Abstract. The change of climate and environmental conditions has obviously affected the evolution and propagation of drought in river basins. The Hun River basin (HRB) is a region seriously troubled by drought in China, so it is particularly urgent to evaluate the evolution of hydrological drought and investigate the threshold of triggering hydrological drought in the HRB. In this study, the standardized runoff index (SRI) was applied to reveal the evolution characteristics of hydrological drought. Meanwhile, based on drought duration and severity identified by the run theory, the copula function with the highest goodness of fit was selected to calculate the return period of hydrological drought. Furthermore, the propagation time from meteorological to hydrological drought was determined by calculating the Pearson correlation coefficients between 1-month SRI and multi-timescale standardized precipitation index (SPI). Finally, based on the improvement of the drought propagation model, the drought propagation thresholds for triggering different scenarios of hydrological drought and its potential influence factors were investigated. The results show that (1) the hydrological drought showed a gradually strengthened trend from downstream to upstream of the HRB from 1967 to 2019; (2) downstream of the HRB were districts vulnerable to hydrological drought with longer drought duration and higher severity; (3) the most severe drought with drought duration of 23 months and severity of 28.7 had corresponding return periods that exceed the thresholds of both duration and severity of 371 and 89 years, respectively; (4) the propagation time from meteorological to hydrological drought downstream of reservoir has been significantly prolonged; and (5) the drought propagation threshold downstream of the HRB was remarkably higher than that upstream in all drought scenarios. Additionally, midstream showed the highest drought propagation threshold at moderate and severe drought scenarios, while downstream showed the highest drought propagation threshold in the extreme drought scenario.

The evolution of meteorological drought under global warming is of great significance to drought risk management. Meanwhile, driving factors that influence hydrological factors and water cycle processes play an important role in meteorological drought risk assessment. The Hun-Taizi River basin (HTRB) is a region seriously troubled by drought in China. Therefore, we reveal the evolution characteristics of meteorological drought and its driving factors. First, the standardized precipitation index (SPI) was adopted to characterize the evolution characteristics of meteorological drought. Meanwhile, copula functions with the highest goodness of fit were established to calculate the return period of meteorological drought. Then, the relationships between the SPI and the climatic phenomena were reflected by the cross-wavelet transform method to reveal the driving factors of meteorological drought. The results showed that (1) the meteorological drought of the HTRB varied greatly in different periods, and drought aggravated during spring and autumn; (2) the meteorological drought in the western, northwestern and southeastern regions of the HTRB was characterized by high frequency, short duration and low severity, while that in the other regions was characterized by low frequency, long duration and high severity; (3) the co-occurrence return periods of mild, moderate and severe drought were 1.9, 4.7 and 32.8 years and the joint return periods were 1.5, 3.0 and 9.3 years, respectively; (4) solar activity (sunspot), El Niño Southern Oscillation (ENSO) and Arctic Oscillation (AO) were strongly correlated with drought evolution in the HTRB.

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