Improved non-stationary SPEI and its application in drought monitoring in China

Abstract

Drought index is the first step in drought monitoring and mitigation. However, nonstationarity in the hydrometeorological data series violates the assumption of stationarity in drought indices. Here we propose a nonstationary standardized precipitation-evapotranspiration index (NSPEIt). We adopt the Penalized Splines (PS) and Stochastic Partial Differential Equations (SPDE) to fit the water deficit (Dt), derive a time-varying log-logistic distribution, and develop two versions of NSPEIt, i.e. NSPEIt-PS and NSPEIt-SPDE. We find that hydrometeorological data series in the Qinghai-Tibet Plateau Region (QTR), East Asian Monsoon Region (EMR) are nonstationary. NSPEIt performed satisfactorily in drought monitoring for both stationary and nonstationary hydrometeorological series. Specifically, NSPEIt-PS had higher drought monitoring performance than NSPEIt-SPDE. We compared standardized precipitation index (SPI), self-calibrated Palmer Drought Severity Index (scPDSI), NSPEIt-PS, NSPEIt-SPDE, soil moisture (SM), and normalized difference vegetation index (NDVI) and found that NSPEIt-PS performed better than NSPEIt-SPDE in drought monitoring and other drought indices considered in this study. However, NSPEIt_SPDE performed better in meteorological drought monitoring but NSPEI_PS was better in agricultural drought monitoring. When compared to historical droughts in 2009, the existing drought indices considered in this study tended to underestimate and/or overestimate drought intensity, whereas NSPEIt-PS and NSPEIt-SPDE captured well droughts with higher intensity, closely describing the spatial evolution of historical meteorological droughts. Results of NSPEIt-PS and NSPEIt-SPDE indicated intensifying droughts in the QTR, and regions with intensifying droughts distributed sporadically in the Northwest Arid and semi-arid Region (NAR) and EMR. Besides, droughts with higher frequency, longer duration, and higher intensity can be monitored in southwest China, the Pearl River Basin and the Yellow River basin.