Duration and severity are two key characteristics commonly used for analyzing streamflow drought (SD); however, without considering important SD time nodes (i.e., drought onset, maximum streamflow deficit, and termination), drought monitoring and characterization remain inaccurate. 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 SD. 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 United States, with a focus on the drought onset, maximum streamflow deficit, and termination. While the average SD durations and severities identified by FDT and VDT were in broad agreement, the key time nodes of SD evolution were completely different between two methods, especially in watersheds with larger streamflow variability within a year. Our results indicated that SD identification by only considering the commonly used drought features (duration, severity) leads to inaccurate drought characterization. It is necessary to improve SD identification and characterization by considering drought onset, maximum streamflow deficit, and termination.
 

水文干旱