Investigation of Droughts Impacts on Water Balance Components Using the Budyko Framework (Case Study: Lake Urmia Basin)

Understanding the hydrological response of the basins to droughts is essential in recognizing different regions’ vulnerability to droughts and drought risk management. In this study, the amount and direction of changes in the components of actual evapotranspiration and runoff in response to the occurred droughts were investigated in the sub-basins of the Lake Urmia Basin in the Budyko framework. To this end, actual evapotranspiration (ET) and potential evapotranspiration (PET) data from the GLEAM database and precipitation data from the MSWEP database were obtained. After determining the drought years using the Standardized Precipitation-Evapotranspiration Index (SPEI), the Fu’s equation (in the Budyko framework) was calibrated separately for drought and non-drought years. The significant difference in empirical probability distributions between drought and non-drought years for the aridity index (PET/P) and the evaporative index (ET/P) was investigated using two-sample Kolmogorov-Smirnov test. In the following, the regime-related and the partitioning-related changes were calculated and examined separately for the actual evapotranspiration and the runoff. In the case of the runoff, the regime-related, the partitioning-related and the total changes of runoff were negative in drought years compared to non-drought years in all the sub-basins and the regime-related changes were more severe than the partitioning-related changes. In the case of the actual evapotranspiration, the partitioning-related changes were positive, the regime-related changes were negative and the total changes were negative in all the sub-basins. In total, the occurrence of droughts in the studied sub-basins has resulted in more decreasing of the runoff (-66.54%) compared to the actual evapotranspiration (-10.45 %). The Budyko framework is effective in the explanation of the nonlinear behavior of water balance during drought.