Subbasin Loss Methods HECHMS Seven Methods l Deficit

Subbasin Loss Methods HEC-HMS

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

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

SCS Curve Number l Theory – Empirical method developed by SCS – Estimates excess precipitation as a function of cumulative precipitation, soil cover, land use, and antecedent moisture. l Equation – Pe = (P-Ia)2 / (P – -Ia + S)

SCS Curve Number l Equation parameters Pe = Excess Precipitation l P = Accumulated rainfall l S = Potential maximum retention l S = (25, 400 – 254 * CN) / CN I = Initial abstraction = 0. 2 * S l CN = Curve Number l a CNcomposite = sum (Ai * CNi) / sum Ai • CN = 30 (very permeable) • CN = 100 (impervious cover)

SCS Curve Number l Required input – Initial loss – Curve number

Gridded SCS Curve Number l 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 l What HEC-HMS does – Computes excess precipitation for each cell independently using SCS equation – Routes excess to watershed outlet using the Mod. Clark transform method

Gridded SCS Curve Number l 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 l Basic Concepts and Equations - The maximum potential rate of precipitation loss is constant througout an event

Initial and Constant

Initial and Constant l If the watershed is in a saturated condition, Ia will approach zero l It is suggested that Ia ranges from 10 -20% of total rainfall forested areas to 0. 1 -0. 2 in for urban areas.

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

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

Deficit and Constant

Soil Moisture Accounting l 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 l 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 l 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

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

l 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