# Responses of Seasonal Indicators to Extreme Droughs in Southwest China
## 3 Results
### Variations in Vegetation  
Variations of spatially averaged SOS, EOS and LOS are illustrated in Figure 5. All the parameters indicated obvious response of vegetation to the drought periods. The SOS generally fell in day of year (DOY) of 122-131 and they were sensitive to drought events with a delay of 3, 4, and 6 days in  
\begin{table}
\begin{tabular}{c c c} \hline
**SWA** & **First Period** & **Second Period** \\  & **(08-2003–06-2007)** & **(09-2009–06-2014)** \\ \hline Time of reduction (months) & 16 (08-2003–12-2004) & 27 (09-2009–12-2011) \\ Amplitude of reduction (km\({}^{2}\)) & 29.65 & 304.169 \\ Speed of reduction (km\({}^{2}\)month) & 1.85 & 11.27 \\ Time of recovery (months) & 29 (01-2005–06-2007) & 29 (01-2012–06-2014) \\ Amplitude of recovery (km\({}^{2}\)) & 275.24 & 376.27 \\ Speed of recovery (km\({}^{2}\)/month) & 9.49 & 12.66 \\ \hline \end{tabular}
\end{table}
Table 2: Variations of seasonal water area (SWA) in two drought periods.  
Figure 4: Variations of surface water area, (**a**) seasonal water area (SWA) and (**b**) permanent water area (PWA).  
year 2005, 2010 and 2012, respectively. The EOS fell in DOY of 324\(\sim\)334, but they were in advanced for 4 days in 2011\(\sim\)2013. The average of LOS in the whole study period was 202 days. Two periods influenced by droughts also showed in the variations of LOS, and the LOS shortened by 5 and 9 days in 2005 and 2012, respectively, but with a slight increase in 2004 and 2011.  
All the advance of EOS, delay of SOS and shorten of LOS suggested that droughts had a significant impact on vegetation phenology. It needs to be mentioned that the response of SOS and EOS to drought were not fully synchronized. The drought in spring generally delayed SOS, but the drought in autumn usually advanced EOS. For example, the drought in 2009 spanned from the autumn of 2009 to the spring of 2010, which made a slight advance of EOS in 2009 and a significant delay (4 days compared to average) in SOS of 2010. In another extreme drought period, which began in the summer of 2011 and lasted until the spring of next year, the EOS was shifted to an early date in 2011\(\sim\)2013 with the influence in two years, also postponed SOS for 6 days in 2012. All the three parameters sensitively and rapidly responded to droughts in SWC.  
Vegetation indices and indicator of carbon cycle also reflected the influence of droughts to plants (Figure 6). Although all NDVI, EVI and GPP show the increasing trend in 2000-2017, they did sharply decline during drought years, such as 2005 and 2012. NDVI and EVI performed highly consistent with each other and GPP showed the same trend as NDVI and EVI, but different amplitude. Influences from droughts are different between greenness and productivity of vegetation, also different in the situations of recovery. An obvious difference is that the drought in 2012 made more reduction in greenness (NDVI and EVI) than that of 2010. While a decrease in productivity (GPP) by drought in 2010 was even as serious as impact made by drought in 2012.  
Figure 5: Variations of vegetation growth: (**a**) start of season (SOS), (**b**) end of season (EOS) and (**c**) length of season (LOS), (DOY means day of year).