Subbasin Loss Methods

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 • 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 0.1-0.2 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