Evolution of Clark's Unit Graph Method to Spatially Distributed Runoff

In 1945, C. O. Clark proposed a method to develop synthetic unit hydrographs for the modeling of watershed rainfall response. His technique utilizes two components: a translation hydrograph and a linear reservoir routing. Clark's three parameters (the time of concentration TC, a storage attenuation coefficient R, and a time-area histogram) can be estimated for gauged basins. For ungauged basins, TC and R are difficult to estimate. TC can be approximated by analyzing physical basin characteristics whereas both TC and R have been successfully estimated using parameter regionalization. Technologies for observing and managing spatially distributed watershed and rainfall information are constantly evolving. The improved availability of areally oriented data brings the challenge of integration of these data into hydrologic models. The Hydrologic Engineering Center (HEC) has developed the program ModClark to take a first step in this integration. ModClark is based conceptually on Clark's 50-year-old method and uses next generation weather radar (NEXRAD) data. The ability of Clark's technique to accommodate the spatially distributed nature of rainfall and runoff illustrates the adaptability of Clark's original methodology. A demonstration of this adapted methodology is provided.