Nonlinear Watershed Behavior Using Variable Unit Hydrograph Parameters for California

Nonlinear watershed response to rainfall has been recognized as an important component in hydrologic routing, especially for large synthetic storms that are commonly used in dam safety evaluations. Recent updates to the US Army Corps of Engineers Hydrologic Engineering Center’s Hydrologic Modeling System (HEC-HMS) software includes a new method referred to as the variable Clark unit hydrograph that expands the classical Clark unit hydrograph by allowing the time of concentration and storage coefficient to vary in response to changes in excess rainfall intensity throughout the simulation. This new method allows modelers to simulate the nonlinear dynamic response more accurately within a watershed when subjected to large excess rainfall intensities. HEC partnered with the California Department of Water Resources, Division of Safety of Dams (DSOD) to develop a method to estimate variable Clark unit hydrograph parameters throughout the state of California for use within dam safety applications. Using results from 2D diffusion wave simulations, variable Clark unit hydrograph parameters were derived for a range of excess rainfall intensities using 16 watersheds in California. The results were then synthetized to develop regression equations that quickly allow estimation of changes in time of concentration and storage coefficient needed for hydrologic routing. Predicted variable Clark parameters derived from these equations were evaluated through a validation process demonstrating that the equations improve predictions compared to classical Clark unit hydrograph theory.