Comparing Threshold Level Methods in Development of Stream Flow Drought Severity-Duration-Frequency Curves

This study compares Severity-Duration-Frequency (SDF) curves (SDFs) of stream flow drought derived from threshold level methods. For this purpose hydrological drought of Roudzard river basin was investigated, based on run theory. Daily runoff data of Mashin hydrometery station (1970–2012) assessed using 70% (Q₇₀), 90% (Q₉₀) of mean daily and 70% of monthly average runoff (monthly) as threshold level methods. Time series of the annual maxima values of duration and volume deficit showed similar trend of increase and decreasing in different thresholds. SDFs were prepared, classifying drought durations to four intervals and fitting statistical distribution to each one. Resulted SDFs showed that, in each period, increasing of duration resulted to increased value of the volume deficit with a non-linear trend while duration and severities from the threshold levels were different. Drought deficit-volume increasing rate was also different in each class of duration-interval. For the additional analysis, the duration-frequency and deficit-frequency curves were also prepared to quantify the extent of drought duration and deficit more. SDFs developed in this study can be used to quantify water deficit for natural stream and reservoir. They could be an effective tool to identify multivariate hydrological drought using severity, duration and frequency.