Evapotranspiration and related calculations for stormwater biofiltration devices; proposed calculation scenario and data

Robert Pitt, John Voorhees and Shirley Clark


When modeling the water and pollutant mass balances for stormwater biofiltration devices, numerous elements must be considered. Obviously, soil characteristics affecting infiltration and pollutant retention is critical. One of the key benefits of biofiltration devices, however, compared to many other stormwater controls is the aspect of additional features associated with plants. Evapotranspiration of water from the saturated soils between events needs to be considered when conducting mass balances of moisture in biofiltration devices. Different soils, plants, and rainfall characteristics all dramatically affect how important the different mechanisms are for a specific design. This paper will outline the calculation scenario being implemented in WinSLAMM, the Source Loading and Management Model for biofiltration devices, stressing the available data and site specific information needs.

The following is a brief listing of the data needs for a continuous biofilter model evaluation:

a) pore volume of soil (porosity) (fraction of voids to total soil mass)

b) root depth (feet)

c) field moisture capacity for soil (% of dry weight of soil)

d) permanent wilting point (% of dry weight of soil; varies for different soil textures)

e) reference monthly ETo values (average inches per day for each month)

f) crop factor for actual crop compared to reference ET values (decimal multiplier)

g) initial soil moisture conditions at beginning of study period

h) supplemental irrigation to be used.

i) soil layer depths (feet).

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