1. Subbasin Loss Methods
HEC-HMS

2. Seven Methods Deficit and Constant Green and Ampt
Gridded SCS Curve number
Gridded Soil Moisture Accounting
Initial and Constant
SCS Curve Number
Soil Moisture Accounting

3. Green 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

4. Green and Ampt Input Initial loss Volumetric moisture deficit
Wetting front suction
Hydraulic conductivity

5. SCS Curve Number Theory Equation 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)

6. SCS 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)

7. SCS Curve Number
Required input
Initial loss
Curve number

8. Gridded 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

9. Gridded 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

10. Gridded 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

11. Initial and Constant Basic Concepts and Equations
- The maximum potential rate of precipitation loss is constant througout an event

12. Initial and Constant

13. Initial 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 in for urban areas.

14. Initial and Constant
The constant loss rate can be viewed as the ultimate infiltration capacity of the soils

15. Deficit and Constant Quasi-continuous model of precipitation loss
Initial loss can recover after a prolonged period of no rainfall

16. Deficit and Constant

17. Soil 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

18. Soil Moisture Accounting

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

20. Gridded SMA

21. Pros and cons of HEC-HMS loss 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

22. Deficit and constant rate
Similar to initial and constant rate
Geen and Ampt
- Parameters can be estimated for ungaged watersheds from information about soils
- Not widely used, not as much experience in professional community

23. 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