3 RESULTS

3.1 Trends of precipitation in the EIB

Figure 2 shows trends in annual precipitation for 2002–2020 and includes gridded precipitation data from CRU and GPM and observed station data from CRU. A comparison of sites and grid data showed that the trends in the observation sites were inconsistent with the CRU grids in some regions (Figure 2(b)). Among the 112 selected sites, trends for more than half the sites (60) were inconsistent with those of the CRU grids. For example, in the mid and downstream regions of the ASB, there was a non-significant (significance level: p < 0.05) increasing trend in the annual precipitation of most sites, while the corresponding CRU grids generally showed a non-significant decreasing trend. In the Mongolia and Inner Mongolia Plateau, located in the northeast EIB, there was a non-significant increasing trend in annual precipitation at most stations, while there was a decreasing trend at the corresponding CRU grids; this decreasing trend was even significant for most grids. The trends for most sites were also inconsistent with the grid results in the northern and western regions of the CSB, with some exhibiting the opposing trends in these regions.
Trends for the observed sites were highly consistent with those of the GPM grids in the EIB (Figure 2(a)). In terms of the annual precipitation trends, the majority of the CRU sites (108/112) were consistent with the GPM grids. This indicates that the GPM satellite product was highly reliable in detecting annual precipitation trends in the EIB. Therefore, the GPM precipitation data were used to analyze the precipitation trends and simulate the monthly AET series. In general, the annual precipitation was increasing in most regions, with the exception of the CSB. The significance of the increasing trend was greater at the headwater and upstream regions, and the magnitude of the trend was higher in these regions compared to the mid and downstream regions. Annual precipitation in most areas of the CSB showed a decreasing trend, particularly in the Caspian Sea and its adjacent regions. However, an opposite trend was observed in the northern region of the Volga River basin.