HEC-HMS Subbasin Elements: Defining Your Watershed

Subbasins are the fundamental building blocks of HEC-HMS hydrologic models. They represent land areas that receive precipitation and generate runoff. Understanding how to properly configure subbasin elements is essential for accurate watershed simulation.

What Is a Subbasin Element?

A subbasin in HEC-HMS represents a land surface area that:

• Receives precipitation from the meteorologic model
• Computes losses (infiltration, interception, depression storage)
• Transforms excess precipitation into a runoff hydrograph
• May include baseflow from groundwater contributions
• Discharges to a downstream element (junction, reach, reservoir, or sink)

Subbasin Properties Overview

Every subbasin element requires configuration of several key properties:

Area and Units

The area property defines the size of the drainage area contributing to the subbasin.

Area Units by Unit System

Determining Subbasin Area

Methods for calculating drainage area:

1. GIS Analysis: Delineate watershed from DEM using HEC-GeoHMS or ArcGIS
2. Topographic Maps: Trace drainage divides on contour maps
3. Survey Data: Use site-specific survey information
4. Planimeter: Measure area on scaled maps

Loss Methods Overview

Loss methods determine how much precipitation infiltrates into the ground, is intercepted by vegetation, or is stored in surface depressions. The remaining precipitation becomes excess precipitation that generates runoff.

HEC-HMS offers several loss methods:

Initial and Constant Loss

The simplest loss method with two parameters:

Where:

• Ia = Initial abstraction (inches or mm)
• f = Constant loss rate (inches/hour or mm/hour)
• t = Time

Use when:

• Limited soil data available
• Rough preliminary analysis
• Calibrating to observed data

SCS Curve Number Loss

The most widely used loss method, based on NRCS (formerly SCS) methodology.

Where:

• Pe = Excess precipitation (runoff)
• P = Total precipitation
• Ia = Initial abstraction (typically 0.2S)
• S = Potential maximum retention

Curve Number Reference Table

Green-Ampt Loss

A physically-based infiltration method using soil hydraulic properties:

Where:

• f = Infiltration rate
• Ks = Saturated hydraulic conductivity
• psi = Wetting front suction head
• theta = Soil moisture deficit
• F = Cumulative infiltration

Use when:

• Soil physical properties are known
• Modeling infiltration physics is important
• Site-specific soil data is available

Deficit and Constant Loss

An extension of Initial and Constant for continuous simulation:

Use when:

• Running continuous simulations over weeks or months
• Soil moisture variation between storms matters
• Recovery of infiltration capacity is important

Transform Methods Overview

Transform methods convert excess precipitation (after losses) into a runoff hydrograph at the subbasin outlet. The relationship between precipitation and discharge involves timing, attenuation, and hydrograph shape.

SCS Unit Hydrograph

The most commonly used transform method, based on the SCS dimensionless unit hydrograph.

The SCS dimensionless unit hydrograph has a peak at:

Where:

• Tp = Time to peak
• D = Computational time step
• tlag = Subbasin lag time

Estimating Lag Time

The SCS lag time equation:

Where:

• L = Hydraulic length (feet)
• S = (1000/CN) - 10
• Y = Average watershed slope (percent)

Alternative relationship with time of concentration:

Use our Time of Concentration Calculator to estimate Tc.

Clark Unit Hydrograph

The Clark method uses two parameters to describe watershed response:

1. Time of Concentration (Tc): Time for water to travel from the most distant point to the outlet
2. Storage Coefficient (R): Linear reservoir coefficient representing storage effects

Use when:

• Time-area relationship is known or can be derived
• More control over hydrograph shape is needed
• Watersheds with significant storage effects

Snyder Unit Hydrograph

A traditional empirical method using two parameters:

Use when:

• Maintaining consistency with previous studies
• Regional Snyder coefficients are available
• Traditional USACE analysis required

ModClark Transform

A distributed version of the Clark method that uses gridded time-area relationships:

• Accounts for spatial distribution of excess precipitation
• Useful with radar rainfall data
• Requires GIS-derived grid cell properties

Use when:

• Using gridded (radar) precipitation
• Spatial variability within subbasin is important
• High-resolution analysis is required

Baseflow Methods Overview

Baseflow represents the contribution of groundwater discharge to stream flow. For event-based modeling, baseflow may be omitted. For continuous simulation or calibration to observed data, baseflow is important.

Constant Monthly

The simplest baseflow method - specify a constant flow for each month:

Use when:

• Long-term average baseflow is known
• Monthly variation is significant
• Quick estimation is needed

Linear Reservoir

Models groundwater as a linear reservoir that releases water proportional to storage:

Where:

• Qbf = Baseflow discharge
• G = Groundwater coefficient
• Sgw = Groundwater storage

Use when:

• Groundwater-surface water interaction is important
• Calibrating to observed streamflow
• Continuous simulation with baseflow variation

Recession Method

Models baseflow recession using an exponential decay function:

Where:

• Qt = Baseflow at time t
• Q0 = Initial baseflow
• k = Recession constant (0.8 - 0.98)

Typical Parameter Values by Land Use

Urban/Developed Areas

Rural/Agricultural Areas

Entering Parameters Correctly

Step-by-Step Configuration

1. Select the subbasin in the Watershed Explorer
2. Set the Area property in the Component Editor
3. Choose a Loss Method from the dropdown
4. Enter loss parameters in the expanded section
5. Choose a Transform Method
6. Enter transform parameters
7. Choose a Baseflow Method (or None)
8. Enter baseflow parameters if applicable
9. Set the Downstream connection

Parameter Entry Tips

• All parameters must use consistent units within the project
• Some parameters have typical ranges - values outside these may indicate errors
• Use the ellipsis button (…) to access sub-editors for complex parameters
• Hover over parameter names for tooltips with descriptions

Common Mistakes and How to Avoid Them

Incorrect Curve Number

Problem: CN values not appropriate for land use and soil type Solution: Use NRCS tables or our calculator; verify soil group from soil survey

Wrong Lag Time Units

Problem: Lag time entered in wrong units (hours vs. minutes) Solution: Check unit system; typical urban lag is 15-60 minutes, rural is 1-10 hours

Unrealistic Initial Abstraction

Problem: Ia too large, resulting in no runoff for small storms Solution: Use Ia = 0.2S or Ia = 0.05S (more recent research suggests 0.05)

Missing Downstream Connection

Problem: Subbasin not connected to downstream element Solution: Always verify Downstream property is set to valid element

Impervious Area Double-Counting

Problem: Impervious already in CN but also entered separately Solution: If CN includes impervious, set Impervious parameter to 0

Subbasin Delineation Guidelines

When to Create Separate Subbasins

Create separate subbasins when:

• Land use differs significantly: Urban vs. rural areas
• Soil types vary: Different hydrologic soil groups
• Outlets differ: Different discharge points in the system
• Calibration points exist: Gage locations within watershed
• Timing varies: Significantly different lag times

Subbasin Size Recommendations

Lumping vs. Distributing

Lumped approach (fewer, larger subbasins):

• Simpler model
• Less data requirements
• May miss spatial variation

Distributed approach (more, smaller subbasins):

• Better spatial representation
• More data requirements
• More complex model maintenance

Next Steps

Now that you understand subbasin elements:

1. Learn calibration: Model Calibration
2. Calculate curve numbers: SCS Curve Number Calculator
3. Review your model: Your First HEC-HMS Model

References

1. U.S. Army Corps of Engineers, Hydrologic Engineering Center. (2024). HEC-HMS User’s Manual. Davis, CA: USACE.

2. U.S. Army Corps of Engineers, Hydrologic Engineering Center. (2024). HEC-HMS Technical Reference Manual. Davis, CA: USACE.

3. Natural Resources Conservation Service. (1986). Urban Hydrology for Small Watersheds, TR-55. Washington, DC: USDA.

4. Natural Resources Conservation Service. (2004). National Engineering Handbook, Part 630: Hydrology. Washington, DC: USDA.

Summary

Subbasins are the hydrologic engine of HEC-HMS models. Key takeaways:

• Area defines the drainage area contributing to each subbasin
• Loss methods (SCS CN, Green-Ampt) determine how much precipitation infiltrates
• Transform methods (SCS UH, Clark) convert excess precipitation to a hydrograph
• Baseflow methods add groundwater contribution (optional for event modeling)
• Curve Number is the most important parameter for most analyses

Accurate subbasin configuration is essential for reliable HEC-HMS results. Take time to properly characterize land use, soils, and watershed geometry before moving to routing and simulation.