## 3 Results
### Drought Evolution and GPP Response to Drought in Yunnan
#### 3.3.3 Response Characteristics of GPPSIF to Drought  
Based on Section 3.3.2, we can understand that the response of GPPSIF to drought is more accurate and faster than the remaining two GPP products. Yet, although GPPSIF responded better to drought than the other two products, we still need to further compare these data between normal and drought periods to assess whether this change accurately reflects the effects of drought. Thus, in order to estimate the value of the effect of drought on GPP (GPPd), we take the monthly average value of GPPSIF in multiple normal years (in this paper we use multi-year monthly averages for 2003, 2004, 2008, 2013, 2014, and 2015) as the benchmark and compare it with the GPPSIF data during the drought period (in this section, we take June 2009 to June 2011 as the drought study period), i.e., GPPd = drought month GPP-normal month GPP. Figure 13 shows the variation in GPPd with drought index sc_PDSI. Figure 13 shows that the GPPd was 10.285 gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) and 8.41 gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) in June and July 2009, indicating that at the beginning of the drought, the effects of drought were not significant and the plants could still grow normally. With the aggravation of the drought, sc_PDSI dropped to about \(-3\) and remained stable, and GPPd decreased significantly, with GPPd in November and December 2009 being: \(-37.64\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) and \(-42.24\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\), respectively, especially in Eastern and Central Yunnan, where there was a larger drop of about \(-62.52\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\). The maximum value of GPPd decline in the province occurred in October 2010: \(-58.35\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\). As the drought gradually eased, the GPPd was 12.26 gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) in June 2011, marking the end of the drought in that period. The overall trend of GPPd was in excellent agreement with the drought index (Figure 13). The monthly average values of GPPd in Yunnan Province in 2009, 2010 and 2011 were: \(-11.37\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\), \(-23.48\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) and \(-17.92\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\), respectively, which corresponded well with the drought index of Yunnan Province in these three years: \(-1.112\), \(-2.722\) and \(-0.932\). Figure 13: Changes in GPPd and sc_PDSI, 2009-06–2011-06. Moreover, we found that GPP decreased rapidly after the onset of drought, but recovered relatively slowly. The monthly average value of GPPd in the province took only six months from the beginning of 10.29 gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) to \(-52.24\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) in the basic drought in the province. In contrast, it took about 11 months when GPPd from \(-18.71\) gC\(\cdot\)m\({}^{-2}\cdot\)month\({}^{-1}\) to GPPd almost returned to normal (close to 0). The length of recovery also has spatial variability, with faster recovery in Southwest Yunnan, where only four months were spent in Region 4, and a slower recovery in the lower elevation areas of the river valley in Northwest Yunnan, which were more affected by the drought and took about nine months. In addition, the response of GPPSift to drought varies across geographic regions, with most areas in Eastern Yunnan responding rapidly and strongly to drought, Southwestern Yunnan responding slowly to drought and with less impact from it, and alpine areas in Northwestern Yunnan having less drought and some areas with positive GPPd. As shown in Figure 14b, the drought in Northwest Yunnan was mild (sc_PDSI \(>\) -1). Most of the remaining areas experienced moderate drought (\(-3<\)sc_PDSI \(<-2\)). Severe drought (\(-4<\)sc_PDSI \(<-3\)) occurred in Southeast Yunnan, Southwest Yunnan, and the Dali region in Western Yunnan, and extreme drought (sc_PDSI \(<-4\)) occurred in a small part of Southeast Yunnan.