Propagation patterns of hydrological droughts in the source region of Yellow River: Insights from standardized versus threshold‐based approaches

Abstract Understanding the propagation patterns of hydrological droughts is crucial for drought prevention, disaster mitigation, and water resource management. Two common methodological frameworks are employed: standardized indices, represented by the Standardized Streamflow Index (SSI), and threshold‐based (non‐standardized) indices, represented by the variable drought threshold (VDT) and fixed drought threshold (FDT). However, differences and similarities between these two types of methods in identifying hydrological droughts and characterizing their propagation patterns (e.g., onset, peak intensity, and termination) have not been systematically examined. To address this gap, the source region of Yellow River basin (SRYB), an area with relatively limited human influence, is selected as a case study. The results reveal several key similarities and distinctions: (i) The average duration of hydrological droughts during 1956–2022 is similar between SSI and VDT, but significantly longer when identified by FDT. The average severity derived from FDT is lower than that from VDT. (ii) All three methods effectively capture the spatial propagation behavior of hydrological droughts across the SRYB, which generally exhibits a decreasing intensity from upstream to downstream. (iii) Marked differences exist in the intra‑annual timing of event occurrences: SSI and VDT show an approximately even monthly distribution, whereas FDT indicates a pronounced concentration of drought events in low‐flow periods. (iv) The onset, peak intensity, and termination of hydrological droughts identified by FDT align well with wet‐dry transitions, a feature not reflected in the results from SSI or VDT. (v) These discrepancies stem from the “relative” nature of SSI and VDT—which evaluate drought events against reference‐period thresholds, leading to similar average durations and uniform seasonal distributions—and the “absolute” nature of FDT, which uses inherent low‐flow thresholds, resulting in longer durations, reduced severity, seasonal concentration, and better alignment with wet‐dry transitions. This study provides critical and actionable insights for selecting appropriate hydrological drought identification methods, thereby supporting sustainable water resource management and enhancing water security under drought conditions.