Subbasin Loss MethodsSubbasin Loss Methods

HEC-HMS

Seven MethodsSeven Methods

Deficit and ConstantGreen and AmptGridded SCS Curve numberGridded Soil Moisture AccountingInitial and ConstantSCS Curve NumberSoil Moisture Accounting

Green and AmptGreen and Ampt

Theory– Combines unsaturated flow from Darcy’s law

with requirements of mass conservation– Initial loss is included to model interception

and depression storage– Excess precipitation is computed using Green

and Ampt equations after initial loss is satisfied

Green and AmptGreen and Ampt

Input– Initial loss– Volumetric moisture deficit– Wetting front suction– Hydraulic conductivity

SCS Curve NumberSCS Curve Number

Theory– Empirical method developed by SCS– Estimates excess precipitation as a function of

cumulative precipitation, soil cover, land use, and antecedent moisture.

Equation– Pe = (P-Ia)2 / (P – -Ia + S)

SCS Curve NumberSCS Curve Number

Equation parameters Pe = Excess Precipitation P = Accumulated rainfall S = Potential maximum retention

S = (25,400 – 254 * CN) / CN Ia = Initial abstraction = 0.2 * S CN = Curve Number

CNcomposite = sum (Ai * CNi) / sum Ai

• CN = 30 (very permeable) • CN = 100 (impervious cover)

SCS Curve NumberSCS Curve Number

Required input– Initial loss– Curve number

Gridded SCS Curve NumberGridded SCS Curve Number

Theory– Similar to SCS curve number method– Basin areas are represented by grid cells– Database in HEC-HMS contains data on grid

cells including location of cell, travel distance from watershed outlet, cell size, cell CN

Gridded SCS Curve NumberGridded SCS Curve Number

What HEC-HMS does– Computes excess precipitation for each cell

independently using SCS equation – Routes excess to watershed outlet using the

ModClark transform method

Gridded SCS Curve NumberGridded SCS Curve Number

Required input– Initial abstraction ratio (0.427 – 2.28)– Potential retention scale factor (0.095 – 0.38)– No percent imperviousness required with this

loss method

Initial and ConstantInitial and Constant

Basic Concepts and Equations

- The maximum potential rate of precipitation loss is constant througout an event

ctat

ctat

at

ctt

fPandIP

fPandIP

IP

fPpe

,

,

0

0

Initial and ConstantInitial and Constant

Initial and ConstantInitial and Constant

If the watershed is in a saturated condition, Ia will approach zero

It is suggested that Ia ranges from 10-20% of total rainfall for forested areas to 0.1-0.2 in for urban areas.

Initial and ConstantInitial and Constant

The constant loss rate can be viewed as the ultimate infiltration capacity of the soils

Deficit and ConstantDeficit and ConstantQuasi-continuous model of precipitation

lossInitial loss can recover after a prolonged

period of no rainfall

Deficit and ConstantDeficit and Constant

Soil Moisture AccountingSoil Moisture Accounting

Basic Concept and Thory

- Continuous model that simulates both wet and dry weather behavior

- The SMA model represents the watershed with a series of storage layers

Soil Moisture AccountingSoil Moisture Accounting

Gridded SMAGridded SMA

The gridded soil moisture accounting method can be used to specify a SMA unit for each gridded cell

Gridded SMAGridded SMA

Pros and cons of HEC-HMS Pros and cons of HEC-HMS loss modelsloss models

Initial and constant rate - ‘Mature’ model that has been used

successfully. - Easy to set up and use - Model is parsimonious - Difficult to apply to ungaged area - Model may be too simple to predict losses

within event

Deficit and constant rate

Similar to initial and constant rateGeen and Ampt

- Parameters can be estimated for ungaged watersheds from information about soils

- Not widely used, not as much experience in professional community

SCS CN - Simple, predictable and stable - Relies on only one parameter - Well established, widely accepted - Predicted values not in accordance with classical

unsaturated flow theory - Rainfall intensity not considered - Infiltration rate will approach zero during a storm of

long duration - Default initial abstraction does not depend upon storm