Determining Hydrological Drought Characteristics Using Severity-Duration-Frequency Curves and Flow Thresholds Levels in Ardabil Province Rivers

Drought is a global phenomenon that can occur anywhere and cause significant damage to humans and natural ecosystems. Therefore, the issue of hydrological drought and reduction of river flow in Ardabil province is also an important issue that requires a comprehensive study.

In this study, hydrological drought characteristics are using Severity-Duration-Frequency (SDF) curves considering four different threshold levels (constant, annual average, seasonal, and environmental flow) in 33 hydrometric stations in Ardabil province. The severities of drought events are calculated using Easy-Fit software in different return periods. In this regard, drought magnitude-duration-frequency curves are calculated and based on that, the values of drought events in different return periods are calculated and analyzed.

The highest drought event is determined for constant, seasonal, yearly, E-flow threshold levels in Samina, Mashiran, Booran, and Samian stations, respectively.  Also, the lowest occurrence of drought is associated with the Vildaragh station. The majority of drought events in all four aforementioned thresholds are mostly observed in shorter durations, especially within one-month periods. The Jonson_SB and General Extreme Value distributions were the most suitable statistical distributions. The highest intensity of drought increases with longer return periods associated with the average annual threshold level, and the lowest intensity of hydrological drought is linked to the threshold of environmental flow. The SDF curves for all stations demonstrate an increasing trend, indicating that with prolonged hydrological drought duration in all studied thresholds, the severity of hydrological drought occurrences also increases. The greatest intensity of hydrological drought is sequentially related to the annual threshold, followed by the seasonal threshold and the fixed threshold, with the threshold of environmental flow being the lowest and least. Consequently, for shorter return periods, the fixed threshold indicates greater intensity or magnitude compared to the seasonal threshold in all four stations, and for longer return periods, the seasonal threshold demonstrates higher intensity of drought events compared to the fixed threshold.

Furthermore, it can be concluded that the magnitude of hydrological drought at a fixed threshold shows less variability in all four selected stations compared to the other thresholds. It should be noted that in defining drought based on fixed, annual, and seasonal thresholds, events will have a higher number and greater intensity compared to the environmental flow threshold. The spatial changes in drought intensity are depicted on the map, indicating that most droughts have occurred in stations located in the northern and north-western regions (Borran and Dostbiglou). Separating the effects of human and climatic factors in drought assessment is a suggestion from this study that could be considered in future research.