Subbasin Loss MethodsHEC-HMS

Seven MethodsDeficit and ConstantGreen and AmptGridded SCS Curve numberGridded Soil Moisture AccountingInitial and ConstantSCS Curve NumberSoil Moisture Accounting

Green and AmptTheoryCombines unsaturated flow from Darcys law with requirements of mass conservationInitial loss is included to model interception and depression storageExcess precipitation is computed using Green and Ampt equations after initial loss is satisfied

Green and Ampt

InputInitial lossVolumetric moisture deficitWetting front suctionHydraulic conductivity

SCS Curve Number

TheoryEmpirical method developed by SCSEstimates excess precipitation as a function of cumulative precipitation, soil cover, land use, and antecedent moisture.EquationPe = (P-Ia)2 / (P -Ia + S)

SCS Curve Number

Equation parametersPe = Excess PrecipitationP = Accumulated rainfallS = Potential maximum retentionS = (25,400 254 * CN) / CNIa = Initial abstraction = 0.2 * SCN = Curve NumberCNcomposite = sum (Ai * CNi) / sum AiCN = 30 (very permeable) CN = 100 (impervious cover)

SCS Curve Number

Required inputInitial lossCurve number

Gridded SCS Curve Number

TheorySimilar to SCS curve number methodBasin areas are represented by grid cellsDatabase 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 doesComputes excess precipitation for each cell independently using SCS equation Routes excess to watershed outlet using the ModClark transform method

Gridded SCS Curve Number

Required inputInitial abstraction ratio (0.427 2.28)Potential retention scale factor (0.095 0.38)No percent imperviousness required with this loss method

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

Initial and Constant

Initial and ConstantIf 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 ConstantThe constant loss rate can be viewed as the ultimate infiltration capacity of the soils

Deficit and ConstantQuasi-continuous model of precipitation lossInitial loss can recover after a prolonged period of no rainfall

Deficit and Constant

Soil Moisture AccountingBasic 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 SMAThe 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 modelsInitial 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