The role of climate change and vegetation greening on evapotranspiration variation in the Yellow River Basin, China

Actual evapotranspiration (ET) plays a key role in the water cycle and energy balance, and it is also sensitive to climate and land use changes due to its explicit link with multiple land surface processes. Understanding spatiotemporal changes of ET and its drivers is of vital importance for water resources management. In this study, we estimated spatiotemporal changes in ET and quantified its drivers in the Yellow River Basin (YRB)-the second largest river basin of China-during the period of 1982-2016 using the modified Penman-MonteithLeuning (PML) model by simply incorporating land use dynamics. The PML model was validated against onsite flux observations, global evaporation modeling results, and water balance-based observations. The validation using annual and monthly data from above-mentioned data showed that the modified PML model performs well for both the entire YRB and sub-watershed scale. During the 35-year study period, the YRB experienced significant climate warming and vegetation greening, reflected by significant increases in air temperature and leaf area index, causing a significant increase of the basin-averaged ET (1.36 mm/yr, P<0.01). We conducted the factorial experiments to attribute ET variations to climate and vegetation greening, and the results show that climate warming, vegetation greening, and increased vapor pressure deficit were the major contributors to the positive changes in ET, accounting for 45.6% (0.62 mm/yr), 31.6% (0.43 mm/yr), and 19.9% (0.27 mm/yr). Variations in solar radiation and wind speed played minor positive and negative roles in annual ET variations. Moreover, vegetation greening significantly contributed (92.3%) to the increase in the ratio of transpiration to ET due to the increased available energy absorbed by canopy. This study provides a comprehensive assessment of the effects of climate and vegetation on ET, and can help in formulating appropriate policies for water resource management in the context of climate warming and revegetation programs.