Irrigation infrastructure prevalence is associated with amplified agricultural water use efficiency drought sensitivity: evidence from Chile's megadrought

Francisco Zambrano , Francisco Fernández, María Molinos-Senante

Agricultural water use efficiency (WUE) is widely expected to improve with advances in irrigation technology, yet its response to prolonged drought remains poorly understood at regional scales. We analyzed spatiotemporal patterns of WUE across 127 Chilean agricultural sub-watersheds from 2001 to 2020, a period marked by an unprecedented megadrought. WUE declined broadly across arid and semi-arid regions, driven primarily by precipitation deficits and intensified atmospheric evaporative demand, with the largest declines concentrated in the Mediterranean-climate zone where irrigated agriculture is most prevalent.

 Crop system composition is associated with WUE drought sensitivity through two contrasting patterns, each constrained by a different form of confounding. Annual-crop prevalence is negatively associated with drought sensitivity and is robust to latitude control; the causal direction, whether annual-crop presence structurally buffers drought sensitivity, or whether drought stress itself reduces annual cultivation in sensitive watersheds, cannot be established from cross-sectional data alone. Irrigation infrastructure prevalence is positively associated with stronger WUE-drought coupling in the full sample (SEM $\beta$ = +0.422, $\rho$ < 0.001), consistent with demand-hardening theory, but attenuates substantially when latitude is explicitly controlled ($\beta$ = +0.109, $\rho$ = 0.327) and cannot be statistically isolated from the latitudinal aridity gradient. Both patterns are hypotheses consistent with the data rather than definitively isolated causal effects.

 These findings suggest that hydroclimatic pressures associated with sustained aridification can outweigh gains from technological adaptation, and that agricultural systems with limited crop-type flexibility may remain structurally exposed to drought-coupled supply failure, with implications for water governance in dryland regions globally.