Using the Budyko Curve for Water Budget Calculation in Long-term Basin Planning Studies

The Budyko curve is a method used in water budget calculation in hydrology. The curve consists of a nonlinear relationship between the evaporation rate and the aridity index and it defines the water and energy-based limits of evapotranspiration. A change in the evaporation rate of the aridity index is expected to trigger a change in the compliance of the basin to the Budyko curve in the long term. In this study, a water budget approach based on the Budyko curve is proposed by conisdering the monthly rainfall-runoff relation. Actual evapotranspiration is taken as the difference between streamflow and precipitation under the assumption that there will be no change in the basin water storage in the long term, and potential evapotranspiration is calculated by the Thornthwaite method. The Kucuk Menderes river basin was selected for the application. The monthly total precipitation, monthly average temperature and monthly average streamflow data of two meteorology and two streamflow gauging stations located in the upstream and downstream of the basin were used. The compliance of a river basin from Turkiye to the Budyko framework was examined for the first time within the scope of this study. The results show that the Kucuk Menderes basin generally complies with the Budyko curve. Based on the Kucuk Menderes river basin case study, it has been understood that the Budyko curve is a practical method in calculating water budget for water resources planning and management.