Mathematical models of flow duration curves

The flow duration curve (FDC) used in water resources studies (such as water supply structures, water intake structures and water quality studies) is the graph showing the percentage of the flow rate in a stream versus the flow equalized or exceeded in a certain period. In this study, flow duration curve (FDC) of the major rivers in Gediz Basin, located in the west of Turkey, is discussed. Within this scope, the FDCs obtained from daily and monthly flows were compared at first. Subsequently, FDCs were obtained by using the Annual Percentages method, which gave healthy results in the dried rivers. Finally, the variation of the flow rates (Q0,05, Q0,50, Q0,95) with significant watershed sizes was investigated. As a result of comparison of daily and monthly flow duration curves, it has been shown that these curves are almost overlapping, so that flow duration curves can be obtained from monthly flows in streams with missing daily flow data. The flow values corresponding to 5%, 50% and 95% exceedances obtained from the FDCs show a strong regression relation (R2%5=0,987, R2%50=0,988, R2%95=0,969) with the mean slope, average elevation and precipitation area expressing the most important characteristics of the basin. Thus, it has been shown that the flows of a certain probability (5%, 50% and 95%) can be estimated successfully with basin mean slope, basin average height and precipitation area.