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Interaction between wind speed and net radiation controls reference evapotranspiration variance in the inland river basin of Northwest China

Exploring the variance in reference evapotranspiration (ET0) and its dominant influencing factors is important for climate change, hydrological cycles and water management. Temperature (T), wind speed (U2), net radiation (Rn) and actual vapour pressure (ea) are the major climatic input variables in Penman–Monteith equation. Previous studies have successfully applied the total differential method to determine the relative contributions of changes in these variables to variation in ET0 on different timescales. However, the interaction of climatic variables has not been incorporated into this method. Taking the inland river basin of Northwest China as the study area, we extended the total differential method to decompose the ET0 variance into temporal variance and covariance of T, U2, Rn and ea on intra‐annual and annual scales during 1960–2017. The results indicated that the variance in ET0 on the intra‐annual scale was larger than that on the annual scale. Among the four single climatic variables, U2 variance made larger contributions to intra‐annual and annual ET0 variance with relative contributions of 6.8% and 1.1%, respectively. However, the interaction of climatic variables played a dominant role in the variation in ET0. Specifically, on intra‐annual scale, coupled U2 and Rn primarily controlled the ET0 variance (76.1%), followed by coupled Rn and ea (9.6%); but their positive effects were weakened by the negative effects of coupled T and ea (−2.3%). On annual scale, coupled U2 and Rn still governed the variance in ET0 (97.3%), but the effects were also weakened by other groups of interaction effects. Furthermore, ET0 was highly related to NDVI on an intra‐annual scale (R2 = 0.68, p = 0.00), indicating the strong effect of seasonal vegetation dynamics on ET0 variance. This study provides new insights to quantitatively assess the interaction effects of environmental factors on key hydrometeorological variables.