The Effects of Climate Change and Greening of Vegetation on Spatiotemporal Variation of Evapotranspiration in the Haihe River Basin, China

Highly accurate evapotranspiration (ET) estimation and understanding the impacts of climatic and land use change on ET are essential for water resources management in the Haihe River Basin (HRB). This study estimated spatial and temporal changes of ET and its drivers over the period 2000-2020, using the Priestley-Taylor Jet Propulsion Laboratory (PT-JPL) model. Validation performed with the observations of 11 eddy covariance sites showed that the PT-JPL model can simulate ET with high accuracy (<i>R</i> ² = 0.64, RMSE = 1.32 mm/day, NSE = 0.57). During the 21-year study period, the mean annual ET in HRB was 583 mm/year and showed an insignificant increasing trend (0.45 mm/year). Canopy transpiration (ET_c, 2.96 mm/year) and interception evaporation (ET_i, 0.74 mm/year) significantly increased whereas soil evaporation (ET_s, -3.25 mm/year) significantly decreased. The mean annual net radiation (Rn), relative humidity (Rh), and wind speed (Ws) showed insignificant decreasing trends. In contrast, mean annual air temperature (Tm), vapor pressure deficit (VPD), and precipitation showed insignificant increasing trends. The significantly increased leaf area index (LAI) demonstrated that vegetation in the HRB is greening. We explored the relationship between ET and its components to climate and vegetation parameters. The results showed that net radiation was the most important parameter for ET variations. Vegetation and temperature had large impacts on ET_c. Vegetation greening in HRB dominates the increasing trend in ET_c. Net radiation and relative humidity showed an important role in changes in ET_s. Temperature and vegetation were key impact parameters for ET_i. The increase in ET_i is mainly located in the region of forests, which is due to the forest protection and afforestation projects in HRB. This study highlights the importance of isolating the contributions of vegetation and climate changes to the changes in ET and its components, which is useful for water resources management in HRB and other regions of the world.