*2.2. Study Area*

The TRHR (31◦39N–37◦10N, 89◦24E–102◦27E) is located in the hinterland of the Tibetan Plateau and in southern Qinghai Province of China (Figure 2). As the source area for the Yellow, Lancang, and Yangtze rivers, the TRHR supplies approximately 40 billion m<sup>3</sup> of water downstream every year. It serves as an important source of freshwater resources in China and Asia and is often referred to as the "Chinese water tower" [26]. The TRHR spans 22 counties and covers an area of about 3.95 × 10<sup>5</sup> km2, and the elevation increases from 1987.5 m in the southeast to 6714.5 m in the northwest, with an average elevation over 4000 m. In 2010, the main land-use types in the TRHR were grassland (68.4%), desert (16.0%), wetland (9.4%), shrub (4.6%), and forest (0.3%), where alpine steppe and alpine meadow were the main types of grassland [27]. The TRHR has major extensive wetlands in China, with abundant lake, river, glacier, and mountain snow resources, and supports the largest alpine wetland ecosystem in the world. Moreover, the TRHR is an important ecological functional zone, a typical ecologically fragile area in China, and is quite sensitive to climate change.

**Figure 2.** Maps of the study area: (**a**) Tibetan Plateau in China; (**b**) Three-River Headwaters region on the Tibetan Plateau; (**c**) locations of meteorological, phenological stations, alpine steppe, and alpine meadow in and near the Three-River Headwaters region.

The TRHR experiences a typical plateau continental climate with large daily temperature differences, small annual temperature differences, intense radiation, and a large number of sunshine hours [28]. The TRHR has cool and dry winters with wet and warm summers, mainly caused by the influence of the Asian monsoon and high elevation [29].Meanwhile, the annual average precipitation in the THRH gradually increased from northwest (262.2 mm) to southeast (772.8 mm), primarily concentrated between June and September owing to the influence of the warm and humid air currents in the southern Bay of Bengal [28,30]. Furthermore, the annual average temperature, sunshine hours, and evaporation of TRHR ranging −5.6 to 7.8 ◦C, 2300 to 2900 h, and 730–1700 mm, respectively [14,26].
