Investigating the Impact of Drought on Flow Duration Curve in Karst Catchments (Case study: Kaka-Reza Catchment)

Flow duration curve (FDC) is one of the most appropriate criteria for recognizing the hydrological response of catchments. Understanding the interaction of drought phenomenon along with FDC indices is a suitable tool for optimal water resources management of catchment. The aim of this study was to investigate the temporal relationship between drought and FDC indices in the Kaka-Reza, karstic catchment. In this study, the common period (1982-2017) was considered for the corresponding hydrometric and rain gauge stations. The standardized precipitation index (SPI) and standardized discharge index (SDI) were then calculated at time scales of 3, 6, 9, and 12 months. Precipitation duration curve (PDC) and FDC were plotted and curve indices were calculated and standardized. The relationships between drought index and FDC indices were investigated and analyzed. The results showed the general trend of changes in SPI and SDI in the respective time scales follow each other. However, the most consistent trends are related to the time step of three months. The rate of relative changes in the section of low flow indices (Q90/Q50) were very small and around 0.6% and in the high flow (Q25/Q50) was around 4.41%. The correlation between FDC and PDC showed coefficient of determination of 0.904. The rate of slope change was from 0.5 to 5% and trend of its changes was a slight upward slope during the research period. The small amount changes of FDC indices and slope of FDC showed the role of karst phenomenon in determining the shape of FDC as well as controlling flow fluctuations and providing stable flow during the drought period.