**5. Conclusions**

Based on the performance evaluation of the GLDAS-NOAH data by using the in-situ data from 20 gauging meteorological stations, the transition characteristics of the dry-wet regime in the YZR basin indicated by the SPEI and runo ff were identified, which were combined with the satellite-based NDVI to investigate the dynamic responses of the vegetation. The hydro-meteorological factors including the soil water content, precipitation, surface air temperature, and PET were taken into consideration to explore the possible driving mechanisms of the dry-wet transition from 1982 to 2015 and the comprehensive impacts on vegetation dynamic variations. The conclusions are as follows:

(1) During the period of 1982–2015, the precipitation and surface air temperature simulated by the GLDAS-NOAH both showed high consistencies with the in-situ observed data in terms of the spatial and temporal variation patterns, implying a reasonable performance of the GLDAS-NOAH data in the YZR basin.

(2) The transition characteristics of the dry-wet regime indicated by the SPEI from 1982 to 2015 revealed that the YZR basin presented an overall wetting tendency, while the spatial and temporal characteristics of the dry-wet conditions reversed before and after 2000, that is, the basin showed a wetting trend before 2000 and a drying trend after 2000; the arid areas in the basin showed a tendency of wetting, whereas the humid areas exhibited a trend of drying.

(3) The NDVI-based vegetation in the YZR basin exhibited an overall increasing trend during 1982–2015, implying an improvement in the vegetation cover. However, a simultaneously reversal phenomenon of the vegetation variation was also detected before and after 2000, i.e., the vegetation cover in the basin increased before 2000 and decreased after 2000. From the perspective of spatial distribution, the area with higher NDVI values showed a degrading trend of vegetation cover, while the areas with lower NDVI values presented a greening trend with the improvement of vegetation cover.

(4) Approximately 70% of the basin area showed a positive correlation between the SPEI and NDVI, and were mainly located in the middle and lower reaches with high vegetation cover. However, the proportion of areas showing a negative correlation between SPEI and NDVI was small and not statistically significant. The high consistencies between the spatial and temporal variation characteristics of the NDVI and SPEI indicated that the dry-wet conditions played an important role in vegetation variations in the YZR basin.

(5) The investigation of possible driving factors for the dry-wet regime demonstrated that 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, emphasizing a grea<sup>t</sup> necessity to explore the physical mechanism of the dry-wet transition associated with the vegetation dynamics.

**Author Contributions:** Conceptualization, L.L.; methodology, L.L. and J.H.; software, J.H.; validation, Q.N., J.H. and L.L.; formal analysis, Q.N. and J.H.; investigation, Q.N. and J.H.; resources, L.L.; data curation, J.H. and H.L.; writing—original draft preparation, L.L and Q.N.; writing—review and editing, L.L. and H.L.; visualization, Q.N., J.H. and H.L.; supervision, L.L.; project administration, L.L. and Z.X.; funding acquisition, L.L. and Z.X.

**Funding:** This research was financially supported by the National Natural Science Foundation of China (91647202, 41890822, 51509247).

**Acknowledgments:** Thank the National Meteorological Information Center of China Meteorological Administration for archiving the observed climate data (http://data.cma.cn/). Thank the GIMMS team and the Global Land Data Assimilation System team for making the data freely available. Authors have grea<sup>t</sup> thanks to the anonymous reviewers for providing so valuable comments.

**Conflicts of Interest:** The authors declare no conflict of interest.
