Impact of climate change and groundwater consumption scenarios on a major transboundary karst water resource - The Western Mountain Aquifer in Israel and the West Bank

Climate simulations indicate that the Mediterranean region will be severely affected by climate change and is often referred to as the most prominent climate change hotspot (Gao and Giorgi, 2008). This study addresses the combined effects of climate change and three groundwater consumption scenarios on the water availability of the Western Mountain Aquifer (WMA) in Israel and the West Bank. Generally applied methods to quantify recharge and water resources rely on linear regressions or simplified models, such as data-driven approaches (i.e., lumped-parameter and black-box approaches). However, they are unfit to assess climate change impacts because the predictive power of data-driven approaches is low, should the variability of, e.g., precipitation expand beyond historically observed fluctuations, such as expected from climate change effects. Furthermore,  they do not honor the physics of flow. Therefore, assessing the impact of climate change requires the application of distributed process-based numerical models that incorporate as many relevant physical flow processes as feasible. For example, when karstified vadose zones measure several hundreds of meters, such as in the case of the WMA, variably saturated flow is a highly relevant flow process controlling vadose storage at large timescales and altering recharge flux at the “control plane” groundwater table.
We simulate the complex dynamics of the dual-domain infiltration and partitioning of the precipitation input signal by employing HydroGeoSphere (HGS) for transient variably saturated water flow. Flow in the limestone rock matrix and secondary high porosity system (i.e., conduits and fractures) is modeled by overlapping individual continua based on the bulk effective Richards’ equation with van Genuchten (VG) parameterization. The model input of this study stems from two coherent dynamically downscaled high-resolution regional climate projections (daily, 3km, and 8km resolution) until the year 2070, assuming the IPCC RCP4.5 climate change scenario. The results indicate that long-term average recharge quantities will decrease by circa 10 % compared to the reduction in average precipitation by 30 %. The mitigated impact on recharge is an effect of the pronounced heterogeneity of karstic flow (i.e., preferential recharge along with karst dissolution features) and increased intensity of individual rainfall events, emphasizing the need to apply spatiotemporally resolved climate models with daily precipitation values as input to the recharge assessment. However, despite the comparatively moderate decrease in recharge, the length and severity of consecutive drought years with low recharge values are likely to increase while freshwater demand is believed to increase during these periods, emphasizing the need to adjust the current management practices to climate change. Finally, the model is used to simulate managed aquifer recharge applications to mitigate the effects of more extended drought periods by strategic freshwater reserves.