Estimation of monthly current and obtaining flow duration curve in Coruh basin

Current estimates, has always been one of the main topics of hydrology. These estimates, which can be made in different ways, have been quite difficult where the data stream of the river does not exist or inadequate. These current estimates are utilized in many areas of the water resources. River flow has a great importance on projecting dams, watershed management, the determination of the hydropower potential, flood protection projects and so on. Therefore, it is important to make the estimation of current correctly. There are many methods for monthly flow estimation in the nonmeasured rivers. These methods may be empirical, deterministic and statistical. Many unknown parameters in a hydrological events have led to a decrease in interest in the deterministic method in recent years and that caused increased tendency to use statistical methods in hydrology due to randomness. Current measurements in rivers in our country are carried out by Devlet Su İşleri (the State Hydraulic Works) (DSI) through established at certain critical points on the river stations. However, since receiving data from the station is hard and costly, and also lack of business problems, as well as for reasons such as the lack of the number of historical data, the current estimates has become a necessity for making the estimates. Drainage area ratio (DAR), which is a traditional and common current estimation method, can cause inability to make accurate estimates especially in some of the major water sources projects although it requires less data and parameters. Therefore new methods are needed to be used. In this study, it is aimed at making estimation about montly flow by taking lack of data in our country into consideration. Methods, which is applicable for this aim and giving reliable results, were used. In this study, the method as a new approach in the monthly flow projections which is published in the Journal of Hydrology of the magazine in 2013 is considered. This method was done by Farmer WH and Vogel RM as "Performance-weighted methods for estimating monthly streamflow at ungauged sites". Located in the northeast of Turkey Coruh Basin was selected as the study area. Monthly flow values which were observed in the gauging stations 2304, 2305, 2315, 2316 and 2323 in the basin between 1965 and 1998 are used as data. In this study, three current transfer method is used which consists of drainage area ratio (DAR), the average current (SM) and the mean standard deviation (SMS) including the standardization of the current. Regional regression was utilized in SM and SMS method. Regional regression is made on an annual and monthly basis using data from the other four stations outside the station to be estimated. In regional regression, parameters for rainfall area is (A) and rainfall area with elevation is (A + K). These parameters' ln terms are used as ln (A) and ln (A + K). In this way, SM and SMS estimation methods which require local regression are made with a total of 12 variants. In this study, 13 estimation methods were used with DAR method and a total of 260 estimation were made. Some of these estimates could not be used. The performance of the estimates made by the methods mentioned above, are evaluated for each station separately with Nash- Sutcliffe efficiency coefficient (NSE) and root mean square error (RMSE). Then the DAR method is compared with other methods (SM and SMS) by obtaining w weight value calculated in blended performance weighted method. DAR method was successful in many stations. Current estimates on the river Coruh in the standardization process is more successful, generally. The methods which uses regional regression method and use only rainfall area (A) has been more successful than the DAR methods. Comparison of DAR method with the other methods is made according to w weight value calculated in blended performance weighted method and superiority is observed for estimations with %50 for DAR method and %50 for other methods. In this study, Flow Duration Curves (FDC) were obtained from selected stations and estimation methods and compared FDC's which were obtained from observed flow values of those stations. In addition, FDC's were obtained using blended methods. Estimations and observed flow values were compared on FDC . Thus, it was determined which methods was successful. Consequently, considering the position of the stations, it may be said that the current estimate of the stations on Coruh River (2305, 2315 and 2316) is made more successful. Since the station no. 2304 is the arm of Coruh River and placed near the source of basin, estimations in some methods could not be done or done wrong. The station no. 2323 is considered to be the same way because of the low success of the estimate since it is also one of the arms of Coruh.