The Irrigation Efficiency Paradox in Transboundary Aquifers: Implications for Groundwater Recharge and Sustainability

The 1944 Water Treaty recognizes irrigation return flows; however, neither the Treaty nor subsequent Minutes (323/2017, 330/2024) regulate irrigation efficiency, creating a critical institutional gap under increasing water scarcity. This review provides a comparative institutional and policy evidence across Mexico–United States transboundary aquifer systems, anchored by the Valle de Juárez/Hueco Bolson system as a representative case study. It identifies a quantifiable irrigation efficiency paradox, whereby increases in field-scale efficiency may reduce basin-scale recharge by decreasing recoverable return flows. In the Valle de Juárez system, irrigation return flows account for 51.1% of total recharge (64.4 mm3/year), while natural recharge contributes only 2.6%, resulting in a current groundwater deficit of −73.3 mm3/year. Scenario comparisons reported in the literature indicate that progressive irrigation modernization may further reduce recharge and increase the deficit by up to 65.9%, despite reductions in applied water. The reviewed evidence suggests that in highly arid transboundary systems strongly dependent on anthropogenic recharge, efficiency-focused policies that do not distinguish between consumptive use and recoverable return flows may unintentionally intensify groundwater depletion. These findings highlight the need for binational management frameworks that explicitly incorporate return flow dynamics, hydrogeological connectivity, uncertainty analysis, and basin-scale water balance for future research.