Direct Runoff Hydrograph Parameters versus Urbanization.

Various rainfall-runoff models are based on the development of unit hydrographs, loss rate functions, and routing criteria. With models of this type, characteristics used to define the unit hydrograph, loss rate, and routing criteria need to be modified to predict runoff that would occur because of future development within a watershed. Certain aspects of this problem, particularly changes in peak flow and lag time due to urbanization, have been treated previously. It was the aim of this note to present additional information regarding the modification of unit hydrograph characteristics due to increased urbanization and to introduce techniques which can be utilized in a practical solution. Relationships presented in this paper can be used as a guide to compute the regional unit hydrograph parameters TCthe time in hours from the end of effective rainfall to the inflection point on the recession limb of the hydrograph and Rthe ratio in hours of the discharge at the inflection point on the recession limb of the hydrograph to the rate of change of discharge at that point, for existing and predicted values of imperviousness. Modified expressions, such as those developed, are applicable for all values of Ithe percentage of impervious surface within a watershed and the ratio of existing to future impervious surface. Author