Enhanced Ecosystem Water‐Use Efficiency Under the More Diffuse Radiation Conditions

Abstract Diffuse and direct radiation can affect ecosystem gross primary production (GPP) and evapotranspiration (ET) differently, leading to their confounding impacts on ecosystem water‐use efficiency (WUE). Currently, how these two types of radiation affect ecosystem WUE through their different impacts on GPP and ET are still not fully understood. Here, we explored how diffuse and direct radiation affects ecosystem WUE at 133 FLUXENT sites taking advantage of a newly derived diffuse radiation fraction (K d ) data set. We found that increases in K d consistently improved WUE for all vegetation types, but the process varied with K d levels: when K d ≤ 0.45, the improved WUE attributed to the greater increase in GPP than ET; when K d in 0.45–0.51, the increase in GPP and decrease in ET jointly improved WUE; when K d ≥ 0.51, the deeper decline in ET than GPP led to a continuously increasing WUE. Furthermore, by dividing radiation during growing seasons into diffuse (K d ≥ 0.75) and direct (K d ≤ 0.25) dominated conditions, the average GPP, ET, and WUE under diffuse conditions were respectively 1.18 ± 0.16, 1.10 ± 0.19, and 1.10 ± 0.21 times higher ( P < 0.001) than those under direct conditions for 133 sites. By using a random forest method, we further found that diffuse radiation dominated the variations of GPP, while direct radiation was the most important factor for ET. We suggest that future research on the ecosystem carbon‐water relationship should distinguish the different roles of diffuse and direct radiation on carbon and water fluxes, especially under high scattering conditions with severe aerosol pollution and cloudy skies.