Subbasin Loss Methods Subbasin Loss Methods HEC-HMS
Jan 13, 2016
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