A Critical Evaluation of Streamflow Estimation of an Ungauged Watershed using a Gauged Watershed of Same River Basin – Case of Nilwala River Basin

Transferring hydrological model parameters of a nearby gauged watershed to an ungauged watershed is being widely practiced to predict streamflow in the ungauged watershed. However, proper conclusions on transferring data from the main to sub-watershed and vice versa need to be established, then only watershed managers can plan where to locate or ensure optimum use of resources when establishing gauging stations. Therefore, the present study aimed to evaluate the performance of spatial transferability by using the Tank Model, which was initially calibrated to daily streamflow from water year 2008/09 to 2017/18 observed at two gauged watersheds in Nilwala River Basin, and then transferred from main to sub-watershed and vice versa. The efficiency of predictions under two parameter transferability scenarios was highly acceptable where the Mean Ratio of Absolute Error (MRAE) values were 0.35-0.39 and, the accuracy of annual water balance was 91.8%-93.3%. Most importantly, the best results were in the intermediate flow regime with an accuracy of 61%-70%. The monthly-scaled streamflow indicated an accuracy of 83%-92%. The present study demonstrated the liability of the Tank Model for successful estimation of daily streamflow with an average accuracy of 63% under spatial parameter transferability thereby with only a single wellmanaged streamflow gauging station, Tank Model can be applied for sustainable water resources management in similar ungauged watersheds of the basin.