The Roles of Winter Versus Summer Precipitation in Supplying Evapotranspiration Across US Ecoregions: A Cross‐Catchment End‐Member‐Splitting Analysis

Abstract Understanding how changing seasonal precipitation will affect ecosystems and water resources can benefit from understanding how precipitation from different seasons contributes to runoff versus evapotranspiration (ET). We use stable‐isotope data from 23 National Ecological Observatory Network watersheds to quantify the fractions of winter and summer precipitation that supply ET, and the fractions of ET supplied by summer versus winter precipitation. Across 20 watersheds, 34%–101% of summer precipitation supplied ET, with 8%–105% of ET supplied by summer precipitation; these end‐member‐splitting solutions were poorly constrained in the other three watersheds. These precipitation partitioning fractions were significantly correlated with many topographic, climatic, and vegetation metrics. This first empirical study of seasonal precipitation partitioning fractions across diverse ecoregions demonstrates that they can be well‐constrained in many locations using existing public data sets, and that partitioning‐fraction variations are largely explained by climate variations.