**3. Results**

#### *3.1. Spatial and Seasonal Patterns of Terrestrial Biophysical Variables in the Three-River Headwaters Region*

### 3.1.1. Climate Variables

Figure 2 shows the spatial distribution of the climate variables (Ta, P, RH, and Rs) over the TRHR at annual and seasonal scales during 1982–2015. Influenced by the typical plateau continental climate, the climate variables have distinctly di fferent spatial patterns. On an annual basis, the annual mean Ta (Figure 2a) of the TRHR ranged from −12 to 6 ◦C, with an average of −4.2 ◦C. The multiyear average P (Figure 2b) varied from 162 to 781 mm, with an average of 424 mm. Both Ta and P presented obvious decreasing trends from the southeast to the northwest, which corresponded to the water and energy gradients of the TRHR. The 400 mm contour lines of annual precipitation roughly divide the TRHR into semi-arid and semi-humid climates from northwest to southeast. As a major part of the Tibetan Plateau, the climate of the TRHR is also influenced by atmospheric circulation and topographical features [50]. Figure 2c shows the spatial distribution of annual mean RH over the TRHR, with an average value of 52.3%. The decreasing trend of RH is noticeable from southeast to northwest, which is consistent with the distribution pattern of cloud cover [51]. By contrast, the annual mean Rs spatially decreased from west to east, ranging from 196 to 232 <sup>W</sup>/m2. There were abundant solar energy resources in the TRHR due to the high altitude, thin atmosphere, and few anthropogenic activities [52].

On a seasonal basis, the climate of the TRHR is characterized by cold and dry winters, and cool and rainy summers. The spatial distributions of seasonal Ta and P were similar to the multiyear patterns of Ta and P as averaged during 1982–2015. The mean Ta was below −5 ◦C, and the P was less than 10 mm/month in winter (DJF, December, January, and February), whereas in summer (JJA, June, July, and August), the average P accounted for more than 80% of the total annual P, and the average Ta was about 5 ◦C. Moreover, we also found an obvious and clear distinction between dry and wet season over the TRHR. The southeastern area of TRHR remained the most humid region in other seasons with the exception of winter. During the Asian summer monsoon period, the mean surface RH of JJA was relatively higher than that of other seasons. The seasonal mean Rs of MAM (March, April, and May) and JJA were approximately 230–290 <sup>W</sup>/m2, which were much higher than the values of SON (September, October, and November) and DJF.

**Figure 2.** Multiyear average and seasonal spatial patterns of the climate variables: (**a**) temperature, (**b**) precipitation, (**c**) relative humidity, (**d**) downward shortwave radiation. (**1**) MAM (March, April, and May); (**2**) JJA (June, July, and August); (**3**) SON (September, October, and November); (**4**) DJF (December, January, and February). Precipitation is in units of mm/month. Radiation is in units of W/m2.

#### 3.1.2. Normalized Di fference Vegetation Index

Figure 3a illustrates the annual mean NDVI, which presents an increasing trend from northwest to southeast over the TRHR during the period 1982–2015. Higher NDVI values were mainly distributed in the southeastern part of the TRHR, where the main land use type is forest and temperate grassland. Meanwhile, this region has su fficient precipitation and warmer temperatures that are suitable for vegetation growth. By contrast, the northwestern part of the TRHR has a relatively cold and dry climate, resulting in a lower NDVI value. Our findings are consistent with those of Zhong et al. [53], who also found that the spatial distribution of the NDVI was influenced by the Asian monsoon over the Tibetan Plateau.

As shown in Figure 3, seasonal NDVI has a decreasing trend from southeast to northwest across all four seasons. However, there were distinctive di fferences in the seasonal average of the NDVI value. During winter (DJF), the NDVI value was below 0.24 in the majority of the region under the dormancy condition of vegetation and lower air temperature. The NDVI reached a maximum value of 0.8, accompanied by increasing precipitation and rising temperatures in summer (JJA). When the rainy season had passed, the NDVI value began to decrease in autumn (SON) and winter (DJF), with an average of 0.15 and 0.09, respectively.

**Figure 3.** (**a**) Spatial patterns of the multiyear average NDVI in the TRHR. (**a1**) MAM (March, April, and May); (**a2**) JJA (June, July, and August); (**a3**) SON (September, October, and November); (**a4**) DJF (December, January, and February).
