HS8.3.1

Heterogeneity plays a major role in subsurface processes from the local scale (preferential infiltration and flow paths, fractures) to the catchment scale (presence of lateral and vertical variability, multiple horizons, bedrock interface, etc.). If high-resolution direct observations are often available through drillholes, CPT or installing in-situ monitoring probes, those local measurements only provide punctual or 1D information. Within this context, geophysical techniques can provide relevant spatially-distributed information (2D, 3D or even 4D) with a much larger coverage than direct measurements. However, geophysical information remains indirect and must be translated into the sought parameter through petrophysical or transfer functions. 

Geophysicists are facing two important issues when imaging the subsurface: 1) Generating images of the subsurface that are consistent in terms of soil or geological structures; 2) Integrating the geophysical information into hydrological models. Both issues will be discussed in this contribution.

Geophysical imaging is the result of an inversion process whose solution is non-unique. This problem is generally solved using a regularization approach introducing some a priori characteristics of the model. The dominant choice is still the smoothness constraint inversion, which often introduces a too simplistic representation of the subsurface, and decreases the potential of geophysics to discriminate between different facies. In the first part of this contribution, we will analyze what can be expected from geophysical methods in terms of characterization of the heterogeneity. We will illustrate how the inversion method affects the discrimination potential of geophysics, and how we can improve the geophysical image by accounting for prior information. We will see how the discrimination potential decreases with the loss of resolution. Finally, we will investigate how recent methodologies using machine learning can improve our ability to image the subsurface.

Given the high spatial coverage of geophysical methods, they have a huge potential to inform hydrological models in terms of heterogeneity. However, the limitations related to geophysical inversion also make the geophysical model uncertain and the risk to propagate erroneous information exists. In the second part of this contribution, we will illustrate how to incorporate geophysical data into hydrological models to unravel their spatial complexity. At the early stage of a project, several scenarios regarding spatial heterogeneity are often possible (orientation of fractures, number of facies to consider, interconnection within one facies, etc.), and this can largely influence the outcomes of the hydrological models. In this context, geophysical data can be used to verify the consistency of some scenarios without requiring any inversion in a process called falsification. Once realistic scenarios have been identified, geophysical data can be used to spatially constrain hydrological models. However, this should ideally account for the uncertainty related to geophysical inversion. One possibility is to use a fully-coupled approach where geophysical data are integrated directly in the hydrological model inversion. This requires nevertheless a transfer function to relate hydrological and geophysical variables. As an alternative, a sequential approach using a probabilistic framework accounting for the imperfect geophysical data can be used. The latter requires co-located measurements.

How to cite: Hermans, T., Michel, H., Lopez-Alvis, J., and Nguyen, F.: Imaging the subsurface to inform hydrological models: a geophysicist’s perspective, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-319, https://doi.org/10.5194/egusphere-egu21-319, 2021.

One of the major challenges in soil hydrological modelling is due to the fact that soils are heterogeneous at all spatial scales. The identification and accurate representation of such heterogeneity can be crucial for quantifying the subsurface hydrological states and water fluxes.

This work presents the results of an integrated approach for process-based soil hydrological modelling for a highly instrumented hillslope site. The approach builds on the integration of classical soil mapping, on accurate monitoring of soil water dynamics as well as on geophysical measurements for characterising subsurface heterogeneity. It finally integrates the gathered information into a physical model for simulating the soil water dynamics with high spatial and temporal resolution.

At the Schäfertal Hillslope site (Central Germany), the soil monitoring network STH-net provides high-quality data about the soil water dynamics and soil properties at 8 instrumented soil profiles and depths within the unsaturated zone. The soil spatial variability, known from local soil description and sampling, was mapped using time-lapse electromagnetic induction measurements. The geophysical inversion of the data provided depth-resolved information about the subsurface structures in terms of soil-bedrock interface, soil horizons and their spatial continuity along the hillslope transect. Based on this, different versions of the subsurface geometry model were produced and associated to soil hydraulic parameterizations derived from different approaches.

We show the performance of the physical model CRITERIA-3D in reproducing the soil water dynamics for different subsurface models with increasing complexity. Specifically, we highlight and discuss the key challenges that need to be addressed when continuous information about the subsurface heterogeneity is to be mapped in the field with high resolution and represented in a numerical model with fine discretization in three-dimensions.

We conclude that linking state-of-the-art experimental methods to advanced numerical tools, and bridging the gap between different disciplines such as pedology, hydrology and geophysics can be the key for improving our ability to measure, predict and better understand the vadose-zone processes. This will provide important knowledge needed for transferring this approach to larger scales where the accurate quantification of the soil water fluxes is required for a more efficient water management in the context of sustainable food production and climate change.

How to cite: Martini, E., Wollschläger, U., Bittelli, M., Tomei, F., Werban, U., Zacharias, S., and Roth, K.: Hillslope-scale mapping and numerical representation of the subsurface heterogeneity towards an improved process-based hydrological modelling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2848, https://doi.org/10.5194/egusphere-egu21-2848, 2021.

Measuring hydraulic properties of stony soils and interpretation of the measured data is a challenge in vadose zone hydrology. The reason is not only the problem of installing suitable sensors but also the systematic measurement errors when sensors are only located in the background soil. A common approach to calculate the hydraulic properties of stony soils is by scaling the properties of the background soil according to the rock fragment content. Such modeling approaches are primarily developed for saturated flow conditions and only consider the amount of rock fragments as an input parameter. However, there is still a gap in knowledge regarding the effective properties of stony soils under unsaturated flow conditions.

Recently, 3D numerical simulation has become a convenient alternative tool to study the transport properties of heterogeneous porous media. The generation of data by numerical models is fast, measurements are repeatable and the simulation of the system under different initial and boundary conditions is easily achievable. We simulated three-dimensional unsaturated water flow in laboratory columns with stony soil material using the Hydrus 2D/3D software. Geometries were generated by assuming different volume fractions of impermeable rock fragments with spherical, cylindrical, or prolate shapes embedded in sandy loam soil. Time series of mean water contents, local pressure heads, and fluxes across the upper boundary were generated in an evaporation experiment, and a multi-step unit gradient simulation was applied to obtain values of hydraulic conductivity near saturation.

The synthetic measurement data were evaluated by inverse modeling, assuming a homogeneous system, and the effective hydraulic properties of stony soils were identified. The results were used to evaluate the scaling approaches for different volumes of rock fragments. A non-linear reduction in hydraulic conductivity by the increase of rock fractions was visible. The results also highlighted the effects of the orientation and shape of rock fragments. The orientation of rock fragments towards flow has a significant effect on the flow reduction, and in the case of prolate spheroids oriented along the flow direction, the reduction in conductivity was less significant. 

How to cite: Naseri, M., C. Iden, S., and Durner, W.: Effective hydraulic properties of virtual stony soils: Forward 3D simulations evaluated by 1D inverse modeling, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12928, https://doi.org/10.5194/egusphere-egu21-12928, 2021.

As midwinter melt and rain-on-snow events become more common occurrences in the northern hemisphere under climate change, incorporating frozen processes when simulating winter-time recharge is increasingly necessary. The activation of infiltration pathways and recharge dynamics of shallow bedrock environments under frozen conditions has received relatively little attention. Over the 2019-2020 winter, hydrogeologic and cryospheric conditions of the surface, unsaturated, and saturated zones were monitored around a low-lying granitic outcrop in eastern Ontario, Canada. Interpretation of the data indicated that the soil-rock contact around outcrop margins was the key pathway enabling midwinter infiltration and recharge. To support this conceptual model and further explore the role of outcrops in enhancing midwinter bedrock recharge, a numerical investigation was undertaken. Measured climate data (hourly time step) was used to govern the surface energy and water balances of a 1D finite difference model that incorporates frozen processes. Measured snow depth, soil moisture content, and soil temperature profiles were simulated. Simulations with vertical infiltration alone could not account for observed increases in moisture content in the deepest soil horizons. This is attributed to additional lateral flow along the unfrozen soil-rock contact that bypasses the frozen soil layers. Preliminary results support the concept that bedrock outcrops provide a window for midwinter infiltration since repeated winter melts reduce frozen soil permeability and inhibits vertical infiltration until the ground thaws. Results from the surface/near-surface simulations are used to guide the development of a 2D finite element model that includes heat and flow transport and ground freeze-thaw. The impacts to bedrock recharge under different rainfall and snowmelt scenarios as well as various outcrop geometries are explored. Results from these numerical experiments help provide greater insight into the processes driving enhanced midwinter bedrock recharge under conditions of warmer winters.

How to cite: Wright, S. and Novakowski, K.: Bedrock outcrops: A window for enhanced midwinter recharge, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8074, https://doi.org/10.5194/egusphere-egu21-8074, 2021.

Artificial recharge is the set of techniques used to increase or facilitate the flow of water to aquifers. It has been a management strategy for centuries to optimize the use of water in regions where the seasonal or inter-annual distribution of surface water produces periods or exceedance and shortage. Water infiltration into aquifers is enhanced such that aquifers serve as short to medium term storage reservoirs. Water is recovered when needed.  Recently, increasing demand of groundwater and the occurrence of more severe and longer droughts in different regions around the world have produced a renewed interest in the application of this management strategy in many countries, particularly in arid and semi-arid regions.

Infiltration wells are a common method to apply artificial recharge, which allows infiltrating water directly into saturated aquifers or to the unsaturated zone. We performed local-scale numerical simulations of unsaturated flow to model the operation of a single well infiltration system. Based on the results of the simulations, we evaluate its performance considering different conceptualizations of the materials present in the vadose zone. We conclude that the performance of similar systems can be significantly different depending on the distribution of subsurface materials and their properties. Hence, the conceptualization and modeling of such systems require some care about how the inherent heterogeneity of aquifers is included in models.  Last, we provide some recommendations for the design and assessment of similar artificial recharge systems. 

How to cite: Herrera, P. and Olivares, Y.: Assessment of an artificial recharge system through numerical simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10013, https://doi.org/10.5194/egusphere-egu21-10013, 2021.

Recharge dynamics within the vadose zone (variable saturation conditions) of consolidated fractured rock formations are an ongoing challenge when it comes to process understanding and predictive modeling. The proper delineation of fast (macropores, fractures, conduits) and slow (matrix) flow components in these systems and their interaction with each other remains a complex puzzle and holds a key to enhance process-based infiltration models.

We conducted laboratory and field experiments to study infiltration dynamics through porous-fractured systems. Laboratory experiments were carried out with analogue fracture networks on meter scale. Orthogonal networks were created by placing equally sized blocks with a constant gap between to glass plates, which were mount by metal clamps. Vertical flow through different network configurations (apertures, intersection types, topology, flow rates) was studied for (1) porous media (sandstone) and (2) non-porous media (glass) to delineate the control of network features on flow dynamics, as well as the effect of fracture-matrix interaction. Matrix imbibition was found to strongly control the preferential flow velocity during flow path evolution. Higher infiltration rates lead to more by-pass at fracture intersections, whereas low infiltration rates favor flow partitioning into horizontal fractures. Vertical flow progression within the non-porous network is significantly faster due to the lack of imbibition. Semi-analytical tools, such as transfer functions, and source-responsive dual-domain models are tested to reproduce the experimental data and to incorporate key features of fracture networks in future modeling approaches. We additionally obtained experimental data from infiltration dynamics at porous-fractured field sites on meter scale to compare them to the well-controlled laboratory experiments and to evaluate the applicability of the results to actual field processes.

How to cite: Rüdiger, F., Bartsch, K., Nimmo, J. R., and Kordilla, J.: Laboratory and field studies of preferential flow dynamics in unsaturated fractured porous media, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9711, https://doi.org/10.5194/egusphere-egu21-9711, 2021.

Understanding and predicting the macro-scale flow characteristics in the fractured vadose zone is of great importance for subsurface hydrological applications. Here we develop a network model to study infiltration in unsaturated fracture networks. We consider an idealized honeycomb-like fracture network composed of a series of Y-shaped and inverted Y-shaped intersections. At the scale of intersections, liquid storage/release and splitting/convergence behaviors are modeled according to local splitting relationships obtained from detailed laboratory work and numerical simulations. By varying the splitting relationships, the influence of local flow behaviors on large scale flow structures is systematically investigated. We find that when the water split tends to split equally at the intersection, a divergent flow structure forms in the network. Conversely, unequal splitting leads to preferential pathways. We also find that an avalanche infiltration mode, i.e., sudden release of a large amount of water from the network, emerges spontaneously, and is modulated by the local splitting behavior. The pathways of preferential flow is controlled by the liquid volume triggered by avalanches and the network structure. The improved understanding from this study may shed new light on the complex flow dynamics for unsaturated flow in fractured media.

How to cite: Xue, S., Yang, Z., and Chen, Y.-F.: Influence of local splitting behavior at intersections on infiltration in an unsaturated fracture network , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3663, https://doi.org/10.5194/egusphere-egu21-3663, 2021.

The locally focused dissolution of the rock material (e.g., below dolines and dry valleys) in karst systems and in general percolating clusters of fractures in consolidated aquifer systems trigger the development of preferential flow paths in the vadose zone. Rainfall events may initiate rapid mass fluxes via macropores and fractures (e.g., as gravitationally-driven films) that lead to source-responsive water table fluctuations and comparably short residence times within the vadose zone. The degree of partitioning into a slow diffuse infiltration component and a rapid localized part depends, amongst others, on the hydraulic interaction of porous matrix and fracture domain as well as the geometrical characteristics of the fracture systems (e.g., persistence, connectivity) that are often difficult to obtain or unknown under most field conditions. Given their importance in water-resource management, specifically in arid and semi-arid regions (e.g., Mediterranean), it is desirable to recover such infiltration dynamics in porous-fractured systems with physically-based yet not overparameterized models. Here, we simulate water table fluctuations in a karst catchment in southwest Germany (Gallusquelle) using a source-responsive film flow model based on borehole and precipitation data. The model takes into account interfacial connectivity between slow and fast domain as well as phreatic zone discharge via classical recession analysis. This case study shows the potential importance of preferential flows while modeling water table responses in karst systems and recognizes the need for formulations other than those applied for a diffuse bulk fractured domain where infiltration patterns are assumed to be homogeneous without formation of infiltration instabilities along preferential pathways.

How to cite: Noffz, T., Kordilla, J., Kavousi, A., Reimann, T., Sauter, M., and Liedl, R.: Predicting preferential flow and water table fluctuations in karst systems using film-flow theory and source-responsive models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14723, https://doi.org/10.5194/egusphere-egu21-14723, 2021.

A fundamental understanding of water infiltration in unsaturated fractured rocks is important in a range of subsurface hydrological, environmental and engineering applications. We perform an experimental and modeling investigation of the gravity-driven liquid slug flow behavior at fracture intersections. In the experiments, we visualize the flow processes and quantitatively analyze the flow dynamics. We develop a novel computational model that adequately captures the splitting dynamics. This model considers dynamic contact angles and solves temporal evolution of interface motion based on force balance with the quasi-static assumption at each time step. We systematically examine the influence of various physical parameters on the flow splitting behavior, including the widths and inclination angles of channels, and slug lengths. Using the local splitting relationships obtained mechanistically, develop a network model to study infiltration in unsaturated fracture networks. Then, the influence of the local flow dynamics on large-scale flow structures is systematically investigated. We find that an avalanche infiltration mode emerges spontaneously and that the local splitting relationship controls the divergent and convergent flow structures.

How to cite: Yang, Z., Xue, S., Hu, R., and Chen, Y.-F.: Gravity-driven liquid splitting behavior at intersections and its control on infiltration in unsaturated fracture networks , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8582, https://doi.org/10.5194/egusphere-egu21-8582, 2021.

A physically distributed water balance model called WetSpass is applied to estimate the recharge for the semi-humid Lake Tana basin in northwest Ethiopia. Lake Tana basin, one of the major sub-basins of the Upper Blue Nile River basin, covers 15,077 km² of which 3,156km² is the lake water body. The basin is regarded as one of the growth corridors of the country, where huge waterworks infrastructure is being developed. The basin has complex volcanic aquifer systems due to the multi-stage volcanism of the Cenozoic and Quaternary eras comprising many dikes, extended volcanic necks, and centers. Hence, estimating hydrological terms such as groundwater recharge considering the high basin physical heterogeneities is difficult, though highly important. In this study, the WetSpass model is developed, and recharge surface, surface runoff, and evapotranspiration at 90 m grid resolution have been developed. The spatial recharge map is cross-validated with water table fluctuation (WTF) and chloride mass balance (CMB) methods. The mean annual recharge, surface runoff, and evapotranspiration over the whole basin using WetSpass are estimated at 315 mm, 416 mm, and 770 mm, respectively. The mean annual recharge ranges from 0 mm to 1085 mm: 0 mm at water bodies and highest on highly fractured Quaternary basalt. Similarly, a high range of recharge is also noted using WTF and CMB methods showing the strongly heterogeneous nature of the hydro(meteoro)logical characteristics of the area. Generally, the recharge is found higher in the southern and eastern catchments and lower in the northern catchments, primarily due to higher rainfall amounts and highly permeable geological formations in the former parts. A fair general correlation between the recharge by WTF and WetSpass is found. However, WetSpass is more effective in the highland areas where the recharge is controlled by rainfall, while the WTF method is more effective in the storage controlled flat floodplain area. CMB is applied in a less spatially distributed way, and hence, the spatial performance of the method is not well evaluated. However, logged water infiltration in the floodplains, and transpiration from the groundwater in shallow water table areas have disturbed the estimated recharge by the CMB method. The land-use change from 1986-2014 brought relatively small hydrological change, although the land use has changed significantly.

How to cite: Yenehun, A., Dessie, M., Nigate, F., Belay, A. S., Azeze, M., Van Camp, M., Fentie, D., Kidane, D., Van Griensven, A., Adgo, E., Nyssen, J., and Walraevens, K.: Spatial and temporal simulation of groundwater recharge amount and cross-validation with point measurement-based estimations on a tropical basin comprising volcanic aquifers: case study of Lake Tana basin, northwestern Ethiopia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9836, https://doi.org/10.5194/egusphere-egu21-9836, 2021.

Nutrient losses in agricultural areas have detrimental effects not only on the surface water quality but are also unfavorable for sustainable agriculture practices. In Denmark, there are currently nitrate regulations applied for ID15 catchments (15 square km scale), nevertheless, it is crucial to know how much nitrogen is retained in the root zone, saturated zone, riparian zone as N-retention varies widely within ID15 catchments. Currently, N-retention mapping does not incorporate N-retention in the root zone on ID15 scale. N-retention in the root zone of subsurface drained clayey areas is potentially influenced by the variation in water table depth. Therefore, we will evaluate the effect of shallow hydrogeology, topography and drain parameters on local (sub-field scale) water table depth variation using a case study in eastern Jutland, Denmark.

The aim of the study was to assess which hydrogeological variables, drain parameters and topographical variables control water table depth variation in the root zone. This analysis was aided by a groundwater flow model code (MODFLOW). For the following purpose, hydrological data (drain flow at the outlet and depth to the water table in piezometers) and geophysical data (subsurface electrical conductivity) were collected. The geophysical data was collected by two ground-based electromagnetic systems (DUALEM and tTEM). The electrical conductivities were directly translated into two zones of homogeneous hydraulic conductivities based on a threshold value. Hydraulic properties were varied for each zone. Areas with no geophysical data were simulated using Direct Sampling, a Multi Points Statistics method. We generated several flow models, which had a varying spatial distribution of hydraulic zones and varying hydraulic properties (input factors). Moreover, boundary conditions (lateral fluxes), topographical smoothing and drain parameters (drain conductance and drain depths) were some of the other input factors we considered in this work. Model boundary conditions data were obtained from the national hydrological model. The variation in input factors was related to variation in simulated water table depths and drain flow at the outlet using a one-at-a-time sensitivity analysis.

Drain flow fraction, depth to the water table and drain discharge are analyzed as the quantity of interest for both wet and dry periods. Drain fraction is calculated as the ratio of the area contributing to the drainage to the area contributing to the recharge within the same area. The results will discover crucial controlling components of water table depth with which variations in N-retention can be estimated between different fields. The emphasis is to discover the connection between hydrogeological, topographical, and drain variables, and water table depth. We will examine potential implications for evaluating drain fraction and potential nitrate reduction.

How to cite: Mahmood, H. and Rumph Frederiksen, R.: Hydrogeological, topographical and drain factors controlling water table depth variation and potential nitrate reduction in subsurface drained clayey till area, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5960, https://doi.org/10.5194/egusphere-egu21-5960, 2021.