Clark Unit Hydrograph and R‐Parameter Estimation

The Clark unit hydrograph is a three‐parameter synthetic unit hydrograph procedure that can be used in flood hydrology and other surface water runoff studies that require the development of a hydrograph or the reconstitution of a flood event. The technique is particularly valuable for unusually shaped watersheds, such as watersheds with large length‐to‐width ratios, or for application to watersheds containing several different physiographic areas, such as plateaus, escarpments, and valleys. The Clark unit hydrograph can be developed completely by a mathematical routing procedure that is computationally very efficient and lends itself to convenient computer applications. Although this unit hydrograph procedure has a strong theoretical basis and is very applicable to many watersheds, it has not gained wide application. Infrequent use by practicing engineers may be because of the difficulty in evaluating the storage coefficient R from recorded hydrographs, and the lack of empirical procedures to estimate R for ungaged watersheds. A procedure is presented to facilitate the evaluation of R from recorded hydrographs using the technique of hydrograph recession analysis, and an application is provided that demonstrates the development of an empirical, regional equation for estimating R for ungaged watersheds.