## 2 Materials and Methods
### Defining a Drought Event and the Drought Duration, Intensity, and Frequency  
There are various drought indices that can be used to quantify the variations in drought characteristics, such as the Standardized Precipitation Index (SPI) proposed by [PERSON] et al. (2016), which is calculated based on precipitation data. The SPI has a wide application mainly because of its simplicity of calculation and because it can be calculated at different timescales to monitor droughts with respect to different usable water resources. However, it overlooked the effects of temperature and PET on the occurrence of drought events. Another widely used drought index, the Palmer Drought Severity Index (PDSI), which is calculated based on both precipitation and temperature data, as well as the water content of the soils, seems more appropriate for the objective and quantitative evaluation of drought severity in the context of global warming. However, because of its fixed temporal scale (yearly timescale), PDSI lacked the multiscale characteristic essential for us to determine the drought characteristics within a growing season period. Therefore, to verify the accuracy of the results of this paper, we analyzed the changes in drought characteristics by using the more accurate grid SPEI data, in which the PET was calculated by the Penman-Monteith equation ([PERSON], 1982). Compared with the Thornthwaite method, the Penman-Monteith equation takes into account the combined effects of wind speed, radiation, and relative humidity on evapotranspiration; it was thus a more accurate, comprehensive, and physically based model for estimating the PET.  
The monthly SPEI-3 (SPEI at a 3-month timescale) series for each pixel during the growing season from 1982 to 2015 was selected to construct the drought duration, intensity, and frequency maps. For example, for a specific pixel, the SOS occurred in March and the EOS occurred in November. The monthly SPEI series from March to November from 1982 to 2015 was then selected for the drought characteristic analysis. The drought event starts and end dates in the month were defined as when the SPEI fell below 0 and when the SPEI returned to a positive value, respectively. However, because the soil moisture can buffer vegetation growth during periods of moisture deficiency, in our analysis, a drought event was defined to occur when the SPEI value fell below 0 for at least three consecutive months. Once a drought event had been determined with its start and end months, the duration of the drought event was calculated by the number of months between the start (included) and end months (not included). In addition, the drought intensity was calculated as the value of the integral area between the SPEI line and the horizontal \(x\) axis (SPEI = 0) from the start to the end month of the drought, which indicated that the more negative the intensity value, the more severe the drought. The drought frequency was defined as the number of drought events that occurred over a defined period.  
To compare the changes in drought duration and intensity before and after the critical time point of 1998, the total drought duration (TDD) and total drought intensity (TDI) were then calculated as the sum of the duration and intensity of the drought events that occurred in 1982-1998 and 1999-2015, respectively.