Simple Rainfall Loss Models for Rainfall-Runoff Modeling

Unit hydrographs are developed for a specific watershed using two basic approaches. If unit rainfall-runoff data are available, then numerous techniques can be applied to estimate a unit hydrograph from the data. If no data are available, then methods of synthetic hydrology must be applied. Current practice is regionalization of measured behavior used to transfer known hydrographs (or other hydrologic entities) from a location where measurements are available to unmonitored watersheds. The regionalization involves determining time parameters for the unit hydrograph procedure, which may include time to peak, time base, or time of concentration. Regionalization also involves development of regional regression equations for time parameters, watershed and/or rainfall characteristics. The principal elements required for the synthetic approach are the determination of characteristic loss features (loss model) and characteristic response time (unit hydrograph model). The authors observe that even if the temporal redistribution of excess rainfall is performed using hydraulic models, which is certainly feasible and often the most appropriate approach, then there is still need to understand how to convert the rainfall signal into an excess rainfall signal, and this will be accomplished with some kind of loss model. In the collective literature losses are correlated with watershed soil properties, while the characteristic response time is associated with physical characteristics such as watershed drainage areas, main channel and overland flow slopes, characteristic lengths, and characteristic velocities. Unit hydrograph parameterization was examined based on topographic information and simple loss models to predict the runoff volume, rate and timing with minimal watershed characterization, using data readily available to drainage engineers. Results from the approach were compared with observed rainfall-runoff responses and are reported herein.