Quantification of groundwater recharge to a semi-arid Mediterranean karst aquifer via high-performance modeling of fully-integrated surface-subsurface, multi-continuum flow dynamics

Recent climate simulations indicate a rapid decrease in groundwater availability in Mediterranean regions due to changes in precipitation patterns and increased evaporation rates. Quantitative approaches for flow and transport dynamics are an important tool for water management practices. Specifically the proper estimation of recharge rates is an essential objective for predictive simulation of the large-scale behavior of Mediterranean karst aquifers.

The main challenges in simulating the recharge process are: (1) the modeling of surface-hydrological processes to estimate groundwater recharge based the particularities of rock-soil landscape (e.g., roughness), ephemeral wadi streams, spatially and temporally highly variant precipitation patterns. (2) Exposed karst features (i.e. sinkholes) facilitating rapid recharge (in addition to classical diffuse recharge) through a thick, several hundreds of meters, vadose zone, requiring appropriate numerical approaches. We employ HydroGeoSphere on a high-performance-computing platform to simulate the terrestrial-based hydrological cycle of the Western-Mountain-Aquifer (9000km², located in Israel and the Palestinian Territories) including fully-integrated surface-subsurface coupling and vadose zone dynamics affected by dual-domain infiltration behavior.

HydroGeoSphere simultaneously solves the 2-D diffusion-wave equation for surface routing coupled to the 3-D Richards equation for subsurface flow under variably-saturated conditions. To naturally account for the partitioning of rainfall into diffuse and rapid direct recharge, e.g., along dry valleys or sinkholes the surface-subsurface flow regimes are coupled via a first-order exchange term including a thin conductive boundary layer. A double-continuum approach is employed to adequately represent the duality of flow in the karst aquifer. Fractures and conduits provide preferential flow paths in karstified rocks, triggering rapid flow, while the rock matrix provides substantial storage and slow seepage. This results in a high space and time dependent variation of infiltration and subsequently for a simulation of the observed high variation in groundwater dynamics.