Characteristics of surface evapotranspiration and its response to climate and land use and land cover in the Huai River Basin of eastern China.

The Huai River Basin (HRB) has experienced significant climate and land use and land cover changes (LUCC) which have impacted the water cycle in recent times. However, little is understood about the impact of climate change and LUCC affecting evapotranspiration (ET). Thus, we investigate how surface ET response to climate change and LUCC in the HRB for the period from 2001 to 2014. ET and land cover types products (i.e., MOD16 and MCD12Q1, respectively) from MODerate-resolution Imaging Spectroradiometer (MODIS) were employed in this research. Water balance method and D20 pan evaporation data (E) as well as eddy covariance (EC) measurements were used to validate the MOD16 product, and the Theil-Sen's slope estimator and Mann-Kendall (M-K) test were adopted to estimate the magnitude and significance of ET trends. Moreover, daily meteorological data of 137 weather stations from 2001 to 2014 were also employed to explore the correlation mechanism between ET and meteorological factors. The results showed that the accuracy of MOD16 product data were convincible and could be used to estimate ET in the HRB. The higher values of ET are mainly distributed in the south and lower values in the north. ET decreased significantly in all seasons except in spring, especially in winter. The results also depicted that the land-use type in the HRB is mainly croplands, followed by forests and grasslands. Croplands area showed a decreasing trend at a rate of -176.2 km·a, grasslands area presents a w-type fluctuation decreasing trend with a rate of -35.8 km·a, urban/built-up area increased at a rate of 138.3 km·a, water bodies area decreased gradually at a rate of -1.38 km·a, wetlands area increased significantly at a rate of 43.6 km·a, and barren area decreased gradually at a rate of -9.5 km·a. The average annual ET is closely related to land-use types and shows a significant downward trend in general. The corresponding ET magnitude is exhibited as follows: forests>grasslands>croplands>wetlands>water bodies>urban/build-up lands>barren. Results of the study also suggest water conditions (precipitation (Pre) and relative humidity (RH) decrease) are major controlling factors in the decline of ET. Overall, the LUCC has a smaller influence on ET than climate change in the HRB. This research will provide a better insight into climate change and LUCC impacts on water resources.