Influence of intensive agriculture and geological heterogeneity on the recharge of an arid aquifer system (Saq-Ram, Arabian Peninsula)

This study assesses the detailed water budget of the Saq-Ram Aquifer System (520 000 km²) over the 2002-2019 period using satellite-gravity data from the Gravity Recovery And Climate Experiment (GRACE). The three existing GRACE solutions (JPL, CSR, GSFC) were tested for their local compatibility to compute groundwater storage variations in combination with soil moisture datasets (VIC, CLSM, NOAH) available from the Global Land Data Assimilation System (GLDAS) land surface models. Accounting for groundwater pumping (15.7 ± 1.1 mm yr-1), artificial recharge (2.2 ±

0.8 mm yr-1) and natural discharge (0.3 ± 0.06 mm yr-1) uniformly distributed over the Saq-Ram domain, the GRACE-derived water mass balance calculation yields a long-term estimate of the domain-averaged natural recharge of 2.4 ± 1.4 mm yr-1, corresponding to 4.4 ± 2.6% of the annual average rainfall.

Beyond the global approach proposed here, spatial heterogeneities regarding the groundwater recharge were identified. The first source of heterogeneity is of anthropogenic origin. Within agricultural plots, irrigation excess is great enough to artificially recharge the aquifer (i.e. 167 ± 83 mm yr-1 distributed over irrigated areas). However, on the outskirts of these crop areas subjected only to the natural recharge but still influenced by pumping drawdown, there is a risk of relative disconnection from the infiltration front with the declining water table (i.e.  the unsaturated zone thickens faster than percolation flows through it), making effective recharge locally zero. The second source of recharge heterogeneity identified here is natural: Volcanic lava deposits (called Harrats on the Arabian Peninsula) which cover 8% of the Saq-Ram Aquifer domain but contribute to more than 50% of the total natural recharge. Hence, in addition to this application on the Arabian Peninsula, this study strongly indicates a major control of geological context on arid aquifer recharge which has been poorly discussed hitherto.