Study region: This research focuses on the Guadalquivir River Basin in southern Spain, a region characterized by high agricultural water demand and increasing vulnerability to drought. The basin serves as a critical case study for semi-arid Mediterranean environments. Study focus: The study evaluates the impact of three distinct water allocation rules: proportional allocation (uniform allotment reductions), proportional economic losses (equalized relative reduction in gross margins), and optimal allocation (prioritization of water-productive crops). Using a hydro-economic model, the research simulates various scarcity scenarios (10% to 70% reductions) and examines the mitigating role of groundwater reserves during periods of drought. New hydrological insights for the region: The findings reveal that while proportional allocation is often perceived as equitable, it results in the most severe and uneven economic losses (16.1% at a 50% water deficit). In contrast, an optimal allocation rule minimizes aggregate losses to 12.0%. Access to groundwater significantly cushions the impact, reducing losses to approximately 5.3%- 7.0%. The study highlights that traditional water-sharing rules are economically inefficient and socially inequitable, advocating for a transition toward allocation models based on economic efficiency and territorial equity to ensure water sustainability in an uncertain climate. Overall, our results provide a policy-relevant quantification of the efficiency-equity trade-offs of drought rationing and show that strategic conjunctive use can substantially increase irrigation resilience in semi-arid, agriculture-dominated basins.