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A warming climate is expected to perturb the hydrological cycle, resulting in changes to both the frequency and duration of drought, especially in arid and semiarid grassland. Although considerable attention has been paid to the responses of key water fluxes to severe drought, few have explored the effects of prolonged drought on evapotranspiration (ET), its partitioning into vegetation transpiration (T) and soil evaporation (E), and its relationship with vegetation productivity. In this study, we used a combination of the eddy covariance (EC) flux technique and satellite remote sensing products to evaluate the effects of prolonged drought on ET components based on the concept of underlying water use efficiency (uWUE). The results showed T accounted for about 51% (standard deviation +/- 0.03%) of ET during prolonged drought lower than those in normal and/or wet years (range, 59-62%). We detected strong positive relationships between the T/ET ratios and aboveground biomass (AGB), leaf area index (LAI), and precipitation (R-2 of 0.80, 0.58, and 0.49, respectively). The specific responses of ecosystem functioning to prolonged drought indicated that grassland ecosystem was able to resist the drought disturbance and retain vegetation growth to a certain extent unless extreme drought hit. The results implied that more intense and prolonged droughts will result in ecological degradation and substantial changes in ecosystem functioning. And these results improve our understanding of how the climate change will affect the function and structure of grassland ecosystem.