Drought structure based on a nonparametric multivariate standardized drought index across the Yellow River basin, China

- A reliable nonparametric multivariate drought index was constructed.
- The drought structure of the Yellow River basin (YRB) was fully investigated.
- The preferred seasons of drought onset and recovery in the YRB were found.
- ENSO events have a strong impact on droughts in the YRB.

SummaryInvestigation of drought structure in terms of drought onset, termination, and their transition periods as well as drought duration helps to gain a better understanding of drought regime and to establish a reliable drought early warning system. In this study, a Nonparametric Multivariate Standardized Drought Index (NMSDI) combining the information of precipitation and streamflow was introduced to investigate the spatial and temporal characteristics of drought structure in the Yellow River basin (YRB). Furthermore, the correlations between the El Niño-Southern Oscillation (ENSO) events and NMSDI variations were explored using the cross wavelet technique. The results showed that (1) The variations of NMSDI were consistent with those of 6-month SPI (Standardized Precipitation Index) and SSFI (Standardized Streamflow Index), indicating that the proposed nonparametric multivariate drought index was reliable and effective in characterizing droughts. (2) The preferred seasons of drought onset were spring and summer, and winter was the preferred season of drought recovery in the YRB. The long-term average drought duration in the whole basin was nearly 5.8 months, which was clearly longer than the average drought onset and termination transition periods. (3) Overall, the drought structure in terms of drought duration, onset and termination transition periods in the YRB remained stable, and no appreciable change trend was found. (4) ENSO events exhibited a statistically negative correlation with NMSDI variations, suggesting that they showed strong impacts on drought evolutions in the YRB. Although the YRB was selected as a case study in this paper, the approach/indicator can be applied in other regions as well.