Comparison of the SCS-CN and Hydrograph Separation Method for Runoff Estimation in an Ungauged Basin: The Izmit Basin, Turke

The separation of surface runoff and base flow is a very specific problem in water balance calculations,particularly if there is not enough measured flow data. In this study, the SCS-CN method is used to estimate the surfacerunoff in the ungauged Izmit Basin. The CNs are estimated using the Hydrologic Soil Groups map, based on soil data ofthe General Directorate of Rural Services of Turkey and land use obtained from the CORINE-2006 database for differentAMCs. The surface runoff was computed using the SCS-CN method for the ungauged Izmit Basin that corresponds to17% and 21% of the rainfall, i.e. 134 mm (for Kocaeli Meteorological Station; rainfall=804 mm) and 171 mm (forSakarya Meteorological Station; rainfall=820 mm). According to SCS-CN method estimates, approximately 41-42% ofthe annual rainfall in the Izmit Basin directly contributes to the total streamflow, and 21-25% of it contributes to baseflow and unmeasured infiltration. To compare the results of the SCS-CN method along with hydrograph separationmethod, the gauged Yuvacık Dam Sub-Basin, which is hydro-meteorologically similar to the Izmit Basin, was selected.The results showed that 16% of the rainfall in the Yuvacık Dam Sub-Basin became surface runoff. Also, it was foundthat about 42% of the annual rainfall in the Yuvacık Dam Sub-Basin directly contributes to the total streamflow and 23%of it contributes to the base flow and unmeasured infiltration. These results confirm that the ratio of surface runoffobtained by the hydrograph separation method in the Yuvacık Dam Sub-Basin matches with the ratio of surface runoffcalculated using the SCS-CN method for the entire Izmit Basin.