Comparative study on bivariate statistical characteristics of drought in Shandong using SPI and SPEI

With global environmental change, an in-depth understanding of the changing patterns in the frequency, duration and severity of drought events is of great significance to regional water resources management and agricultural production. The standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI) are two frequently used meteorological drought indices in characterizing the drought characteristics. The elucidation of the differences between the two indices is an important work to reduce the uncertainty in drought hazard analysis and give suggestions of the appropriate index for the regional drought detection. Shandong Province is considered to be one of the major agricultural production bases in northern China. However, the comparative analysis of the suitability of SPI and SPEI for this area and the spatial variability of drought hazard have not been systematically studied, which forms the basis for this research. In this study, we identified drought events according to run theory with consideration of merging neighboring medium-term drought events. The results showed that both SPI and SPEI can efficiently describe drought in the study area and drought events identified by these two indices were quite similar. The drought duration and severity of the most severe drought event identified by SPI were 11 months and 12.8, respectively, with a joint return period of 90 years, and the drought duration and severity of the most severe drought event identified by SPEI were 12 months and 12.98, with a joint return period of 65 years. The duration and severity of drought given by SPEI, however, were generally longer and larger than those given by SPI at the same meteorological station. A comparison of the return period using the bivariate copula function indicated that minor differences existed for the same drought event in the same meteorological station of SPI and SPEI. The spatial distributions of mean drought duration and severity as well as the return periods suggested that the northwest part of the study area and Weifang city were more likely to experience longer and more severe drought events, which also indicated a relatively higher potential for drought hazard in these areas.