Hydrological drought characterization considering onset, maximum streamflow deficit, and termination

Duration and severity are two key characteristics commonly used for analyzing hydrological drought (HD); however, without considering important HD time nodes (i.e., drought onset, maximum streamflow deficit, and termination), drought monitoring and characterization remain inaccurate. For example, any HD index can identify the duration and severity of certain drought events. However, the onset and termination times of drought events identified may vary significantly among different types of drought indices. This discrepancy presents an obstacle within the realm of drought forecasting and management. The methods of fixed drought threshold (FDT) and variable drought threshold (VDT), which apply an absolute and relative threshold for drought definition, respectively, are commonly used to identify HD. We studied the difference and applicability of the FDT and VDT methods for different thresholds (i.e., streamflow percentiles Q10, Q20, Q30, Q40, and Q50) based on monthly streamflow records of 219 watersheds in the United States, with a focus on the drought onset, maximum streamflow deficit, and termination. The results show that, while the average HD durations and se-verities identified by FDT and VDT were in broad agreement, the key time nodes of HD evolution were completely different between two methods, especially in watersheds with larger streamflow variability within a year. The FDT and VDT methods were determined to be suitable for watersheds with lower seasonal flow variability based on the key time nodes of HD evolution. However, the FDT method seems to be more appropriate for watersheds with notable seasonal variations in streamflow, when compared to the VDT method. Our results indicated that HD identification by only considering the commonly used drought features (duration, severity) leads to inaccurate drought characterization. It is necessary to improve HD identification and characterization by considering drought onset, maximum streamflow deficit, and termination.