WMS

Infiltration and Evapotranspiration Considerations

ToolsInfiltration HEC-1 Loss Methods Rational Method HSPF TR-20 & TR-55

Evapotranspiration CASC2D

All event based

HEC-1 LOSS METHODS

Uniform Exponential Green-Ampt Holtan SCS Curve Number

HEC-1 Input

Initial &Uniform Loss Rate

Initial loss (in) Uniform loss rate (in/hr) Useful when you can calibrate values

from an actual event Some tables in Hands on HEC-1

manual show loss rate relation to storm frequency, soil type, CN

Exponential Loss Method

Empirical method Relates loss rate to rainfall intensity and

accumulated losses Determined using optimization routines

with actual event data

Green-Ampt Initial loss Volumetric moisture deficit Wetting front suction K at natural saturation

– USDA tables– Soil type and land use coverages– Automated computation in WMS

Holtan

Use where agriculture is dominant use Growth index (infiltration capacity &

maturity of ground cover) Long term equilibrium loss rate Initial avail. soil moisture capacity (in)

– Typical values in Hands on HEC-1

SCS Curve Number

Initial Abstraction (based on CN) Curve Number

– Soil-type & land use data– Automated computation for composite CN

Composite Curve Number Calculations

RATIONAL METHOD Q=CiARequired parameters: C= runoff coefficient

– various standard tables, land use coverage I= rainfall intensity (in/hr)

– IDF curves, NOAA maps (avail on line) A= basin area (ac)

– automated computation in WMS tc = time of concentration (min)

– automated computation (several methods)

HSPF

Water Quality Model WMS interface can read in HSPF input

files created in other programs Soil type and land use data used in

computing non point source runoff

TR-55 Urban runoff model Parameters

– Basin area, CN, tc, rainfall data TR-20 is similar

– Not for urbanizing watersheds– Specify reservoir data

CASC2D Physically based 2D finite difference

rainfall/runoff model Uses rectangular grid cells to represent

distributed watershed and rainfall domains

Includes an advanced soil moisture accounting procedure based on Green-Ampt

CASC2D Evapotranspiration Options Deardorf

– Land surface albedo– Wilting point water content

Penman– Above parameters– Vegetation height– Vegetation radiation coefficient– Canopy stomatal resistance