# Satellite-Observed Global Terrestrial Vegetation Production in Response to Water Availability
## 3 Results  
### Validation of LUE GPPs' Accuracy, Dynamics Trends and Drought's Effect on GPP  
In this study, special attention was given to determining water stress (including atmosphere vapor pressure deficit, soil moisture content and the humidity deficit) in the LUE model to estimate the global GPP. By comparing with the FLUXNET GPP site data, we found that different LUE models have a good fit in estimation of LUE GPP. The fitted R\({}^{2}\) of VPD\({}_{\text{GLO}}\)-SM, VPD\({}_{\text{MOD}}\)-SM, VPD\({}_{\text{GLO}}\)-ETR, and VPD\({}_{\text{MOD}}\)-ETR were 0.7739, 0.7399, 0.7427, 0.7459 and 0.7628, respectively (Table 1). The average goodness of fit of multiple models reached 0.78 (Figure S2). Overall, LUE GPP had a greater response to the atmospheric vapor pressure deficit, followed by soil water content, and the humidity deficit. The LUE GPP showed greater sensitivity to soil moisture content than the vapor pressure deficit only in evergreen needleleaf forest (ENF), mixed forest (MF) (R\({}^{2}\) = 0.7833 and 0.8479, respectively, RMSE = 1.6399 and 1.3825, respectively). The annual average spatial distribution of GPP in multiple models is shown in Figure S1. From 1982 to 2015, the global average annual GPP of terrestrial vegetation continued to increase at a mean rate of 0.134 Pg C a \({}^{-1}\) (\(p<0.001\)), but its growth rate declined after the mid-1990s (Figure 1a). From the spatial distribution of the mean annual GPP change trend, it can be seen that the global mean annual GPP still had a largely increasing trend, and the significant increase area accounted for 36.83% of the global terrestrial vegetation area, located primarily in the mid-high latitudes of the northern hemisphere, India, Southeastern China, Southeastern South America, Northern and Southern Africa, and Eastern Australia. 11.62% of the regional GPP showed a significant downward trend, largely in Eastern Brazil and Central and Southern Africa (Figure 1b). Taking SPEI-12 as an example, a drought episode with an SPEI value less than \(-1\) and no less than 3 consecutive months was considered a drought event. We counted the total frequency of droughts from 1982 to 2015 in each grid around the world. It can be seen that the global drought-prone areas are concentrated primarily in Central Russia, Southeast Asia, the Mediterranean coast and most parts of Africa, and the frequency of droughts has increased more than 10 times (Figure 2a). We divided the entire time period from 1982 to 2015 into a dry period and a non-dry period, and compared the changes in LUE GPP during the dry period relative to the non-dry period. It can be seen that in the high latitudes of the Northern hemisphere, Eastern Asia, the Amazon basin and Central Africa, vegetation productivity under drought conditions increased (accounting for 28.09% of the total global land vegetation area), while in 71.91% of the total land vegetation area in the world, the occurrence of drought led to a decrease in GPP (Figure 2b). To reveal the cause of this phenomenon further, we considered the effect of climate factors (including temperature, solar radiation and soil moisture) on vegetation productivity. As shown in Figure 3, temperature and solar radiation play important roles in the changes in GPP in the high latitudes of the Northern hemisphere, Eastern Asia, the Amazon basin and Central Africa during drought periods, while in other regions, soil moisture limits GPP primarily. In arid and semiarid areas, GPP is correlated negatively with temperature and solar radiation, while in humid and semi-humid areas, the converse is true. GPP is correlated positively with soil moisture globally.