*2.1. Study Area*

The Three-River Headwaters Region (31◦38–36◦20N, 89◦31–102◦14E) is located in southern Qinghai Province, the hinterland of the Tibetan Plateau (TP) (Figure 1). This region is the headstream of three major Asian rivers, including the Yellow, Yangtze, and Lantsang Rivers, and is known as the "Chinese water tower", supporting approximately 40% of the world's population [34]. The TRHR covers an area of 350,000 km2, which supplies 49% of the total water of the Yellow River, 25% of the total water of the Yangtze River, and 15% of the total water of the Lantsang River. The TRHR is a central part of the highest and largest plateau in the world, and constitutes mountainous landforms with an average elevation of more than 4000 m. Due to its unique location and complex topography, the TRHR is characterized by a typical plateau climate with a low air temperature, high daily temperature range, and strong solar radiation. The climate of the TRHR is wet and moist in summers, and cool and dry in winters, with distinct wet and dry seasons. The average mean temperature ranges from −5.6 to 3.8 ◦C and the annual rainfall ranges from 262.2 to 772.8 mm with a notably decreasing trend from southeast to northwest [35]. The TRHR has the richest biodiversity, and contains the largest Chinese alpine wetlands ecosystem. The main ecosystem type of the TRHR is grassland including alpine meadow and alpine steppe, accounting for approximately 76% and 23% of the grasslands, respectively [36].

**Figure 1.** Maps showing the location of the study sites. ( **A**) The distribution of land cover types. (**B**) The Tibetan Plateau (TP) and Three-River Headwaters Region (TRHR). ( **C**) The digital elevation model (DEM) data of the TRHR with a spatial resolution of 250 m. The study area consists of 17 counties and cities delineated by the Ecological Protection and Restoration Program, including Zeku, Tongde, Henan, Xinghai, Maqin, Gande, Jiuzhi, Dari, Banma, Maduo Chengduo, Yushu, Nangqian, Qumalai, Zaduo, Zhiduo, and Tanggulashan.

## *2.2. Climate Data*

In this study, the China Meteorological Forcing Dataset (CMFD) was developed by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences [37,38]. This product covers the period of 1982–2015, with a spatial and temporal resolution of 0.1◦ every 3 h. The instantaneous 2 m air temperature, surface pressure, specific humidity, and wind speed were produced by merging 740 meteorological observations and environmental data sources. The precipitation dataset was developed by merging three precipitation datasets including the in situ observations dataset, the Tropical Rainfall Measuring Mission (TRMM) 3B42 precipitation products, and the Asian Precipitation—Highly Resolved Observational Data Integration Towards Evaluation of the Water Resources project. The CMFD downward shortwave radiation dataset was constructed by the Global Energy and Global Energy and Water cycle Experiment-Surface Radiation Budget (GEWEX-SRB) radiation data and meteorological station measurements [39].

To obtain the surface net radiation (Rn), we calculated Rn using the method proposed by Wang et al. [40]. This method accurately simulates the Rn value combined shortwave radiation measurement with meteorological observations, which is suitable for various land cover types with a correlation coefficient of 0.99. We also used the model-derived soil moisture (SM) dataset provided by the National Centers for Environmental Prediction (NCEP), which contains monthly SM from 1982 to 2015, with a spatial resolution of 0.5◦ × 0.5◦. The specific dataset sources and detailed information on the datasets are provided in Table 1.


**Table 1.** Datasets for the meteorological reanalysis data and satellite data used in this study. NDVI, normalized difference vegetation index; ET, evapotranspiration.
