## 5 Discussion
### Regional Climate Characteristics  
The correlation between the meteorological drought index and TVMPDI is about 0.4 in the middle and west of the study area and some eastern regions, because the dryness and humidity in Tibet are mainly affected by precipitation [35]. The Himalayan mountains to the southwest of the study region often block atmospheric moisture carried to Tibet by the dominant westerly winds [59]. Previous studies [60; 61; 62; 63] also proposed that the decrease in potential evapotranspiration and insufficient precipitation in Tibet were the main reasons for the arid environment.  
The relationship between GPP and drought indices varies from region to region, reflecting the different patterns of driving drought in different regions [64]. In the western part of southeastern Tibet, shown in Figure 6a,b, the meteorological drought index (SPI and SPEI) has a weak positive correlation with GPP. According to the study by [65], these locations are known as vegetation-water stress zones because the principal factor limiting seed germination is water availability, and GPP increases with wetness and decreases with dryness. However, there was a strong negative correlation in the east, which was considered as the vegetation water surplus area. Because vegetation growth is largely limited by other factors (e.g., solar radiation and anthropogenic influence), increasing levels of rainfall may, in turn, expose plants to waterlogging. However, low temperatures allow the atmosphere to hold less moisture from the air and the low evapotranspiration allows the soil to hold more water in southeastern Tibet [35], which is also the reason why the negative correlation in the east is stronger than the positive correlation in the west.  
In Shigatse and other southern areas of the study area, the correlation decreased or was negative. In the dry environment, there is a vicious cycle between plant nutrition and SM supply due to land-air interaction [65]. In contrast, the relation coefficient was above 0.4 in the central areas. Instead of precipitation, SM in terms of the amount of accessible water influences ecosystem production (GPP), and a lack of SM can cause droughs in the agriculture sector [38; 66; 67].  
The low and negative correlations of some areas may be caused by the complex topographical features and atmospheric conditions. Some studies [68] have shown that the precipitation difference in the Qinghai-Tibet Plateau is the main reason for the north Atlantic oscillation, and the north Atlantic Oscillation and Pacific interannual oscillations has a negative correlation, enhancing the La Niiia phenomenon that influenced Indian monsoons. These convective systems and active water vapor transport jets are not alone in creating delays in precipitation and dryness over southern Tibet due to increased water vapor from the Bay of Bengal. Similar delays or decoupling have been reported in some studies in the Midwestern United States [69]. Due to the stronger mesoscale convective system in this region, land-air decoupling is more likely to occur. It is also worth mentioning that the gradual deterioration of the relationship between plant growth and water supply caused by this decoupling phenomenon has become more and more serious [70; 71].