HS8.2.3

Karst aquifers provided 9.2 % of the world’s population with fresh water in 2016 (Stevanović, 2019), but due to their dual flow behavior they are highly vulnerable to anthropogenic impacts and shifts in climate. In the near future, 52 out of 356 Mediterranean aquifers will be exposed to more extreme climatic conditions, which will enhance their water stress if the water usage is not adapted to available water resources (Nußbaum, 2020). Therefore, accurate and high resolution numerical - and empirical models are essential to calculate the groundwater recharge and water availability in complex karst aquifers that cover ~ 14 % of the earth’s ice free land (Stevanovic, 2019).

During the last decades, several empirical equations have been developed to calculate the recharge for Israel´s most important source of freshwater, the Western Mountain Aquifer (WMA). These equations calculate annual groundwater recharge of the entire 1.812 km² recharge area based on annual or monthly precipitation data. We analyzed the applicability of several new methods, such as Soil & Water Assessment Tool (SWAT), HydroGeoSphere (HGS) and Hydro- / Pedo- Transfer Functions (HPTF) to estimate groundwater recharge with  a higher resolution as this is essential to calculate proper water fluxes though the vadose zone of karstic aquifers when precipitation is affected by a high variability in space and time.

The hydrologic balance models,  e.g. SWAT (Neitsch et al., 2009),  calculate the water balance on a daily basis for specified Hydrologic Response Units (HRUs), while generalized HPTFs (Wessolek et al., 2009) use soil-, land cover-  and climate data to calculate  annual percolation rates on a coarse grid, in our case 500 m grid size. The dual continuum model using the code HGS (Brunner et al., 2011) is able to simulated based on Richards’s flow equation down- and upward water fluxes in the unsaturated zone accounting for both, a rapid flow component though the high permeable conduit and a slow flow component through the rock matrix.

The comparison of these empirical and new methods for groundwater recharge estimation show significant differences for hydrological extreme years, while results are similar during years with precipitation rates near the average value. For example, the empirical equation of Guttman & Zukerman (1995) gives  highest recharge values of all approaches during wet years, while the equation of Abusaada (2011) and the SWAT-model calculates  highest recharge values of all approaches during  dry years. Overall, the mean recharge ranges from 120 to 177 mm/a which equals 25 – 37 % of the average precipitation between 1990 – 2018.

These recharge rates are calculated based on IMS climate data. However, for recharge values used in water resources management regional climate projections are needed. For Israel a high resolution CORDEX-MENA climate projection (Hochman et al., 2018) is available for RCP4.5, showing an increase in temperature and decrease of precipitation during the winter of 2.5 °C and 40 %, respectively. Based on these climate projections the  SWAT-model estimates, that the average groundwater recharge for 2050 – 2070 will be 16 % lower than the reference period between 1980 – 2000.

How to cite: Hepach, P., Banusch, S., Bresinsky, L., Somogyvári, M., Bucchignani, E., Sauter, M., and Engelhardt, I.: Comparison of methods for recharge estimation and prediction in karstic aquifers under Mediterranean Climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11825, https://doi.org/10.5194/egusphere-egu21-11825, 2021.

Present methods to quantify recharge in karst aquifers in many cases rely on spatially and temporally aggregated precipitation values, neglecting the highly erratic, non-linear nature of infiltration dynamics that give rise to a dual-domain behavior with a slow diffuse and fast focused recharge component. Here, we demonstrate the applicability of integrated surface-subsurface flow models to simulate diffuse and preferential infiltration within the large scale Western-Mountain-Aquifer (WMA) in Israel and the Palestinian territories. A semi-arid climate region with a highly pronounced seasonality of precipitation and intense short-duration rainfalls, such as the Mediterranean region, emphasizes the importance of understanding and accounting for the complex dynamics of dual-domain infiltration and partitioning of the precipitation input signal via spatially discretized overland flow processes.

We apply HydroGeoSphere as a dual-continuum flow simulator for transient variably-saturated water flows, discretizing the rock matrix and secondary porosity (i.e., conduits and fractures) as separate overlapping continua. Flow is respectively computed via the Richards' equation with volume-averaged van Genuchten parameters, assuming that the Richards' equation is valid for both domains. The presented model accounts for surface flow via the two-dimensional Saint-Vénant equations under nonexistent inertial forces. We apply precipitation directly to the overland flow continuum and naturally account for the partitioning into Horton overland flow and percolating water. However, modeling of unsaturated flow through the conduit/fracture continuum with the van Genuchten parameterization is often limited, as the Richards' equation describes flow solely in terms of capillary forces, leading to high matric suction in the matrix continuum as a result of the smaller pore spaces (and hence constant exchange from the fracture continuum to the matrix system). In a natural system, non-linear transfer processes govern the transfer between fracture/conduit and matrix flow, such as inertia-driven infiltration (i.e., droplet, rivulet, and film flow) that initially retains itself from equilibration of capillary pressure heads and avoids instant matrix imbibition. This study demonstrates parametrization strategies to allow for infiltration through the fracture/conduit continuum using small-scale process-based simulations. Further, we offer procedures that help to achieve convergence of complex catchment-scale variably-saturated simulations.

How to cite: Bresinsky, L., Kordilla, J., Engelhardt, I., and Sauter, M.: Distributed modeling of groundwater recharge in a semi-arid carbonate aquifer using the integrated surface-subsurface flow simulator HydroGeoSphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14446, https://doi.org/10.5194/egusphere-egu21-14446, 2021.

Groundwater is a valuable resource throughout the world. It supplies the needs of many sectors everywhere. Providing high spatial resolution groundwater data is important for climatic, hydrological and agricultural applications, to ensure sustainable groundwater management. The scarcity of high-resolution groundwater data over large scales at the required accuracy is a significant limitation for such applications. This study was undertaken in the Mediterranean region, which is recognized as one of the world's most sensitive regions to water scarcity due to both climate change and consistently increasing anthropogenic pressures. Groundwater is considered a strategic freshwater reserve in the Mediterranean region; however, its status remains poorly characterized. This study investigates the feasibility of downscaling outputs of three global groundwater models (Reinecke et al. (2019), de Graaf et al. (2017) and Fan et al. (2013)) to higher resolution.

Steady-state results of the three models were compared with in-situ groundwater level observations, and an aggregation method was developed for downscaling. Observations from a long-term groundwater monitoring network over different regional studies around the Mediterranean were employed. Results showed that there is a significant discrepancy between the three compared model outputs. More specifically, the de Graaf et al. (2017) model presents a deeper water table than Reinecke et al. (2019) and Fan et al. (2013), while de Graaf et al. (2017) generally shows more significant variability in simulated water table depth. A detailed comparison between simulated and measured water table depth of different Mediterranean aquifers having different climatic, geologic and anthropogenic conditions will be presented.

The results of this work will contribute to advance the understanding of how to combine large-scale groundwater modelling with local in-situ data as a crucial tool to improve groundwater management in data-scarce regions.

This work was supported by the German Federal Ministry of Education and Research (BMBF, Germany, Grant 01DH19015) under the Project Sustain-COAST, co-funded by EU PRIMA 2018 programme.

References

• ­ Reinecke, R. et al. Challenges in developing a global gradient-based groundwater model (G³M v1.0) for the integration into a global hydrological model. Model Dev 12, 2401-2418 (2019).
• ­ de Graaf, I. et al. A global-scale two-layer transient groundwater model: Development and application to groundwater depletion. Water Resour 102, 53-67 (2017).
• ­ Fan, Y. et al. Global patterns of groundwater table depth. Science 339, 940-943 (2013).

How to cite: Ben-Salem, N., Reinecke, R., Gómez-Hernández, J. J., Karatzas, G., Rode, M., and Jomaa, S.: Groundwater assessment in the Mediterranean region: Regional modelling and in-situ data across scales, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9792, https://doi.org/10.5194/egusphere-egu21-9792, 2021.

The Mediterranean region faces water security challenges with increasing water demand and climate change effects. Groundwater has become a key resource for water supply and economic development in the last decades, however, its budget remains poorly understood. The distribution of piezometric data in the Mediterranean has a very contrasting distribution. A large portion of them is not centralized and openly accessible, resulting in lack of detailed assessment of groundwater trends and their controlling factors at the regional Mediterranean scale. The objectives of this work are: i) the creation of a long-term and, possibly, the most comprehensive database for groundwater dynamics in the Mediterranean region, ii) the identification of trends and clusters on groundwater levels, and iii) the identification of the relationship between trends and climatic and hydrogeological drivers.

Over 10,000 time series of groundwater level have been collected from national and regional authorities and the literature. The data come from eight countries in the Mediterranean region and have been post-processed into a common format. A search for seasonal patterns and long-term trends is performed then clustered accordingly. Furthermore, the influence of controlling factors such as precipitation and hydrogeology on the groundwater dynamics and trends are assessed. Significant groundwater level changes have been identified at a regional scale and used to provide insight into groundwater level change drivers’ in the Mediterranean region. The database is the result of a unique joint effort between regional groundwater experts to collect groundwater status information in the Mediterranean region and will serve as the foundation for future research on the influence of anthropogenic drivers and the prediction of groundwater depletion hotspots.

This work was developed under the scope of the InTheMED project. InTheMED is part of the PRIMA programme supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement No 1923.

How to cite: Chavez Garcia Silva, R., Reinecke, R., Varouchakis, E., Gómez-Hernández, J., Rode, M., and Jomaa, S.: Long-term groundwater database and assessment for the Mediterranean region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10989, https://doi.org/10.5194/egusphere-egu21-10989, 2021.

Approximately 10% of the global population rely on groundwater from karst aquifers. Due to complex karst structures, these aquifers have high infiltration capacities and hydraulic conductivities, which makes them vulnerable to pollution and, as prediction and management are complicated, overexploitation. As populations are growing and demand rises, we assess the current level of groundwater stress in karst aquifers with Mediterranean climates and their vulnerability (defined as the change in groundwater stress) to expected changes in temperature and precipitation on the global scale.

Our approach is based on a Groundwater Stress Index (GSI), which is calculated for 356 karst aquifers (as identified in the World Karst Aquifer Map) that have some of their area located in Mediterranean climate zones (Csa, Csb, and Csc after Köppen/Geiger). GSI are calculated from seven indicators: groundwater recharge, storage, and abstractions, surface runoff, climatic water balance, water-intensity of crops, and groundwater-dependent ecosystems. Each indicator is spatially and temporally averaged to describe a recent trend on aquifer level, resulting in one complex attribute table for the 356 aquifers. GSI is calculated as the average of the normalized indicators for each aquifer, ranging from 0 (no water stress) to 1 (extreme water stress).

Aquifers are then grouped based on similarities in two classification parameters – degree of karstification (P1) and land cover (P2). Comparison of aquifers with similar classification parameters allows to focus more directly on the relationship between groundwater stress and climate, disregarding relatively constant influences. For each group (e.g., well-developed karst, primarily agriculturally used), we plot calculated GSI values with current temperature and precipitation data. By investigating four Shared Socioeconomic Pathways (SSPs) until 2100, we identify aquifers that mimic future climate conditions for others with similar P1 and P2. We then measure the difference in groundwater stress accompanied by altered climatic factors. This change is interpreted as vulnerability to climate change.

This approach, which relies on present-day observed conditions, allows us to predict the effect of a changing climate without the need to develop a complex numerical model, which requires large amounts of data and functional understanding of aquifer behavior. While analysis is currently ongoing, we expect both groundwater stress and vulnerabilities to be high. Predicted climate zone shifts by Beck et al. (2018) indicate that, out of 356 karst aquifers with Mediterranean climates, 52 could move to more extreme arid climate zones by 2100.

Results will be visualized in the form of vulnerability maps that may serve as an “early-warning system”. For particularly threatened aquifers, we will derive recommendations for more sustainable management by suggesting strategies to lower groundwater stress. This is done by taking a closer look at the aquifer’s indicator values and identifying factors that currently contribute the most to groundwater stress.

How to cite: Nußbaum, P., Somogyvári, M., Conrad, C., Sauter, M., and Engelhardt, I.: Calculating groundwater stress and climate change-induced vulnerability of karst aquifers on a global scale, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-929, https://doi.org/10.5194/egusphere-egu21-929, 2021.

Determining subsurface hydraulic and geomechanical properties crucially underpins groundwater resource investigation and management. While standard practice relies on active testing, passive approaches require less effort and cost but are underutilised. We present the new Python package named HydroGeoSines (HGS) which quantifies hydraulic and poroelastic subsurface properties using the groundwater response to natural forces (such as Earth tides and atmospheric pressure changes) embedded in standard measurements. All implemented methods are drawn from the peer-reviewed literature. The package includes basic handling of time series, such as joining and aligning records and handling gaps. HGS uses standard atmospheric and groundwater pressure records to estimate the Barometric Response Function (BRF) groundwater state of confinement, hydraulic conductivity, specific storage, barometric efficiency (BE) and porosity. If Earth tides are required, they can be calculated on-the-fly using the PyGTide package which is based on ETERNA and included. HGS allows easy compensation and correction of pressure or hydraulic heads from barometric pressure or Earth tide influences. Further, HGS includes import from and export to common data formats as well as visualisation of data and results. We demonstrate the use of HGS using example datasets from around the world. Since HGS unlocks sophisticated methods for use by anyone with Python skills, we anticipate that it will support subsurface investigations and add value to standard monitoring practice.

How to cite: Rau, G., Schweizer, D., Turnadge, C., Blum, P., and Rasmussen, T.: A new Python package to estimate hydraulic and poroelastic groundwater properties using standard pressure records, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4679, https://doi.org/10.5194/egusphere-egu21-4679, 2021.

The study area located in the semi-arid western India is the tenth most populous Indian state with an estimated population of 60.3 million (~5% of Indian population) and population density of 308 persons/km². About 42.6% of the population lives in the urban area and 57.4% in rural areas. Surface water paucity, seasonal availability and a varying amount of rainfall in this region make groundwater an important and most preferred source to fulfil the demand for drinking water, agricultural, industrial and sustain an important native terrestrial ecosystem. The over-dependency on groundwater leads to various problems related to quality and quantity of groundwater like a rapid decline of the water table, mining of static groundwater, seawater intrusion and groundwater contamination by geogenic and anthropogenic sources. Sustainable water management across the state must be underpinned by the clear understanding of groundwater recharge characteristics, relationship between recharge sources and groundwater, factors controlling the interaction between surface water and groundwater, deletion of the areas not receiving a freshwater influx. Shallow groundwater samples were collected during IWIN nation programme (2008 to 2013)  for the year 2009 to measure the stable isotopes of oxygen and hydrogen in groundwater from 205 locations during post-monsoon (November ) season and 207 locations in pre-monsoon season ( May-June) to understand the factors governing Spatio-temporal variation in isotopic composition and obtain insights about the spatially variable recharge characteristics and possible controlling factors. The oxygen and hydrogen isotopic values and their spatio-temporal variations in the study area demonstrate that (1) d¹⁸O depletion in post-monsoon GW compare to pre-monsoon infer the seasonal recharge of GW (2) d¹⁸O of post-monsoon GW ( -2.3‰) lie in between the d¹⁸O of pre-monsoon GW ( -1.9‰) and d¹⁸O of southwest monsoon rainfall (-4.1‰) indicates post-monsoon GW is the mixture of these two components (3) Seasonal variation in d¹⁸O deduce that 56% ( ~109773 km2) of the total land area (~ 196773 km2) shows seasonal GW recharge while 32% (62727 km2) is not receiving any freshwater influx (4) 36% of post-monsoon samples while 40% of the pre-monsoon samples have negative d-excess values and shows a decreasing trend with d¹⁸O infer evaporation of sub-surface water prior to recharge (5) Alluvial aquifer of north Gujarat has depleted d¹⁸O compare to adjacent high elevated hard rock aquifer indicates irrigation return flow of deep static water.

How to cite: Pandey, A., Padhya, V., and Deshpande, R. D.: Isotopic Characterization of groundwater in semi-arid Western India: Insights into Hydrogeological Processes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10826, https://doi.org/10.5194/egusphere-egu21-10826, 2021.

Quantification of long-term hydrologic change in groundwater often requires the comparison of states pre- and post- change. The assessment of these changes in ungauged catchments is particularly difficult from a conceptual point of view and due to parameter non-uniqueness and associated uncertainty of quantitative frameworks. In these contexts, the use of data assimilation, sensitivity analysis and uncertainty quantification techniques are critical to maximise the use of available data both in terms of conceptualisation and quantification. This paper summarises findings of a study undertaken in the Lake Muir-Unicup Natural Diversity Recovery Catchment (MUNDRC), where a number of techniques were applied to inform both conceptual and numerical models. The MUNDRC is and small-scale endorheic basin located in southwestern Australia listed under the Ramsar Convention as a Wetland of International Importance and have been subject to a systematic decline in rainfall rates since 1970. Conceptual and numerical frameworks have been development to understand and quantify impacts of rainfall decline on the catchment using a variety of metrics involving groundwater and lake levels, as well as fluxes between these compartments and mass balance components. Conceptualisation was facilitated with the use a novel data-driven method relating rainfall and groundwater response running backwards in time, allowing the establishment of baseline conditions prior to rainfall decline, estimation of net recharge rates and providing initial heads for the forward numerical modelling. Parameter and predictive uncertainties associated with data gaps have been minimised and quantified utilising an Iterative Ensemble Smoother (White, 2018), while further refinement of conceptual model was undertaken following results from sensitivity analysis, where major parameter controls groundwater levels and other predictions of interest were quantified.

How to cite: De Sousa, E., Hipsey, M., and Vogwill, R.: Data assimilation, sensitivity analysis and uncertainty quantification in a semi-arid terminal catchment subject to rainfall decline, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14038, https://doi.org/10.5194/egusphere-egu21-14038, 2021.

The environmental fate and transport of nitrogen and phosphorus nutrient species leached from agroecosystems are largely influenced by the hydrogeological setting, which dictates the distribution of groundwater flow pathways, residence times, and physio-chemical properties of the subsurface. Traditional conceptual models tend to oversimplify these relationships, and their application towards river catchment nutrient management promotes insufficient characterisation of hydrogeological heterogeneity, which is subsequently not accounted for. Until recently, very little hydrogeological information and conceptual understanding existed for groundwater systems within the postglacial basement terranes of Scotland and Northern Ireland, due to an abundance of surface water resources and prevalence of poorly productive bedrock aquifers. Recent research has demonstrated the role of geological heterogeneity in determining the contaminant transport behaviour of these hard-rock aquifers, where the presence of weathering and fracturing can potentially result in the rapid delivery of nutrients to rural water supplies and groundwater-dependent ecosystems.

We aim to further elucidate the role of hydrogeological setting in river catchment nutrient dynamics to improve agricultural sustainability in geologically heterogeneous agricultural regions. This will be achieved by developing conceptual models of nutrient fate and transport for two contrasting agricultural river catchments. Here, we present preliminary conceptual models based on a literature review of groundwater systems within the same geological terranes, analysis of hydrochemical monitoring data, and accounting for catchment-specific features through desk studies of geological and airborne geophysical surveys.

The River Ythan is a groundwater-dominated lowland catchment within Scotland’s arable belt, designated a Nitrate Vulnerable Zone due to the eutrophication of its estuary. This catchment is geologically complex, with a variably metamorphosed and sheared Precambrian basement with igneous intrusions ranging from ultrabasic rocks to granite. This complexity is enhanced by the significant preservation of Tertiary weathering profiles and an extensive but discontinuous cover of glacial deposits derived from the saprolites. The superficial deposits create a shallow aquifer system characterized by oxic, well-mixed groundwaters with high nitrate concentrations. The bedrock groundwater bodies feature lower nitrate concentrations with variable denitrification rates, resulting from the relationships between lithology, tectonics, and weathering.

Two upland headwater sub-catchments of the Upper Bann River (Co. Down, Northern Ireland) drain either side of the contact between a granodiorite laccolith and Lower Palaeozoic metasedimentary rocks within an elevated drumlinoid landscape. Here, diffuse phosphorus exports to surface waters have not experienced the same extent of decline observed in storm runoff phosphorus following the implementation of nutrient management policies. Anoxic groundwaters favourable for denitrification may result in the release of previously adsorbed (legacy) phosphorus following the reductive dissolution of Fe (hydr)oxides. These conditions are generated by (a) confinement by thick, drumlinised clayey tills; and (b) bedrock structures promoting deep groundwater flow.

The site-specific conceptual models will be further developed through multi-scale geophysical characterisation of hydrogeological heterogeneity and constrained by the catchment-scale distribution of residence times derived from stable (²H, ¹⁸O) and radioactive (³H) isotope compositions of groundwaters. These refined conceptual models can guide the development of numerical groundwater models and spatially targeted nutrient management.

How to cite: Johnson, H., Comte, J.-C., Ofterdinger, U., Cassidy, R., and Troldborg, M.: Preliminary conceptual models of groundwater and nutrient dynamics in typical agricultural river catchments underlain by hard-rock aquifers in Scotland and Northern Ireland: The River Ythan and Upper Bann River Catchments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9281, https://doi.org/10.5194/egusphere-egu21-9281, 2021.