Trends in evapotranspiration and their responses to climate change and vegetation greening over the upper reaches of the Yellow River Basin

This study comprehensively analyzed long-term (1960–2014) variations of potential and actual evapotranspiration (ET) and quantified their inter-annual and seasonal sensitivity and attribution to climate and vegetation growth changes in the upper reaches of the Yellow River Basin (UYRB). Satellite-derived vegetation greenness and in situ measured hydrometeorological datasets were used. Average annual reference evapotranspiration (ET₀) showed significant decreasing trend (p < 0.05) by 0.32 mm year⁻¹ over the past 55 years. Results obtained from recovered stationary series and sensitivity analyses showed that reduction of wind speed was the major driving force for decreasing trends in annual and seasonal ET₀. Water budget-derived actual evapotranspiration (ET_a) increased dramatically during the study period, whereas potential evapotranspiration (ET_p) calculated from Penman equation and pan evaporation (ET_pan) decreased significantly, suggesting the existence of evaporation paradox over the UYRB. ET_a and ET_p or ET_pan exhibited complementary behavior, while the complementary relationship was asymmetric. During the period 1982–2014, vegetation greening occurred in most areas (65.6%) of the UYRB and was mediated by elevation. Interestingly, non-uniform climate warming imposed opposite impacts on vegetation growth, namely inter-annual and seasonal NDVI was stimulated by daytime temperature but suppressed by nighttime temperature. Furthermore, the positive impacts imposed by daytime temperature on NDVI was much larger in spring than in autumn and summer. Large-scale coherent vegetation greening would be the primary factor affecting inter-annual ET_a in comparison with climate factors. These results help in deep understanding the hydrological response to climate change in cold mountain regions.