Modelling ungauged catchments using the catchment runoff response similarity

Various types of regionalization approaches have been proposed in the last several decades for predictions in ungauged basins. The most commonly used methods are based on the proximity of catchment centroids and physiographic and/or climatic conditions of the catchments. However, the proximity of the catchment centroids and catchment physical attributes do not necessarily translate into similarities in hydrologic behavior. It is also difficult to identify the key attributes that favor hydrologic similarity. Therefore, in this study, we proposed a new method called catchment runoff-response similarity (CRRS), in the view of reducing the hydrologic process predictive uncertainty and to solve the problem of the key attributes identification that favor hydrologic similarity. The CRRS has a two-step approach: 1) the commonly used regionalization approach is used to temporarily transpose the calibrated model parameter from gauged to ungauged catchments, and 2) the runoff response of each smaller delineated subbasin of the gauged and ungauged basins are obtained based on the parameter value computed in the first step. The similar subbasins of the gauged and ungauged basins are then identified based on their runoff response similarity. The final parameter value in the ungauged subbasins are determined based on the notion that similar subbasins with runoff responses to similar input rainfall could have similar model structure settings. The applicability of the proposed approach was verified for the Geum River Basin (GRB) of South Korea and the Lake Tana Basin (LTB) of Ethiopia. Leave-one-out evaluations of the proposed parameter transfer approach at various test-gauging stations showed that the CRRS approach outperformed the other widely used methods. The CRRS in the GRB and LTB, reached 91% and 77% during the calibration and 86% and 67% during the validation period, respectively, of the at-site calibrated model performance on average over the test stations. The overall worth of the CRRS over the second best commonly used regionalization approach were also found to be 10% and 14% during the calibration and 7% and 4% during the validation period in the GRB and LTB, respectively. In general, it can be concluded from the overall result that the runoff predictive uncertainty in the ungauged catchments of the two study basins was significantly reduced by the CRRS approach. Therefore, the proposed approach can be used as an alternative method for runoff prediction in the ungauged basins worldwide.