Spatio-temporal changes evaluation of Flow Duration Curve Seasonal Indexes (FDCSI) during four decades in Namak Lake Basin

Low flow is an important component for investigating the available water, especially in arid regions, which is applied to manage drought and to reduce it. The spatio-temporal changes of hydrological drought phenomenon based on Flow Duration Curve Seasonal Indexes (FDCSI) including Q70, Q80, Q90, Q95 and Q99 in Namak Lake basin were investigated in the present study. For this purpose, 18 stations with the maximum long-term data (43 years) were selected and FDCSI values were extracted based on summer (June to November) and winter (December to May) seasons. Non-parametric tests including Mann-Kendal, Modified Mann-Kendal and Sen Slope Estimator were performed to determine the time trend. Furthermore, Hurst exponent was applied to investigate the long-term memory. Based on Hurst results, most of the time series have memory but uniform patterns are not recognized for them. The significant decreasing trend was identified based on summer and winter seasonal indexes at 55-66% and 38-50% of the total stations, respectively. In addition, a slight increasing trend was observed at 5-11% of study stations at a significant level of 95 and 99%. The evaluation of the results of seasonal indexes showed that summer flow had more serve drought due to lack of base flow during 1970-2012. Severe decline trend during the summer season might occur due to the time patterns of rainfall, increasing evapotranspiration, changing land use and finally, over-harvesting of surface and groundwater resources for agricultural purposes.