Changes in irrigation practices may deplete aquifers faster and more severely than meteorological droughts: A numerical modeling approach

Groundwater availability worldwide is threatened by a changing climate. Aquifers in intensively irrigated systems may present peculiar vulnerability to climate change related to changes in irrigation practices triggered by potential surface water scarcity. This work aims to provide a quantitative assessment of the major drivers of aquifer depletion in agricultural areas: hydrological droughts and changes to more efficient irrigation practices as a response to reduced surface availability. Based on a site-specific conceptual model, a three-dimensional combined steady-state and transient numerical groundwater flow model was developed and calibrated using groundwater level and groundwater-river exchange flow data to reconstruct the 2015–2017 dynamics of an intensively irrigated hydrogeological system, where irrigation return flow dominates the recharge mechanism. Two hypothetical scenarios were simulated: (1) a two-year meteorological drought and (2) a change in irrigation practices from surface irrigation method to the more efficient drip irrigation technique, while maintaining all other conditions the same as in the baseline simulation. The drought scenario leads to a significant reduction of the recharge processes, resulting in a total groundwater storage loss of 2.34 × 105 m3/d over the two simulated years. However, the relative dynamics and seasonal patterns of groundwater storage, groundwater heads, lowland springs discharge, and surface water-groundwater interactions observed in the baseline simulation are preserved. In contrast, the scenario representing the reduction in irrigation return flow determines a disruption in the seasonal pattern over the two simulated years, leading to a loss in groundwater storage up to 2.77 × 105 m3/d and critical impacts on lowland springs and connected surface water bodies. The comparison of baseline conditions and the two scenarios demonstrates that surface-water irrigation return flow is critical to sustaining groundwater balance and the ecological functioning of groundwater-dependent systems in intensively cultivated areas. Therefore, the results indicate that potential policymakers’ adaptation measures to address surface water scarcity induced by climate change may have a more significant impact on groundwater resources than the direct effects of climate change itself, highlighting the crucial role of scientific evidence in informing and guiding policymakers.