Identification of annual and decadal changes in drought trends of the Karoun basin using the SPEI index

This study used the SPEI drought index to identify drought and wet period occurrences, the trend of decadal changes and the area affected in the Great Karoun Basin. Since the SPEI drought index estimates the water balance in terms of precipitation, temperature, and evapotranspiration, thus, the precipitation, temperature, and potential evapotranspiration data were used based on the Torrent White method to detect temporal variations in the frequency and severity of annual droughts and the spatial variability of the drought occurrence over the study period (1345-46 to 1395-96). These data were extracted in a 44*44 km grid for the Great Karoon Basin and calculated by applying the SPEI drought index of drought affected zones over five decades and the areas with high potential for drought risk were identified. Trends in time variations showed that the frequency, duration, and severity of droughts increased with the increase in temperature over the last decade. The worst drought occurred in the fifth decade in the year 2007-2008. The results showed that the fifth decade was the driest, so that 13%, 53% and 29% of the basin area were associated with very high drought, high drought, and moderate to severe drought. Overall, approximately 95% of the basin area is facing drought and it was only in a very small range of about 5% of normal conditions. The knowledge of the process of these changes can help water resource management decision makers to integrate watershed management.