HEC-HMS is a numerical model (computer program) that includes a large set of methods to simulate watershed, channel, and water-control structure behavior, thus predicting flow, stage, and timing. The HEC-HMS simulation methods, which are summarized in the table below, represent:

  • Watershed precipitation and evaporation. These describe the spatial and temporal distribution of rainfall on and evaporation from a watershed.
  • Runoff volume. These address questions about the volume of precipitation that falls on the watershed: How much infiltrates on pervious surfaces? How much runs off of the impervious surfaces? When does it run off?
Summary of Simulation Methods included in HEC-HMS

Category

Method

Precipitation







User-specified hyetograph

User-specified gage weighting

Inverse-distance-squared gage weighting

Gridded precipitation

Interpolated precipitation

Frequency-based hypothetical storms

Standard Project Storm (SPS) for Eastern United States

Hypothetical storm

Evapotranspiration







Monthly Average

Priestly-Taylor

Penman Monteith

Hamon

Hargreaves

Interpolated ET

User-specified 

Snowmelt


Temperature Index

Gridded Temperature Index

Runoff-volume










Initial and constant

SCS curve number (CN)

Gridded SCS CN

Green and Ampt

Layered Green and Ampt

Exponential

Smith Parlange

Deficit and constant

Gridded deficit and constant rate

Soil moisture accounting (SMA)

Gridded SMA

Direct-runoff








User-specified unit hydrograph (UH)

Clark's UH

Snyder's UH

SCS UH

ModClark

Kinematic wave

User-specified s-graph

2D Diffusion Wave

Baseflow




Constant monthly

Exponential recession

Linear reservoir

Nonlinear Boussinesq

Routing





Kinematic wave

Lag

Modified Puls

Muskingum

Muskingum-Cunge

Water control structures


Diversion

Reservoir/detention pond

  • Direct runoff, including overland flow and interflow. These methods describe what happens as water that has not infiltrated or been stored on the watershed moves over or just beneath the watershed surface.
  • Baseflow. These simulate the slow subsurface drainage of water from a hydrologic system into the watershed's channels.
  • Channel flow. These so-called routing methods simulate one-dimensional open channel flow, thus predicting time series of downstream flow, stage, or velocity, given upstream hydrographs.

The HEC-HMS methods are described in greater detail in the HEC-HMS Technical Reference Manual (HEC, 2000). That manual presents the concepts of each method and the relevant equations that are included. It discusses solution of the equations, and it addresses configuration and calibration of each method.