Comparison of methods for recharge estimation and prediction in karstic aquifers under Mediterranean Climate

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.