Rainfall–runoff modelling approach for ungauged catchments: A case study of Nzhelele River sub-quaternary catchment

This paper presents a rainfall–runoff (RR) modelling method aimed at generating natural streamflow from the modified nearest neighbour regionalization approach applied to two ungauged sub-quaternary catchments (SQCs) nested within an ungauged quaternary catchment. It differs from the commonly used nearest neighbour regionalization approach involving a gauged quaternary catchment and an ungauged quaternary catchment. This approach ensures improvement in homogeneity of the estimated hydrological parameters. Lack of gauged streamflows hampers water resources planning and management, and water resources systems operation including allocations for environmental flows. The method has been applied in the Tshiluvhadi and the Nzhelele Rivers SQCs in quaternary catchment A80A of the Nzhelele River Catchment in the Limpopo River Basin. The modelling approach involved computing inflow hydrograph from a water balance model for Mutshedzi Dam. The hydrograph was then used in the calibration and verification of the RR model for the Tshiluvhadi River SQC using the Mike 11 NAM and Australian Water Balance Model (AWBM) in order to determine the model with better performance. The performance of each of the two models assessed by using the Root Mean Square Error, Nash Sutcliffe coefficient of efficiency, the correlation coefficient, % Bias and the overall water balance error was good and comparable. The two models, however, tended to underestimate the high flows. The models were used to simulate runoff hydrographs for the ungauged Nzhelele River SQC using model parameters obtained from the Tshiluvhadi SQC RR modelling. The streamflow hydrographs for the Nzhelele River SQC simulated from both the models are comparable and show behaviour similar to that reported in earlier studies. They also correlate well with the areal rainfall for the Nzhelele River SQC. The modelling results show that the approach is reasonably good and therefore can be used in predicting runoff in ungauged catchments. The simulated runoff hydrographs can be used in water resources planning and management, and water resources systems operation.