*4.1. Consistency Between the NDVI and Satellite Precipitation Data*

To determine the delay in the response of the NDVI to precipitation, Table 1 presents the correlation coefficients (CCs) between the NDVI and the observed precipitation at all stations in the study area during the warm seasons from 2014 to 2018. The results showed that the highest correlations were observed with no time lag. Figure 4 shows the distributions of the monthly average NDVI and IMERG precipitation in the study area during the warm season of 2016. According to Figure 4b, the Ili Valley was the area with the most abundant precipitation, while the eastern area received the least precipitation. This is because the Ili Valley is surrounded by mountains to the north, east, and south (Figure 1), which is beneficial for the collection of water vapor from the Atlantic Ocean to the west, resulting in abundant precipitation in the piedmont zone. According to Figure 4a, there was good consistency between the NDVI and IMERG precipitation data. The high resolution of the NDVI revealed detailed spatial variations and contributed to the improvement of the downscaled precipitation estimates.

**Table 1.** The correlation coefficients (CCs) with different delays between the NDVI and the observed precipitation at all stations during the warm seasons from 2014 to 2018.


**Figure 4.** (**a**) The monthly average NDVI and (**b**) the monthly average IMERG during the warm season of 2016.
