Application of an electronic analog computer to the evaluation of the effects of urbanization on the runoff characteristics of small watersheds

In the synthesis of hydrograph characteristics of small urban watersheds, the distribution of the water among the various phases of the runoff process is attempted by the concept of "Equivalent Rural Watershed". The criteria for transforming the urban watershed into an equivalent rural watershed requires that, for a given input into both the models (urban and its equivalent rural watershed), the outputs must be identical. The hydrograph of outflow from an urban watershed is obtained by chronologically deducting the losses due to interception, infiltration and depression storage from precipitation on the equivalent rural watershed and then routing it through the surface and channel storages. This procedure of computing the synthetic outflow hydrograph is accomplished with the analog computer at the Utah Water Research Laboratory, Utah State University at Logan. Testing and verification is done with rainfall and runoff data from the Waller Creek watershed at Austin, Texas. In the verification process, coefficients representing interception, depression storage, and infiltration are determined by trial and error so that the simulated hydrograph is nearly identical to the measured hydrograph of the prototype. The variation in the values of these coefficients from year to year is assumed to be due to the corresponding variations in the characteristics of urbanization defined by the percentage impervious cover and the characteristic impervious length (ratio of the mean length of travel between the center of the impervious area and the discharge measuring point to the maximum length of travel on the watershed). This study attempted to develop the relation between these coefficients and the urbanization parameters.Waller Creek Working Grou