Subbasin Loss Methods HEC-HMS

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

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

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

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)

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)

SCS Curve Number Required input Initial loss Curve number

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

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

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

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

Initial and Constant

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.

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

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

Deficit and Constant

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

Soil Moisture Accounting

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

Gridded SMA

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

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

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