**4. Results and Analysis**

#### *4.1. Isotopic Composition of Different Water Bodies*

The differences in the Local Meteoric Water Line (LMWL) are explained by the differences in topography, underlying surface, climate, and other natural environmental factors. From sampling points A to D, both the temperature and the evaporation decrease because of the increasing elevation. The slope of the LMWL gradually increases, and the extent of soil water and plant water deviating from the LMWL also increases. These factors indicate that as the elevation increases, the evapotranspiration of soil and vegetation gradually decreases.

In sampling point D (River source), the δ2H and δ18O in different water bodies are similar. The δ2H and δ18O of soil moisture, vegetation moisture, and surface runoff of each sampling site fall to the lower right of the LMWL, indicating that different water bodies are all recharged by precipitation in the source areas, which is significantly different from sampling point A, B, and C.

The characteristics of δ2H and δ18O are similar in the different water bodies in the mountain area in sampling points A, B, and C. The δ2H and δ18O of the surface water samples plot near or above the LMWL (Figure 2), and the soil and vegetation water plot to the lower right of the LMWL (Figure 2). The distribution of samples indicates that the land surface water in the mountain area is mainly recharged by precipitation, while soil water and plant water experience different degrees of evaporation during water body migration and transformation.

**Figure 2.** The relationship between δ2H and δ18O in different water bodies in the Xiying River. Basin.

#### *4.2. The Path of Moisture Transport*

In spring, there are two main air mass movement paths moving from the western section of the study area: (1) The air mass originating from Central Asia moves along the edge of the Qinghai-Tibetan Plateau after entering the Tarim Basin; (2) The air mass originating from the West Siberian Plain, which travels the western arid region of China and then arrives at the XYR basin. In summer, the air mass mainly comes from the east of the study area, and the air mass moves from the east of the study area, similar to the pathway in spring. However, as air mass from the west of the study area decreased in occurrence, the air mass from the southeast increased and the air mass from Central Asia affects the study area along the northwest edge of the Qinghai-Tibetan plateau. In autumn, there are two main air mass movement paths in the west of the study area: (1) Air mass originating in Central Asia and the air mass originating in the Xinjiang move along the edge of the Qinghai-Tibetan Plateau; (2) The other is the air mass arises in Central Asia, crosses the Kunlun Mountains and the Qinghai-Tibetan Plateau, moves through the Qaidam basin and then reaches the study area. In winter, the air mass in the study area is controlled by the westerly wind, while the east wind of the study area has little effect on the region (Figure 3).

**Figure 3.** Movement path and cluster of the Xiying River basin sampling station (A, B, C, D from April 2016 to October 2018).

#### *4.3. Spatial and Temporal Differences of Recycled Moisture*

In spring (Figure 4a), the calculated contribution of recycled moisture in different sampling points was 9.9% and 16.5%, higher than that in other seasons. The contribution was largest in sampling point B (Arbor belt), and lowest in sampling point D (river source area). The melting of snow leads to higher soil moisture content in spring, which leads to higher evaporation of the soil. Plants at low elevations region begin to grow first in spring, which increases the vegetation evapotranspiration in the low elevation region. The portion of recycled moisture decreased with elevation increased (Table 2).

In summer (Figure 4b), the contribution of recycled moisture for different sampling points varied between 3.52% and 12.87%. The contribution was highest for sampling point D (River source) and the lowest for sampling point B (Arbor belt). At the sampling point D (River source), where the plant begins to grow first in July, the contribution of local transpiration *fTr* was 4.86%, and the contribution of evaporation *fEv* was 7.99%. The soil moisture content is high in the river source area, and the frozen soil has thawed in summer, so that the evapotranspiration increases rapidly, which leads to a larger fraction of recycled moisture in precipitation in the river source area. Since there is less precipitation and soil water in the low-elevation regions, the contribution of recycled moisture is higher in high-elevation areas than in low-elevation areas in summer.

In autumn (Figure 4c), the contribution of recycled moisture in the study area varied between 2.05% and 9.63%, which is lower than in spring and summer. The contribution was highest at sampling point C (Shrub belt) and lowest at sampling point B (Arbor belt) the vegetation growth in the river source stagnated, and the soil began to freeze. The

sampling point C (Shrub belt) has high evapotranspiration, and the evapotranspiration was reduced due to the lack of soil moisture in sampling points A and B.

**Figure 4.** Schematic diagram of recycled water vapor contribution rates for each season.

In winter, due to the influence of the cold westerly air mass, the rate of evapotranspiration weakened. The external air mass exerts a dominant influence on the moisture in winter.

Overall, the contribution rate of recycled moisture is subject to local vegetation cover, soil moisture content, and other climatic and hydrological conditions. At all sampling points, the contribution *fTr* moisture was higher than that of *fEv* moisture (Figure 4).
