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## HEC-HMS

HEC-HMS is an abbreviation for Hydrologic Engineering Center’s – Hydrologic Modeling System. HEC-HMS is hydrologic simulation software for modeling precipitation-runoff processes for a dendritic watershed. The basic components of the HEC-HMS are the basin model, meteorologic model, control specifications, time-series data, and paired data.

The basin model was used for physical representation of the watershed. Available elements in the basin model were: subbasin, reach, junction, reservoir, diversion, source, and sink. Computation proceeded from the upstream elements in a downstream direction. The meteorologic model performed meteorologic data analysis and included precipitation, evapotranspiration, and snowmelt. The control specification specified the time span of the simulation. Control specifications included a starting date and time, ending date and time, and a time interval. Combining a basin model, meteorologic model, and control specifications formed a simulation run.

Rainfall-runoff simulation using HMS consisted of modeling four basic components of the hydrologic cycle. Each component was modeled using one of several available models. A few limitations on the model combinations are described in the HMS users manual (version 3.3, September 2008).

1. Models to compute runoff volume

a. Initial and constant-rate

b. SCS curve number (CN)

c. Gridded SCS CN

d. Green and Ampt

e. Deficit and constant rate

f. Soil moisture accounting (SMA)

g. Gridded SMA

2. Models of direct runoff (overland flow and interflow)

a. User-specified unit hydrograph (UH)

b. Clark’s UH

c. Snyder’s UH

d. SCS UH

e. ModClark

f. Kinematic wave

3. Models of baseflow

a. Constant monthly

b. Exponential recession

c. Linear reservoir

4. Models of channel flow

a. Kinematic wave

b. Lag

c. Modified Puls

d. Muskingum

e. Muskingum-Cunge Standard Section

f. Muskingum-cunge 8-point Section

g. Confluence

h. Bifurcation

All of the mathematical models in the HMS uses constant parameter values, are uncoupled and are deterministic. Hence, the parameters are assumed to be time stationary and the models are solved independently and every time a simulation is computed it will yield exactly same results computed in the previous run.

The basin model was used for physical representation of the watershed. Available elements in the basin model were: subbasin, reach, junction, reservoir, diversion, source, and sink. Computation proceeded from the upstream elements in a downstream direction. The meteorologic model performed meteorologic data analysis and included precipitation, evapotranspiration, and snowmelt. The control specification specified the time span of the simulation. Control specifications included a starting date and time, ending date and time, and a time interval. Combining a basin model, meteorologic model, and control specifications formed a simulation run.

Rainfall-runoff simulation using HMS consisted of modeling four basic components of the hydrologic cycle. Each component was modeled using one of several available models. A few limitations on the model combinations are described in the HMS users manual (version 3.3, September 2008).

1. Models to compute runoff volume

a. Initial and constant-rate

b. SCS curve number (CN)

c. Gridded SCS CN

d. Green and Ampt

e. Deficit and constant rate

f. Soil moisture accounting (SMA)

g. Gridded SMA

2. Models of direct runoff (overland flow and interflow)

a. User-specified unit hydrograph (UH)

b. Clark’s UH

c. Snyder’s UH

d. SCS UH

e. ModClark

f. Kinematic wave

3. Models of baseflow

a. Constant monthly

b. Exponential recession

c. Linear reservoir

4. Models of channel flow

a. Kinematic wave

b. Lag

c. Modified Puls

d. Muskingum

e. Muskingum-Cunge Standard Section

f. Muskingum-cunge 8-point Section

g. Confluence

h. Bifurcation

All of the mathematical models in the HMS uses constant parameter values, are uncoupled and are deterministic. Hence, the parameters are assumed to be time stationary and the models are solved independently and every time a simulation is computed it will yield exactly same results computed in the previous run.