Continuous Soil Moisture Accounting in the Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS)

A soil moisture accounting (SMA) algorithm was developed by the Hydrologic Engineering Center for the Hydrologic Modeling System (HEC-HMS). HEC-HMS succeeds the widely used HEC-1 hydrologic model as part of the next generation (NEXGEN) of hydraulic and hydrologic modeling software under development at the U.S. Army Corps of Engineers Hydrologic Engineer Center (HEC). As originally released in 1998, HEC-HMS was an event based simulation model with the exception of a simple deficit/constant loss function. SMA was implemented as a loss rate method in the HMS version released in the year 2000 (Version 2.0) and allows for long-term continuous simulation of hydrologic processes that occur and change over time in a watershed. This is achieved by simulating the movement of precipitation through storage volumes that represent canopy interception, surface depressions, the soil profile and two groundwater layers. Computational components of this algorithm also include evapotranspiration (ET), surface runoff, and groundwater flow. This paper details the computational steps and equations used in the SMA loss method to simulate these processes. Results of model application to simulate runoff for two watersheds are described. The watersheds are near Tifton, Georgia, and Riesel, Texas, and periods of simulation were eleven and seven years, respectively.