Unit Hydrograph Theory

Unit hydrograph theory is a technique for modeling the transformation of excess precipitation to runoff at the watershed scale. When the watershed is gaged with both observed rainfall and flow data available for a storm event, the unit hydrograph relationship can be derived directly. When the watershed is ungaged, the unit hydrograph relationship cannot be derived directly; therefore, many synthetic unit hydrograph methods have been developed to help with ungaged watersheds (though not limited for use to just ungaged watersheds).  HEC-HMS contains six different unit hydrograph methods.  Within this workshop, the Clark/ModClark unit hydrograph method is applied.


In this workshop, you will become familiar with the Clark/ModClark unit hydrograph method that is available within HEC-HMS.  Specifically, you will:

  • Estimate time of concentration (Tc) and storage coefficient (R) using GIS information.
  • Enter the initial parameter values within a basin model and compute the simulation.
  • Calibrate the initial parameter values to achieve a better fit to the observed data.

Questions will be provided throughout the workshop.


In this tutorial group, the study area is designated as the Middle Fork American River watershed upstream of the French Meadows Reservoir near Foresthill, CA.  This study area was detailed here.