Accuracy Assessment of Unit Hydrographs: Point versus Spatially Integrated Rainfall Input

Abstract

A unit hydrograph (UH) reflects the hydrologic response of watershed processes to a rainfall event. Most UH analyses use measured rainfall from a single rain gauge even though rainfall from a single gauge may not adequately reflect either the spatial or temporal variation during a storm event. The intent of this study was to better understand factors that influence UH accuracy, especially the spatial characteristics of rainfall. Analyses were made by using runoff and rainfall data, including radar rainfall measurements, from the densely monitored Walnut Gulch watershed, Arizona. Transmission loss (TL), as a physical process, is shown to be an important factor that influences UH shape. Multiple-storm, multiple rain gauge analyses show that rainfall from a single gauge is unlikely to yield a UH that reflects the watershed response. A comparison of UHs spatially averaged by using the temporal Thiessen method agree with UHs developed by using radar rainfall measurements. Intra-watershed analyses indicate that spatial variation of the physical processes, including channel processes that control transmission losses, can greatly influence the shape of a UH, thus suggesting that watershed subdivision is appropriate for analysis and design on even moderately-sized watersheds.