Exploration of virtual catchments approach for runoff predictions of ungauged catchments

Abstract. We explore unit hydrograph (UH) properties influenced by catchment geomorphology that could be used in ungauged catchments. Unlike using gauged catchments, a robust approach with virtual catchments was adopted in deriving UH equations. Over 2000 virtual catchments were created from the baseline model of the Brue catchment, UK. A distributed model, SHETRAN, was used to generate runoff in these catchments. Using virtual catchments is feasible to control catchment geomorphologies, which could not be done with the real catchments due to their vast heterogeneity. Catchment characteristics of average slope, drainage length and a new index of catchment shape were examined of their influence on UH properties. The agreement of the results with the hydrological principles is a useful validation of the approach (e.g., the increasing slope led to quick response to peak (Tp) and high peak volume (Qp) of UH, whereas the drainage length presented an opposite trend). Catchment shape was shown to have a significant effect on UH properties. Compared with the widely used empirical equation from the U.K. Institute of Hydrology, the drawn conclusion recommends more indicators to be included to derive more comprehensive equations: apart from catchment geomorphologic properties, storm patterns including storm intensity and temporal distribution are also influential on the UH shape. The indicators in this study were limited to generate a sophisticated equation for use. However, these results can be considered as a testing case to gain more understanding in hydrologic processes for ungauged catchments with the help of the virtual catchment approach.