Evaluation of parameter setting for two GIS based unit hydrograph models

Summary For watersheds where flow data are unavailable, the geomorphology–hydrology relationship can be used to estimate the direct flow response to excess rainfall. Two of the most common approaches used to compute this response are Geomorphological Instantaneous Unit Hydrographs (GIUH) and Spatially Distributed Unit Hydrographs (SDUH). In the former, the hydrograph is determined from the input of morphometric parameters and an average channel velocity, where in the latter a time–area relationship is used to compute the hydrograph. Generally, both approaches involve an estimate of velocity to parameterize the Unit Hydrograph (UH), however, information on this parameter is generally limited for watersheds where these methods are most appropriate, when there is no flow data to derive the UH directly. The aim of this work is to investigate if the velocity parameters involved in GIUH and SDUH methods can be estimated directly from watershed characteristics, and allow these methods to be applied in ungauged watersheds. Four watersheds in southern Australia with daily flow records have been considered, to allow the observed direct flow response to be determined. It was found that both approaches could be calibrated to accurately represent the expected response for all four watersheds considered. The SDUH model implemented considered hillslope and channel flow processes separately, which allowed the velocity parameters involved to be estimated from the watershed using Manning’s equation. However, the GIUH model combines these flow processes into one average velocity parameter, and due to this averaging a relationship between the calibrated value and the watershed characteristics could not be determined. The results suggest that the SDUH model can be directly parameterized for a given watershed in the absence of flow data, however, further work is required to investigate if the relationships proposed are suitable for a wider range of watersheds.