Budyko Curve as a Tool for Long-term Water Budget Calculation

Deficit in water budget has a negative effect on society, environment and economy. In order to mitigate these negative effects, it is important to known how precipitation falling on the basin is shared by streamflow, evapotranspiration, baseflow and infiltration; i.e., What is the share of precipitation transferred to flow, returned to the atmosphere through evapotranspiration or contributed to the basin storage system through infiltration? Sharing water resources among drinking and utility water, agriculture, animal husbandry, industry, tourism, etc., is necessary and important for the sustainability. In a simple water budget, the basic and traditional approach can be accompanied by the methods such as Budyko curve, a method that can be used in water budget calculation in hydrology. The Budyko curve consists of a nonlinear relationship between the evaporation rate and the dryness index and it defines the water- and energy-based limits of evapotranspiration. In this study, a water budget approach based on the Budyko curve is proposed by considering the monthly rainfall-runoff relation. Firstly, the aim of the study is to determine whether Kucuk Menderes River Basin in western Turkey complies with the Budyko curve; and if yes, secondly, whether it changes over time, and if it does, how it changes. The monthly total precipitation, and monthly average temperature ​​of meteorological stations and monthly average streamflow were used. Assuming that there will be no change in the basin water storage in the long-term, actual evapotranspiration will be taken as the difference between the streamflow discharge and precipitation, and the potential evapotranspiration will be calculated by the empirical Thornthwaite method. From the application on the upstream and downstream reaches, the river basin was found consistent with the Budyko framework in general. However, according to the calculations made by using the Budyko curve, higher flows than expected were obtained for the upstream reach and lower flows than expected for the downstream reach of the basin. Based on the results of the case study, the potential of Budyko curve as a method to use in planning and management of river basin was demonstrated. The study will also investigate the possibility of extending the use of the Budyko curve to ungauged or nested basins.