Unraveling Local and Upwind Controls on Terrestrial ET-Runoff Partitioning in China: Grid-Scale Analysis and Explanatory Diagnostics

Abstract. How water is divided between evapotranspiration and runoff controls regional water availability, but this partition is usually attributed mainly to local climate and land-surface conditions. Because part of precipitation is supplied by evaporation from upwind land areas, downwind partitioning may also reflect the ecohydrological state of terrestrial moisture-source regions. Here we analyze monthly evapotranspiration-runoff partitioning across mainland China from 2008 to 2017 using a geographically weighted random forest that separates local hydroclimatic and land-surface covariates from upwind land-surface covariates aggregated along atmospheric moisture trajectories. The evapotranspiration share varies non-monotonically with aridity, remaining near 0.80–0.82 from hyper-arid to dry sub-humid regions before declining to 0.62 in humid regions. Local controls dominate the explainable variability nationally, with mean cross-validated R2 increasing only from 0.468 to 0.471 when upwind covariates are added. However, the terrestrial upwind signal is geographically and seasonally concentrated, being most evident in dry sub-humid transition zones and in autumn. Upwind vegetation density, soil wetness, and precipitation show inverted-U relationships with the downwind green-water share, whereas upwind evaporative demand shows a monotone-negative relationship. These patterns persist across alternative evapotranspiration and runoff product combinations, but their interpretation remains limited by gridded product uncertainty, the terrestrial-only definition of upwind covariates, and the observational attribution design. The results suggest that assessments of blue-green water partitioning may benefit from considering upwind land-surface conditions alongside local hydroclimate, particularly in dry sub-humid transition zones.