Annual crops buffer, irrigation infrastructure does not: crop composition and WUE drought sensitivity in Chile

Francisco Zambrano , Francisco Fernández, María Molinos-Senante

Agricultural water use efficiency (WUE) is widely expected to improve under technological intensification, yet its response to prolonged aridification at regional scales is poorly understood. We analyzed spatiotemporal patterns of WUE across 127 Chilean agricultural sub-watersheds from 2001 to 2020, a period spanning an unprecedented megadrought. WUE responses were spatially heterogeneous: arid and semi-arid sub-watersheds showed predominantly negative ΔWUE, while irrigated Mediterranean sub-watersheds showed positive ΔWUE; negative long-term trends and structural breaks concentrated in the 30-35°S band. Precipitation deficit indices dominated cross-sectional WUE sensitivity; at the 36-month accumulation scale, SPEI and SPI showed practically equivalent explanatory power (TOST $p$ < 0.001, 90% CI [−0.004, +0.003]).

 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 robustly and negatively associated with drought sensitivity across all latitude specifications (SEM $\beta$ = −0.322 to −0.485, $p$ < 0.01); whether this reflects structural buffering or drought-driven reduction of annual cultivation cannot be established from the observational data alone. Irrigation infrastructure prevalence is positively associated with WUE-drought coupling in the full sample (SEM $\beta$ = +0.422, $p$ < 0.001), but this association attenuates to non-significance when latitude is explicitly controlled ($\beta$ = +0.109, $p$ = 0.327) and cannot be statistically isolated from the latitudinal aridity gradient; within the warm-summer-Mediterranean zone the association is marginal ($\beta$ = +0.264, $p$ = 0.053). Both patterns are associative; causal inference is not possible given the observational design and spatial confounding.

 These patterns are consistent with crop-portfolio flexibility reducing structural drought exposure under prolonged aridification, and with WUE-based efficiency metrics being insufficient indicators of agricultural resilience where irrigation supply is hydrologically coupled to the same signals that suppress crop productivity. These findings have implications for water governance and agricultural planning in snowmelt-dependent dryland regions globally.