.-P., and Cohard, J.-M.: Saturated hydraulic conductivity spatialization strategy to model recharge and hydrogeological transfers on an industrial site in France, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16238, https://doi.org/10.5194/egusphere-egu24-16238, 2024.

Infiltration is a central concern in soil physics. The advective and diffusive redistribution of event water depends on various factors such as initial wetting, the establishment of film connectivity, and capillary gradients and hydraulic conductivity. Non-uniform infiltration patterns are prevalent. However, direct infiltration measurements do not account for this reality and tracer experiments require a destruction of the experimental plot. We developed a data acquisition strategy based on time-lapse 3D ground-penetrating radar (GPR) to monitor fast and small-scale subsurface flow processes during irrigation in a non-invasive manner.

The technique combines an irrigation pad (1 m² drip irrigation to simulate moderate, non-erosive rain events) with a GPR measurement platform (manually driven two-channel GPR antenna array with positioning guides). We will present a systematic field experiment consisting of two recurrent irrigations (40 mm/2 h, 1 irrigation per day) and a respective replicate. For evaluation, the GPR measurements were sidelined with soil moisture measurements (TDR profile) and tracer applications (dye and salt). Our data show that the achieved high resolution of less than 5 cm in space and 10 minutes in time makes it possible to monitor and quantify infiltration processes in their spatial and temporal non-uniformity.

The experiment supports the hypothesis from earlier experiments at various sites: Non-uniform infiltration patterns and dynamically connected flow-fields are highly heterogeneous but share stochastic features, such as distribution, randomness, and skewness. Our approach opens new options for repeated, spatially resolved infiltration measurements and theory development for soil hydrology and land surface models.

How to cite: Jackisch, C., Stephan, S. M., Tronicke, J., and Allroggen, N.: Infiltration as dynamic non-uniform stochastic flow field – repeatable, high-resolution 4D GPR measurements at the plot scale, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19703, https://doi.org/10.5194/egusphere-egu24-19703, 2024.

We applied neutron imaging techniques to unveil pore scale flow processes occurring during desaturation of a homogeneous, saturated sand pack. For this purpose, we used a small glass flow cell (2 cm x 2 cm x 0.1 cm) filled with pure, artificial S250 quartz sand. The pore space of the sand was initially fully saturated with double distilled water (DDW). The saturated flow cell was subjected to a series of suction phases with increasing suction tensions to extract water via a bottom outlet, controlled by a vacuum pump. In the first phase, a tension of 0.016 MPa (low suction) was applied for 248 min, followed by 1.14 MPa (mid suction) for 227 min, and finally, 10 MPa (high suction) for 397 min. Throughout the entire duration of the experiment, the flow cell was continuously exposed to neutrons. A back-end detector collected the neutron beams passing through the different matters (sand, water, air) contained in the flow cell and generated snapshot images of the internal pore structure and the water distribution with a pixel resolution of 5 µm at one-minute intervals.

The resulting images revealed that water did not redistribute homogeneously during the desaturation of the flow cell, over dimensions of a few millimeters. Despite using “perfectly homogeneous” sand under initially fully water-saturated, controlled conditions, heterogeneous patterns of stable water pockets were observed inside the pore space of the sand, where water became immobilized.

These experiments demonstrate that truly homogeneous flow does not occur even under controlled laboratory conditions in a “perfectly homogeneous” porous medium.

Subsequent simulations of the experiments with common Darcy-Richards models showed that the macroscopic 1D desaturation time series of the flow cell could be realistically depicted. However, even after parameter calibration and the manual addition of heterogeneity, the microscopic, heterogeneous 2D distribution of water observed inside the flow cell could not be reproduced.

This highlights limitations on the applicability of Darcy-Richards models, which may be effective at a macroscopic level but simultaneously fail to represent accurately the internal dynamics of the system. This insight is crucial for the application of Darcy-Richards models and the interpretation of their results.

How to cite: Sternagel, A., Rajyaguru, A. D., Trevisan, L., Loritz, R., Berkowitz, B., and Zehe, E.: Neutron imaging unveils heterogeneous flow patterns in homogeneous porous media and limitations of Darcy-Richards models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7939, https://doi.org/10.5194/egusphere-egu24-7939, 2024.

Saturated hydraulic conductivity (K_sat) is one of the most fundamental parameters in soil hydrology. It governs the rate of saturated flow through porous media and functions as a scaling factor for unsaturated flow. Knowledge of K_sat is key to understanding the movement of water in soils, transport and recharge of groundwater, suspended and dissolved transport in soils, and soil-air water exchange. In hydrology and climate modeling K_sat is often estimated through pedotransfer functions. A large effort has been committed to the development of these models, using an array of differing algorithms and methods. However, estimating K_sat has been somewhat troublesome, since the commonly measured soil properties, such as soil texture, bulk density and organic matter content, used as predictor variables in PTFs do not explain K_sat variation well. Instead, K_sat is largely controlled by pore-network characteristics especially in highly-structured soils. Using an extended, methodologically homogeneous dataset of commonly measured soil physical properties, 3-D X-ray computed tomography imaged pore-network parameters, and quasi-continuous particle-size measurements using the Integral Suspension Pressure method, we assess the benefits of using combined soil textural and structural information on the estimation of K_sat. Using this dataset, we have built models that estimate K_sat using a boosted random forest algorithm (XGboost) and used explanatory model analysis to tune and evaluate the models. Three input data scenarios included (i) basic soil inputs only (ii) imaged pore metrics only, and (iii) their combination. Using or adding imaged pore metrics as inputs greatly improved the K_sat estimations that were reflected, for example, by the respective coefficients of determination, evaluated using a cross-validation scheme (R² = -0.05/0.60/0.58 for the three input scenarios respectively). 3-D imaging of soil and the subsequent characterization of its pore-space may serve multiple research purposes, but such data are still not routinely collected due to cost of measurement and general lack of access to equipment. Our study confirms, however, that when collecting such metrics will become economically feasible through e.g. better automation of image processing using tools like SoilJ, having those metrics will show great potential towards improving the estimation of the soil’s water transport properties. 

How to cite: Låker, E. E., Nemes, A., and Hirmas, D.: Can 3-D X-Ray tomography imaging improve the estimation of saturated hydraulic conductivity of soils?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18554, https://doi.org/10.5194/egusphere-egu24-18554, 2024.

Continuous monitoring of root-zone soil moisture status is important to ensure the effective management of water resources for agricultural production, and to improve our understanding of land-atmosphere interactions in a changing climate. Utilizing gamma radiation to monitor soil moisture at the field scale is an emerging non-invasive technique that can also bridge the gap between point and remote sensing measurements. The measurement principle relies on the increased attenuation of gamma radiation emitted from soil with increasing soil moisture content. In a previous study, we successfully obtained soil moisture estimates from low-cost environmental gamma radiation (EGR) detectors. However, since EGR detectors provide the bulk response to gamma radiation over a wide energy range (0 to 3000 keV), EGR signals are influenced by several confounding factors, e.g., skyshine radiation, atmospheric and soil radon variability. To what extent these confounding factors deteriorate the accuracy of soil moisture estimates obtained with EGR is still not fully understood. Therefore, the aim of this study is to compare EGR measurements with K-40 gamma radiation (at 1460 keV) measurements that are much less influenced by these confounding factors. For this, two different kinds of gamma radiation detectors were installed close to each other at an agricultural field in Selhausen, Germany: an EGR detector based on a G-M counter tube (MIRA, ENVINET GmbH) and a spectroscopic scintillation-based detector (SARA, ENVINET GmbH). The field was also equipped with in-situ soil moisture sensors to measure reference soil moisture and a climate station to measure meteorological parameters. The EGR measurements were corrected for atmospheric radon-washout during precipitation events and the contributions of meteorologically influenced secondary cosmic radiation were also eliminated. In case of the spectroscopic measurements, no further corrections were applied as the analysis was only focused on the K-40 energy window. Both sets of gamma radiation measurements were related to reference soil moisture using a functional relationship derived from theory. We found that daily soil moisture can be predicted more accurately from K-40 gamma radiation (RMSE 4 vol.%) than from EGR (RMSE 6 vol.%). Regardless of the higher prediction uncertainty obtained due to the influence of the confounding factors at low energy, the long-term availability of ERG data, e.g., in Europe via EURDEP, makes it interesting for continental scale analysis of soil moisture. 

How to cite: Akter, S., Huisman, J. A., and Bogena, H. R.: Comparing the accuracy of soil moisture estimation derived from environmental and spectroscopic gamma radiation measurements , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-10596, https://doi.org/10.5194/egusphere-egu24-10596, 2024.

The matric potential plays a pivotal role in understanding of water movement, plant water availability, and mechanical stability. In lack of direct measurements, the matric potential dynamics must be deduced from soil water content values, using the soil water retention curve. This approach is of particular importance at larger scales where only the water content (but not the potential) can be deduced from satellite data. However, because the relationship between water content and matric potential in natural field soils is highly ambiguous, not unique and dynamic, the prediction of matric potential from water content data is a big challenge. This ambiguity is related to different structures controlling drainage and wetting, dynamic effects, and seasonal changes of structures controlling the water distribution. In this study we present an autoencoder neural network as a new approach to analyze the soil moisture dynamics and to predict matric potential from water content data. The autoencoder compresses the water content time series into a site-specific feature (denoted as autoencoder value, AUV) that is representative of the underlying soil moisture dynamics. The AUV can then be used as predictor of the matric potential and the highly hysteretic soil water retention curve. The approach was tested successfully for nine soil profiles in the region of Solothurn (Switzerland). Three sites were chosen to establish the connection between AUV and the ambiguous soil water retention curve using a deep neural network, that was then applied to predict the matric potential dynamics of the other six sites. This method offers the potential to (i) deduce matric potential dynamics by relying solely on soil water content measurements (including satellite data), even when strong seasonal effects challenge standard methods, and (ii) serves as a warning system for changes in soil properties and in the intricate relationship between soil water content and matric potential dynamics.

How to cite: Aqel, N., Carminati, A., and Lehmann, P.: The dynamics of field soil water retention curves predicted by autoencoder neural network, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11928, https://doi.org/10.5194/egusphere-egu24-11928, 2024.

Soil data is a crucial component for hydrological models operating at the catchment scale, such as the Soil and Water Assessment Tool (SWAT). Nevertheless, the reliability of these models is heavily contingent upon the quality and spatial resolution of the soil information employed. This study addresses the pressing need for robust soil data in SWAT modeling by evaluating various soil mapping techniques.

The first objective was to prove different mapping techniques, such as textural class combination and clustering approach using soil grid data to have different soil maps. The performance of those maps, together with WRB, WSR, Zobler, HWSD v2.0 and DSOLMAPS, was evaluated to improve the accuracy and reliability of the SWAT hydrological model. To achieve this, we conduct a comprehensive investigation involving multi-objective calibration, utilizing both flow data and soil moisture data to calibrate the model. Finally we incorporate a  pedotransfer function to include the landcover effect on Saturated Hydraulic Conductivity to improve the reliability of soil hydrological processes in the SWAT model.

The study area, situated within the Cauquenes River Catchment, presents a complex hydrological system characterized by substantial spatial heterogeneity in soil properties. The soil mapping techniques under evaluation encompass traditional soil survey data integrated with remotely sensed soil information and machine learning-based soil mapping methodologies. These methods are compared in their ability to enhance the SWAT model's representation of the catchment's hydrological dynamics.

In the case of the kmeans clustering approach the results of soil clusters are equivalent to soil units. A number of clusters from 3 to 100 were evaluated with the lowest DB index. Clusters from 3 to 16 presented an optimal range. The SWAT model calibration was performed under multi-objective evaluation, with kmeans soil cluster and DSoilMaps with better result for daily simulations.

The work to correct the application of soil data, including in situ observation, satellite data and machine learning approach, provides a valuable approach to improve the calibration and validation processes of hydrological models in semi-arid regions, important for cacthment management and decision making processes, and to correctly assess the impacts of land use changes, climate variability and extreme events on water resources. 

How to cite: Gimeno, F., Zambrano-Bigiarini, M., Galleguillos, M., and Marinao, R.: Evaluation of Soil Mapping Methods for SWAT Hydrological Modeling through Multi-Objective Calibration, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12291, https://doi.org/10.5194/egusphere-egu24-12291, 2024.

The location of dry layer interface (LDI), which varies during soi drying process, is a key parameter for characterizing soil water evaporation process. Recently the heat pulse (HP) technique has been applied to estimate the LDI indirectly. However, errors may occur with the HP method when analytical solutions are used because of soil heterogeneity across the measurement plane, especially when a heterogeneous interface lies between the two probes. In this study, we propose a numerical inversion-based heat pulse method for estimating the LDI under five configurations. The inputs are thermal properties of dry and wet soils, and the temperature rise-by-time curves at two locations from the heat source. The heat source was positioned at different distances from the LDI. The inversion method was evaluated with temperature rise-by-time curves from 17 scenarios obtained from numerical simulation and 14 scenarios obtained from laboratory measurements. Results demonstrated that the new approach produced reasonable LDI estimates within the range of 0.1 to 5 cm to the soil surface, with relative errors (REs) less than 0.30, except for the situation that the LDI was close to the heat source. The proposed method has significant implications in groundwater management and modeling hydrological processes in unsaturated soils.

How to cite: Liu, L., Xie, X., Lu, Y., and Ren, T.: Locating the dry layer interface during soil water evaporation by using numerical inversion-based heat pulse method, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13980, https://doi.org/10.5194/egusphere-egu24-13980, 2024.

Soil moisture is an important state variable with high spatiotemporal variability depending on land and climate variables. The importance of various physical controls on soil moisture varies depending on the scale and extent of the study. At a fine scale, soil properties are proven to be critical in defining spatiotemporal variability of soil moisture. In the context of agricultural applications in India, soil moisture estimates at the farm scale (finer spatial resolution) over various root depths are essential.  Traditional Land Surface Models (LSMs) are limited to large spatial scales (in the order of tens of kilometers). They have been designed for synergistic coupling with Earth system models. Besides, they do not account for the vertical heterogeneity of soil. LSMs, including Noah-MP, use a lookup table to obtain soil properties corresponding to soil texture while assuming vertically homogeneous soil texture. Recent studies proved that accounting for vertical heterogeneity in the soil using state-of-art soil maps and pedotransfer functions in LSM can significantly improve the surface soil moisture estimations. However, the effects of incorporating vertical heterogeneity in soil properties on deeper layer soil moisture simulations are yet to be explored. Considering the importance of farm scale root water uptake processes, understanding soil moisture heterogeneity at deeper layers is essential. In this context, the present study hypothesizes that a hyperresolution LSM, which accounts for subgrid spatial heterogeneity while maintaining soil heterogeneity between layers, can improve the characterization of rootzone soil moisture. 

In this work, we used HydroBlocks, a semi-distributed hyper-resolution LSM, which uses Noah-MP at its core, and the concept of Hydrologic Response Units (HRU) to simulate the land surface variables. The analysis is carried out for the first time in India over the Upper Bhima Basin, for the year 2020. Initially, we investigated the benefit of vertical heterogeneity in soil properties to simulate soil moisture at five different layers till one meter deep using HydroBlocks. We used SoilGrids data for different layers to calculate soil hydraulic properties using PTFs and feed them as inputs in the HydroBlocks model. We compare HydroBlocks surface and rootzone soil moisture to existing reanalysis and satellite products, including GLEAM, ERA5-Land, SMAP-L3, and SMAP L4 statistically in terms of bias, ubRMSE and R². Further, an intercomparison of surface and rootzone soil moisture simulations is made between the two cases of Hydroblocks model, first with vertically homogeneous soil properties, and second, with vertically heterogeneous soil properties. The objective of this step is to emphasize the role of vertically heterogeneous soil layers in a hyper-resolution LSM towards addressing the spatiotemporal variability of soil moisture. Finally, a soil parameter sensitivity analysis (using Sobol technique) is carried out using HydroBlocks for five soil layers (up to 1 meter depth), for the first time, to assess the influence of eight soil textural parameters such as wilting point, porosity, pose size distribution, and likewise, on soil moisture simulations. In this process, we also assessed the seasonal variability of parameter sensitivity.

How to cite: U Krishnan, V., Vergopolan, N., Jayaluxmi, I., and Lanka, K.: Examining the benefits and sensitivity of vertically heterogeneous hyper resolution land surface model towards simulating a farm scale soil moisture profile in Upper Bhima Basin, India, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18945, https://doi.org/10.5194/egusphere-egu24-18945, 2024.

The loose, stratified composition of pyroclastic soil covers typically consists of layers of air-fall ashes and pumices. When these deposits are resting on steep slopes, they pose a significant geohazard due to slope instabilities. This scenario is evident in the carbonate massifs covered by pyroclastic soils in Campania (southern Italy), an extensive area of about 400 km² prone to landslides. In these porous deposits, the soil suction in unsaturated conditions plays a crucial role in enhancing the slope stability by providing additional shear strength.

This study aims to present a comprehensive overview of the hydraulic and shear strength characteristics observed in different layers of pyroclastic ashes across various sites in the Campania study area. By gathering datasets from previous studies and introducing new experimental data, the relationship between soil index, hydraulic properties and the shear strength in unsaturated conditions is examined.

The findings highlight notable differences in hydraulic properties of soils originating from the same location but belonging to different layers: ashes from intermediate layers within the soil profile, where failure surface usually occurs; weathered ashes in direct contact with the carbonate bedrock, responsible of water exchange with deeper systems. The water retention curves of intermediate ashes exhibit air entry values at approximately 4 kPa, while those in contact with the bedrock show values around 25 kPa. Conversely, soils from the same layer but from different sites exhibit relatively similar hydraulic characteristics. For example, intermediate ashes from the same layer typically display air entry values varying between 0.5 kPa and 4 kPa. The same behavior also appears regarding the influence of soil suction on the shear strength of the investigated materials, which can be estimated directly from the water retention curves.

How to cite: Roman Quintero, D. C., Damiano, E., Olivares, L., and Greco, R.: Overview of water retention and suction stress properties of layered pyroclastic ashes in landslide prone areas of Campania, southern Italy, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12041, https://doi.org/10.5194/egusphere-egu24-12041, 2024.

Extensive cropland-to-orchard transition alters water flow and nitrate transport in the vadose zone (VZ) of the Earth’s Critical Zone (CZ), which may impact groundwater recharge and threaten future water quality from intensive nitrogen fertilizer application. Understanding the regional unsaturated water and nitrate fluxes and travel times in the deep VZ is crucial for the sustainable management of the groundwater system. Here, a regional-scale model was developed to estimate the recharge and nitrate transport in the cultivated loess CZ of China’s Guanzhong Plain (CGP), where cropland-to-orchard transition has been extensively promoted in the past few decades. Besides, uncertainties and sensitivities in estimated fluxes of water and nitrate induced by variations in soil hydraulic parameters (SHPs) were evaluated. A comparison between model simulations and observations at 12 sites exhibits good simulation performance. Comparing the measured SHPs, SHPs from Rosetta and Global SHPs products introduced 86.28% and 48.94% uncertainties in the simulation of nitrate leaching fluxes from cropland and orchard, respectively, as well as 44.76% uncertainties in the simulation of groundwater recharge fluxes from the orchard. Application over the CGP based on measured SHPs indicates that the central and eastern CGP were the hotspots for groundwater nitrate contamination. By comparing traditional cropland and orchard scenarios, simulations reveal that cropland-to-orchard transition results in a 39.3-fold increase in nitrate leaching fluxes and a 9.8% decrease in groundwater recharge fluxes. Modeled nitrate travel times through the deep VZ range between decades and centuries under both land use scenarios; however, the cropland-to-orchard transition would extend the time (~22.4 years) it takes for nitrate to reach the aquifer. Although cropland-to-orchard transition delays nitrate transport to the aquifer, the increased nitrate leaching flux will increase the risk of nitrate groundwater pollution, especially in areas with shallow VZs and coarse soil texture. This study provides valuable information for assessing the future vulnerability of groundwater resources under agricultural land use and management changes in the cultivated loess CZ.

How to cite: Niu, L. and Jia, X.: Future Orchard Expansion May Decrease Groundwater Recharge and Increase Nitrate Contamination in An Intensively Cultivated Loess Critical Zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14516, https://doi.org/10.5194/egusphere-egu24-14516, 2024.

Frost weathering is considered the primary cause of erosion in periglacial environments. This process is initiated by the freezing of water within rock pores and its subsequent expansion, which generates substantial forces leading to the physical fragmentation and disintegration of the rock structure. To detail the mechanism and predict the patterns of rock fracturing, this study has developed a specialized numerical model. In previous study, researchers typically studied the mechanical failure of rocks via macroscopic numerical methods. However, these methods often face limitations in depicting mesoscale forces, particularly in the context of multiphase flow processes of water migration. Moreover, the influences of various hydrothermal conditions on the mechanical behavior of rocks are frequently overlooked. In this study, a coupled lattice Boltzmann model (LBM) was developed to simulate the freezing process in rocks. The porous structure with complexity and disorder was generated by using a stochastic growth method, and then the Shan-Chen multi-phase model and enthalpy-based phase change model were coupled by introducing a freezing interface force to describe the variation of phase interface. By utilizing the developed model, the ice growth process in rock pores can be well depicted under porous conditions characterized by varying contact angles, porosities, and specific surface areas. Building on this foundation, our work advances the understanding of the complex interaction between thermal dynamics and mechanical processes in periglacial environments, shedding light on the mechanisms of frost weathering and the predictive modeling of rock fracture patterns under varying hydrothermal conditions.

How to cite: Wang, Z., Zhang, C., Zhang, Y., and Li, B.: A hydrothermal model for unsaturated frozen rocks based on lattice Boltzmann method, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13600, https://doi.org/10.5194/egusphere-egu24-13600, 2024.

The continuous and frequent occurrence of the Shallow Water Table (SWT) in urban regions aggravates the severity of geotechnical and environmental problems. Exploring possible measures that effectively reduce the negative impact of SWT in urban aquifers is of extreme importance. This research investigated the effectiveness of bioengineering techniques in lowering SWT in the framework of hydropedological factors (soil structure in the vadose zone, where most pore water fluxes take place). The methodology includes physical models (sand tank experiments) and field-scale studies. A total of nine sand tanks were utilized, segmented into three distinct groups, to investigate the reduction of the water table through different mechanisms: (1) evapotranspiration drawdown by soil water uptake by the roots of vegetation (specifically Reeds), (2) evaporation by bare homogeneous topsoil, and (3) evaporative “soil siphoning” in tanks that were made as “smart composites” with a fine-textured vertical “moisture chimney”. The dynamics of SWT were monitored over 6 months (March–September 2023). Each tank measures 100 cm in length, 70 cm in height, and 15 cm in width. All tanks were packed with two horizontal soil layers (sand and clay) to simulate a perched aquifer, common prototypes of which were explored in Muscat, Oman. In the siphoning experiment, a small trench was made and packed with silt loam soil. This trench extended the entire thickness of the aquifer to enhance capillarity and, hence, evaporation. Analysis of the results showed that the drawdown of the water table ranged from 75% to 300% (winter to summer seasons) in the tanks containing plants (Reeds) compared with the control tanks. In addition, the SWT in the tanks with “soil siphons” was reduced in the range of 22%-46% compared with the control tanks. Another experiment with Reeds was conducted on a larger field scale using Concrete Agricultural Basins (CABs) with dimensions of 1000 cm length, 200 cm width, and 60 cm depth. The experiment spanned three months (June-September 2023) and aimed to investigate the impact of Reeds on SWT levels in larger-scale 3D pore water dynamics (in sand tank experiments flows were 2D). Overall, the large-scale experiment showed that over the three months, the evapotranspiration from Reeds reduced the water level by 16.7%, 66.7%, and 116.7% more than evaporation from bare soil during the first, second, and third months, respectively.

This investigation highlights the significant influence of bioengineering through phreatophytic Reeds, seasonal variations of weather conditions, and the hydropedology of the root zone on checking the SWT levels. The influence of fine-textured soil lenses, strata, and engineered soil siphons in controlling water levels was studied. While the presence of Reeds plays a crucial role in influencing water levels, seasonal fluctuations, usually modeled by ET₀-ET_c, also contribute, with drastic differences between summer and winter. The investigated drainage techniques are ecologically and environmentally benign: no electricity or fuel is used for the reduction of waterlogging because only soil capillarity and solar energy maintain the processes of evaporation and transpiration; the Reeds’ biomass, accumulated during the ecoengineering process, additionally intercepts and sequesters CO₂.

How to cite: Al-Yaqoubi, S., Al-Maktoumi, A., Kacimov, A., Abdalla, O., and Al-Ismaily, S.: Investigating the feasibility of Bioengineering and Hydropedological techniques in controlling shallow water table problem in urban areas, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-3493, https://doi.org/10.5194/egusphere-egu24-3493, 2024.

Drywell infiltration is a common approach to recharge groundwater and reduce load from drainage systems. Due to rapid acceleration in urban developments, as well as climate change that predicts an increase in frequencies and magnitude of extreme precipitation events in the Mediterranean area, it is critical to predict the drywell infiltration capacity, i.e., its response to anticipated precipitation events. The infiltration capacity of a drywell is mainly determined by the geometrical parameters, i.e., diameter and depth, and the soil hydraulic parameters, i.e., hydraulic conductivity, porosity, and water retention. Predictions of drywell infiltration capacity are commonly conducted using models that solve the unsaturated flow in the subsurface using complex and costly numerical schemes. This work proposes a different approach based on a semi-analytical model that relies on a sharp interface wetting front assumption. The proposed model can predict the water levels in the well and the subsurface wetting front location during and after an infiltration event. The semi-analytical model was tested and compared with numerical simulations of Richards' equation and with data from a field experiment and proved to be sufficiently accurate. The typical run times of the semi-analytical model are smaller than 1 second and about three orders of magnitude shorter than the numerical model of Richards' equation. The field data was further utilized to calibrate the soil hydraulic properties by matching the semi-analytical model's outcomes to the measured water levels in the well. A sensitivity analysis of the drywell response to variable hydraulic properties, climatic scenarios, and well configurations (depth and diameter) was conducted, demonstrating some practical applications for analysis, which may be used for adequately determining site-specific drywell design.

How to cite: Moreno, Z., Paster, A., and Kamai, T.: A Wetting-Front Model for Vadose Zone Infiltration via Drywells, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1708, https://doi.org/10.5194/egusphere-egu24-1708, 2024.

Variably saturated subsurface flow models have been widely used in the context of water resources management as they conceptualize and simulate water flow in the unsaturated and saturated zone. By solving the Richards equation and using infiltration flux as an input, these models do not require groundwater recharge. As the models simulate the infiltration dynamics through the unsaturated zone, recharge is expected to be reliably extracted from such kinds of models. In this study, we explore to what extent variably saturated subsurface flow models can actually be used to extract groundwater recharge. In this context, we implement numerous definitions of groundwater recharge in a simple, variably saturated 1D model, extract groundwater recharge for a wide range of infiltration and groundwater dynamics imposed through boundary conditions, and assess the reliability of the extracted values. The results show that the value of recharge cannot be uniquely obtained from such kinds of models. The problem is attributed to the storage dynamics in the capillary fringe above the water table. However, it is important to keep in mind that if a variably saturated subsurface flow model of a project area is available, extracting recharge is superfluous as the model is capable of representing all the relevant flux and dynamics.  

Keywords: Variably saturated subsurface flow models; Groundwater recharge; unsaturated zone; Water resources management.

Reference: Gong, Chengcheng, Peter G. Cook, René Therrien, Wenke Wang, and Philip Brunner. "On groundwater recharge in variably saturated subsurface flow models." Water Resources Research 59, no. 9 (2023): e2023WR034920.

How to cite: Gong, C., Cook, P., Therrien, R., and Brunner, P.: Extraction of recharge in variably saturated subsurface flow models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15146, https://doi.org/10.5194/egusphere-egu24-15146, 2024.