WMS Infiltration and Evapotranspiration Considerations

Tools Infiltration 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=CiA Required 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