3.1.3. Evapotranspiration

Figure 4a shows the spatial distribution of the multiyear (1982–2015) average ET over the TRHR, and the annual mean ET was approximately 230.23 mm/year. As shown in Figure 4a, the multiyear average ET decreased from southeast to northwest, which was similar to the spatial patterns of Ta and P. Higher ET mainly occurred in the moister and warmer regions, including the eastern and southern parts of the TRHR, whereas the northwestern part of the TRHR with less P and lower Ta had the lowest ET value. Furthermore, the spatial pattern of ET was also affected by land management, such as agricultural irrigation, that caused a positive trend of ET in the cropland areas.

**Figure 4.** (**a**) Spatial patterns of the multiyear ET of the TRHR. Multiyear seasonal patterns of ET: (**a1**) MAM (March, April, and May); (**a2**) JJA (June, July, and August); (**a3**) SON (September, October, and November); (**a4**) DJF (December, January, and February). ET is in units of mm/month.

As shown in Figure 4, the multiyear average seasonal patterns of ET exhibited obvious seasonality with reasonable seasonal cycles (higher ET in the summer wet season and lower ET in the winter dry season). Distinct fluctuations of ET throughout the four seasons corresponded to the plateau mountain climate system. In spring (MAM) and autumn (SON), the ET was less than 26 mm/month due to the lack of available energy and temperature. The seasonal ET reached the largest value (26–66 mm/month) in summer (JJA), accompanied by the maximum Ta and P in the whole year. By contrast, ET dropped to its lowest value in winter (DJF), which is when vegetation turns to dormancy, and the temperature declines.

#### *3.2. Interannual and Seasonal Variation of Terrestrial Biophysical Variables in the Three-River Headwaters Region*
