Irrigation management and groundwater recharge in Mediterranean intermontane basins: a multi-method evaluation of...

Groundwater recharge in irrigated agricultural landscapes and surrounding watersheds is critical for sustainable water management and environmental flows. In irrigated Mediterranean regions, quantifying this process is complicated by substantial interannual and spatial variability in precipitation, irrigation practices, and evapotranspiration (ET), which introduces significant uncertainty. Here, we assess field-scale spatiotemporal variability in potential and actual contributions to aquifer replenishment across Mediterranean intermontane irrigated basins. Potential estimates were derived from a remote sensing ET water-balance residual (RSET-WB) and soil water balance modeling (SWBM), whereas the actual component was inferred from groundwater-level fluctuations using the water-table fluctuation method (WTFM). Results reveal strong spatial and crop-specific contrasts among basins and fields. In SWBM, irrigation-season variability was primarily associated with soil available water storage (AWS) and crop type, whereas non-irrigation season patterns were explained largely by interbasin differences in wet-season precipitation. Crop-specific patterns differed between methods, with alfalfa dominating RSET-WB residual estimates and grain and pasture lands showing greater SWBM-derived dry-season deep percolation below the root zone. Within SWBM, low-AWS fields also showed enhanced growing-season drainage. WTFM estimates indicated relatively balanced water table recharge between wet and dry seasons across most basins, contrasting with the wet-season dominance shown by RSET-WB and SWBM. Long-term averages (2008-2023) from RSET-WB and SWBM suggest that the dry season accounted for about 29-34% of annual potential recharge, while the wet-season fraction ranged from 54% to 78% of precipitation. Collectively, these findings underscore that irrigation return flow and late-season precipitation are critical to sustaining groundwater potential recharge in Mediterranean agricultural lands, supporting managed aquifer recharge strategies such as early- or off-season irrigation in low-AWS pasture grasslands.