Spatiotemporal variability of standardized precipitation evapotranspiration index in mainland China over 1961-2016

A good understanding of spatiotemporal characteristics of historical droughts is important for managing and tackling drought hazards. This paper aims to investigate spatiotemporal variations of drought in mainland China over 1961-2016 based on the standardized precipitation evapotranspiration index (SPEI) at the 1- and 12-month timescales. The trend significance and magnitude of 12-month SPEI were analysed using the modified Mann-Kendall (MMK) method and Sen's slope values. The empirical orthogonal function (EOF) and ensemble empirical mode decomposition (EEMD) were applied to analyse the spatial patterns and periods in 1-month SPEI series. The results showed that the temporal fluctuations of SPEI were weaker as the timescale increased. The droughts in 1960s were the severest than in other decades. The sites with SPEI < -5 (denoting the extreme droughts) mainly located in the northwestern and southwestern China. An overall wetter trend was detected in most regions of China, except the belt from northeastern to middle and southwestern China. The 1-month SPEI had shorter than 1.20-year of mean periods for the seven sub-regions and China, while had 0.22-2.95 years of mean periods at different sites. Residuals (decomposed from 1-month SPEI) showed increasing trends in most of the sub-regions except sub-region 3. In northern and northwestern China, longer periods of SPEI were detected. The highest values of EOF1 and EOF2 illustrated the eastern and northeastern China were drought-sensitive areas. From the correlations between 1-month SPEI and the atmospheric circulations, the AO was the main physical factor that influenced drought occurrence in mainland China. The identified spatiotemporal characteristics of SPEI could offer references for drought prevention and management in China.