**4. Discussion**

#### *4.1. Implication of the Soil Water Content for the Dry-Wet Regime*

The soil water content is the link between the water cycle, carbon cycle, and energy cycle [73]. It is also a key factor in associating vegetation growth with precipitation [74]. Although drought is are caused by the below-average level of precipitation over a long period of time, it is also accompanied by the rising temperature, decreasing atmospheric humidity, increasing evapotranspiration, and declining soil water content [75]. Moreover, the soil water content is an important indicator of drought which could be used to efficiently identify dry-wet conditions, and the acquaintance of the soil water content variation is of grea<sup>t</sup> significance to agricultural production, ecological environment, resource allocation, and social-economic development in China [76]. To further identify the spatio-temporal characteristics of the dry-wet transition in the YZR basin, the soil water content variation associated with the SPEI was investigated in this study. The soil water content was characterized by the sum of soil moisture in four soil layers (0–10 cm, 10–40 cm, 40–100 cm, and 100–200 cm) extracted from the GLDAS-NOAH dataset, which was represented by the water depth in millimeters.

#### 4.1.1. Spatio-Temporal Variation of the Soil Water Content

The spatial distributions of the mean annual and growing season soil water content in the YZR basin are shown in Figure 13. It could be seen that the regions with higher soil water content were mainly concentrated in the high-altitude areas such as the western upstream and the boundary of the middle and lower reaches. Except for the above areas, the soil water content decreased gradually from northwest to southeast. To analyze the spatial variation trend of the soil water, the *slope* was calculated at the pixel scale by utilizing the mean annual and growing season soil water content from 1982 to 2015. As shown in Figure 14, the regions with higher soil water content showed a downward tendency, while the regions with lower soil water content exhibited an increasing trend, which was consistent with the spatio-temporal variations of the SPEI. According to the results of the significance test, the areas showing an extremely significant decrease and significant decrease in soil water content were mostly distributed in the upstream and southeastern downstream regions. The areas with a non-significant increase and non-significant decrease in soil water content accounted for a small percentage of the total basin, and were mainly concentrated in Gongga, Lhasa, and Namling, while the areas where the soil water content increased significantly and extremely significantly were mainly situated in the midstream and northern downstream regions.

**Figure 13.** Annual (**left**) and growing season (**right**) spatial distributions of the soil water content.

**Figure 14.** Annual (**a, b**) and growing season (**c, d**) spatial variation trends of the soil water content with the significance test.

#### 4.1.2. Relationship Between the Soil Water Content and SPEI

In order to further elaborate on the transition of the dry-wet regime in the YZR basin during 1982–2015, the correlation analysis with the significance test between the annual and growing season soil water content and SPEI are illustrated in Figure 15. The correlation coefficient between the mean annual soil water content and the SPEI ranged from −0.204 to 0.923 in the basin, and nearly 99.59% of the total area showed a positive correlation between the soil water content and the SPEI. According to the results of the significance test, the area where the soil water content was extremely significantly positively correlated with the SPEI in the basin accounted for approximately 92.17% of the basin area. The correlation coefficient between the growing season soil water content and SPEI ranged from −0.206 to 0.938, and the soil water content was positively correlated with the SPEI in nearly 99.65% of the basin area, among which the areas with an extremely significant positive correlation reached 97.66%. Such close relationship between the soil water content and SPEI indicates the significantly important role of the soil moisture for identifying the dry-wet condition in the YZR basin [56,67,76].

**Figure 15.** Annual (**a, b**) and growing season (**c, d**) correlation analysis between the SPEI and soil water content with the significance test.

#### *4.2. Possible Climatic Drivers for the Dry-Wet Regime*

The dry-wet transition is a comprehensive effect of climate change. Global warming leads to an increase in the terrestrial temperature, while the rising temperature also generates an increase in evapotranspiration, which results in drought and other meteorological disasters [77]. To further investigate the possible physical mechanism of the reversal phenomenon of the dry-wet regime in the YZR basin before and after 2000, the spatial distributions of variation trends for the mean annual and growing season precipitation, temperature, and potential evapotranspiration (PET) were comparatively analyzed. As shown in Figures 16 and 17, the spatial variation trends of the annual and growing season precipitation, temperature, and PET during 1982–2015 showed identical consistencies in the YZR basin, i.e., the precipitation in the western upstream and southeastern downstream gradually decreased, while the temperature and PET exhibited an increasing trend in the same areas; the increasing trends of the precipitation, temperature, and PET all occurred in the western upstream and eastern midstream; and a tendency for the increase in precipitation in the eastern midstream appeared, while there was a declining tendency for temperature and PET. Compared to the spatial variation characteristics of the SPEI (Figures 7 and 8), in the humid regions within the basin, the precipitation presented an increasing trend while the temperature and PET showed a decreasing trend, whereas, the reversal phenomenon occurred in the arid region within the basin, i.e., there was a decreasing trend in the precipitation and an increasing trend in the temperature and PET. To sum up, the spatial distributions and variation trends with the significance test of the precipitation, temperature, PET, and soil water content showed high consistencies with those of the dry-wet regime indicated by the SPEI in the YZR basin, especially the soil water content.

**Figure 16.** Analysis on the spatial variation **trends** of the annual precipitation (**a, b**), temperature (**c, d**), and PET (**e, f**) with the significance test.

**Figure 17.** Analysis on the spatial variation trends of the growing season precipitation (**a, b**), temperature (**c, d**), and PET (**e, f**) with the significance test.
