Impact of the False Intensification and Recovery on the Hydrological Drought Internal Propagation

Understanding the influence factors of hydrological drought internal propagation (HDIP) is crucial for early detection of hydrological drought. However, the ‘false recovery’ (FR) during development of hydrological drought and ‘false intensification’ (FI) during the recovery stage were not considered. Here, the definitions of FR and FI were firstly introduced in detail. We define the FR that occurs during the drought intensification period and does not recover the hydrological drought to a normal pre-drought condition, and the FI that occurs during the drought recovery period and does not allow the hydrological drought to develop into maximum drought intensity. Then, we designed a numerical algorithm to assess the roles of the FR and FI during the hydrological drought by comparing two scenarios: 1) considering the FR and FI and 2) not including these two variables. The monthly streamflow and precipitation records with at least 40 years of data for five unregulated and rainfall-driven basins with minimal human activities, located in southern China, were taken for the case study. Three evaluation indicators were considered, i.e., the average of relative error (Ave.RE), coefficient of determination (R2), and the Nash-Sutcliffe efficiency (NSE) coefficient, to evaluate the differences in tracking effect of HDIP under considering FR (FI) and without considering FR (FI). Results showed that the FI and the FR influence the hydrological drought intensification and recovery by changing the drought severity. The higher the FR is, the lower severity and slower intensification the drought has. A greater FI leads to a hydrological drought that has larger severity and slower recovery. Considering the substantial influence of FR and FI can significantly improve tracking effect of HDIP. The Ave.RE decreased by 36.01% (24.98%) on average, R2 and NSE increased by 22.22% (14.06%) and 39.19% (24.02%) on average in the development (recovery) period of hydrological drought. The FR (FI) during the hydrological drought is mainly caused by the occurrence of short duration precipitation events (precipitation shortage) in the study basins. Our findings highlight the role of FI and FR in hydrological drought and provide valuable scientific insights for tracking hydrological drought in real time.