Parallel Cascades Model for Urban Watersheds

The conversion of rainfall to surface runoff in urbanized watersheds is represented by a model composed of three main elements. The first element receives the total rainfall hyetograph as input and produces two rainfall excess hyetographs as output. These are used as inputs to the other two elements which are in parallel. The two elements, representing the impervious and the pervious portions of the watershed, were taken to be composed of cascades of linear reservoirs. The elements produce as their outputs two hydrographs that are added to produce the direct surface runoff hydrograph of the watershed. The first element of the model is nonlinear but the other two are linear. The model, viewed as one unit converting total rainfall to runoff, is thus nonlinear. Data from a semi-arid urban watershed in Arizona were used to demonstrate the use of the model.