Abstract
The arid and semi-arid regions of the world are facing limited freshwater resources, minimal amounts of rainfall, and increasing population pressure and water demands. These resources, often groundwater, are vulnerable to both natural variability and anthropogenic interventions. Here, we develop and apply an integrated approach using geophysical, geochemical, and remote sensing observations to quantify the recharge rates of arid region aquifers that are witnessing rapid groundwater depletion. Focusing on the Saq aquifer system in the Arabian Peninsula, our study was three-fold: (1) to examine the areal extent of aquifer recharge domains using geologic, climatic, and remote sensing data; (2) to investigate the origin of, and modern contributions to, the aquifer system by examining the isotopic composition of groundwater samples; and (3) to estimate the magnitude of modern recharge to the aquifer utilizing estimates from the Gravity Recovery and Climate Experiment (GRACE) data and a continuous rainfall-runoff model, the Soil and Water Assessment Tool (SWAT) model. Isotopic examination of the groundwater from Saq revealed that deeper aquifers (fossil water) are more depleted compared to shallow reserves (alluvial aquifer; mixed waters). Analysis of GRACE-derived terrestrial and groundwater storage estimates indicates that the aquifer system received an annual recharge rate of 5.21 ± 0.10 km3/yr (11.85 ± 0.22 mm/yr), equal to about half of the withdrawal rate from wells during the investigation period (2002–2016). Analysis of SWAT results indicates that, from 1998 to 2014, the investigated watersheds received an average annual precipitation of 19.20 km3, of which approximately 51% is partitioned as potential recharge. The temporal variations in storage, depletion and recharge are related to the changes in regional groundwater extraction, rainfall, and extent of irrigated areas.