# Responses of Seasonal Indicators to Extreme Droughs in Southwest China
## 4 Discussion  
### Responses of Seasonal Indicators to Droughs  
In spite of many studies on the droughs in SWC, this study made the effort to explore the responses of seasonal factors to droughs in SWC. Our results indicate that there were two major drought periods during 2000-2017. In the first period, most months were severely or moderately dry while almost all months were extremely dry in the second period (Figure 8). Considering both drought severity and the speed of occurrence or recovery, the second drought period was more serious, also had much more impact on the surface water and the vegetation (Table 2). This is consistent with the conclusion obtained from the study using gravity satellites to analyze the water reserves in SWC [4]. Cyclical fluctuations of SWA indicated that seasonal water bodies shrank or disappeared during the dry period, confirming that the obvious effect of drought on surface water. Moreover, signs of drought captured by LOS were highly consistent with that of SWA (Figure 8). Advance of EOS and delay of SOS in drought years shortened the LOS significantly, indicating that drought has a significant impact on vegetation phenology. The conclusion is in agreement with papers studied in western North America and semi-arid regions of northern China [69; 35; 68]. In addition, our study distinguished spring-drought and autumn-drought presented evidence, in which spring drought delayed SOS and autumn drought advanced EOS in SWC (Figure 4). A similar pattern was observed in Northern China, Canadian Praires and Chile [71; 35; 69; 70]. Same as variations of vegetation phenology, vegetation greenness and productivity also showed a significant decline in several years (Figure 6). Growth of plants was directly controlled by water and carbon processes and these changes could be explained  
Figure 8: Variations of seasonal surface water (SWA), length of season (LOS) and PDSI in the study area. by drought [72; 73]. Serious water scarcity made insufficient available soil water for carbon synthesis during the photosynthesis process, resulting in insufficient supply for normal growth of plant [74; 75]. This made decline of greenness and productivity, therefore slowed the growth of vegetation and delay of SOS. While, lack of water and high temperature in autumn drought prompted leaves to close the stoma, which decreased the transpiration and photosynthesis rates, but respiration still kept high rate to accelerate carbon degradation [75; 76]. As a result of deficient supply and EOS advanced in autumn [35]. A continuous growth of PWA was observed in the SWC during the period of 2000-2017, which demonstrates a growing trend of the surface water reserves in this region. This increase might be closely linked to global climate warming and human activities. Large water infrastructures changed the location and lasting of water, which influences the conversion among seasonal water, permanent water and no water pixels. Most countries' reservoir construction contributed to the increase of PWA in the world according to [PERSON]'s analysis [9]. In this region, more than half of increased PWA (58.5%) came from no water pixels, and this change matched to the water infrastructure project well. We found that the number of reservoirs in SWC increased quickly and kept a close relationship with PWA with correlation coefficient of 0.85 (Figure 9). Building reservoirs and long-term storage of water made no water pixels converted into water pixels, resulting an increase of PWA. The other part of increased PWA came from SWA, which is related to many reasons, such as the fluctuated rainfall, temperature due to climate change and other aspects [77; 78]. Moreover, similar to the PWA increased during 2000-2017, our study found that the greenness and carbon cycle also increased in this region, this is identical to the recent studies, in which the SWC is greening and carbon stocks is raising [79; 80].