When Irrigation Cannot Keep Pace: Aridification, Crop Composition, and the Spatial Concentration of Agricultural Water Use Efficiency Decline in Central Chile

Francisco Zambrano , Francisco Fernández, María Molinos-Senante

Water scarcity under sustained aridification is among the most consequential threats to agriculture in Mediterranean-climate regions. Central Chile's ongoing megadrought (~2010-present), characterized by persistent precipitation deficits and intensified atmospheric evaporative demand-provides a natural experiment for examining how agricultural water use efficiency (WUE) responds to prolonged climatic stress. Using remotely sensed estimates of net primary production (NPP) and evapotranspiration (ET) across the 76 agricultural watersheds (2001-2020), we characterize WUE dynamics along multiple dimensions of aridification and assess the role of crop composition in modulating watershed-scale sensitivity.

Contrary to expectations that technological adaptation might buffer efficiency outcomes, we find a persistent, spatially coherent decline in agricultural WUE across much of central Chile, with structural break points clustering near 2010 in coincidence with megadrought onset. Hydroclimatic pressures-particularly reduced water availability and elevated atmospheric demand-appear to have outweighed any efficiency gains from improved irrigation practices, suggesting that under sustained drought, water and energy limitations constrain agricultural productivity at regional scales beyond the reach of field-level adaptation. Precipitation deficit was the dominant aridification signal explaining WUE variability, with atmospheric demand contributing modest independent explanatory power. Crop composition explained more than half of the spatial variance in drought sensitivity, with cereal-dominated systems showing lower WUE-SPEI sensitivity relative to forage systems, while irrigated area share was positively associated with drought sensitivity.

These results challenge the assumption that irrigation efficiency improvements can adequately buffer agriculture against intensifying hydroclimatic stress, and underscore the need for water governance frameworks that operate at the landscape and regional scale.