Attributing the changes of grass growth, water consumed and water use efficiency over the Tibetan Plateau

•Elevated CO2 and warming dominated the uptrend of GPP in most parts of grassland.•The reduction of grazing intensity enhanced ET in the east.•The positive population growth rate weakened GPP in the east. Degradation of grassland over the Tibetan Plateau (TP) has become increasingly intense, producing a wide impact on the local and downstream environment and society. There is a pressing demand to explore contributions of climate change or human activity. Based on the simulations at 1-km spatial resolution during 2001–2017 from the VIP distributed eco-hydrological model, the trends of grass growth (expressed as gross primary productivity (GPP)), water consumption (expressed as evapotranspiration (ET)), and water use efficiency (WUE) were studied to identify the drivers by using the ridge regressions. The results show that 18.75% and 12.43% of grassland showed upward and downward ET trends, respectively. Elevated atmospheric CO2 concentration (eCO2), increasing precipitation (PRE), leaf area index (LAI), and wind speed (U) collectively dominated the uptrend of ET, and their contributions were 62%, 17%, 9%, and 9%, respectively. eCO2 and the diminished PRE and LAI were the principal causes of the decrease of ET. Their contributions were −42%, −9% and −7%, respectively. 81.13% of the grassland showed an uptrend in GPP, while only 0.04% showed downward tendencies. Rising air temperature (Ta) and eCO2 have jointly promoted GPP, with their contributions being 36% and 58%, respectively. The water vapor pressure (EA), U, and PRE reduction and warmer Ta caused a decrease in GPP, and their contributions were −25%, −23%, −9%, and −25%, respectively. 36.60% of the grassland showed a positive WUE trend, while only 3.98% showed a downtrend. The contribution of human activities is relatively small compared to that of climate change. Clarifying the contributions to ET, GPP, and WUE in grassland will support the policy-making for ecological management and environmental protection over the plateau.