Calculating credits for tree trenches and tree boxes

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The credit information provided here is summary in nature. More detailed information will be provided in spring, 2014.

Credits are discussed for volume, phosphorus and total suspended solids (TSS).

Volume credit

Volume credits for tree trenches and tree boxes includes

• water that infiltrates into the underlying soil,
• water that is intercepted by the tree canopy, and
• water that is taken up and evapotranspired by trees.

Infiltrated water

• For a tree BMP without an underdrain, the volume credit for infiltration equals the amount of water stored between soil saturation and field capacity.
• For a tree BMP with an underdrain, the volume credit for infiltration equals the amount of water stored between soil saturation and field capacity and stored below the underdrain.
• For a tree BMP with an underdrain at the bottom, the MIDS calculator includes a volume credit equal to the soil infiltration rate times the drawdown time. The default soil is a D soil with an infiltration rate of 0.06 inches per hour.

Interception

Water intercepted by a tree canopy may evaporate or be slowly released such that it does not contribute to stormwater runoff. An interception credit is given by a simplified value of the interception capacity (Ic), as presented by Breuer et al. (2003) for deciduous and coniferous tree species.

Ic coniferous = 0.087 inches (2.2 millimeters)Ic deciduous = 0.043 inches (1.1 millimeters)

Evapotranspiration

Two calculations are needed to determine the evapotranspiration (ET) credit. First is the volume of water available for ET. This equals the water stored above an underdrain between field capacity and the wilting point.

The second calculation is the theoretical ET. The theoretical volume of ET lost per day is given by (Lindsey and Bassuk, 1991)

\(ET = (CP) (LAI) (E_{rate}) (E_{ratio})\)

Where:

CP is the canopy projection area (square feet);

LAI is the Leaf Area Index;

E_rate is the evaporation rate per unit time (inches per day); and

E_ratio is the evaporation ratio.

The canopy projection area (CP) is the perceived tree canopy diameter at maturity and is given by

\(CP = Π (d/2)^2\)

where:

d is the diameter of the canopy as measured at the dripline (feet)

CP varies by tree species. Please refer to the Tree Species List for these values.

The leaf area index (LAI) should be stratified by type into either

• deciduous tree species (LAI = 4.7), or
• coniferous tree species (5.47).

These values are based on collected research for global leaf area from 1932-2000 (Scurlock, Asner and Gower 2002).

The evaporation rate (Erate) per unit time can be calculated using a pan evaporation rate for the givenarea, as available at NOAA. This should be estimated into per day value. The evaporation ratio (Eratio) is the equivalent that accounts for the efficiency of the leaves to transpire the available soil water or, alternately, the stomatal resistance of the canopy to transpiration and water movement. This is set at 0.20, or 20 percent based on research by Lindsey and Bassuk (1991). This means that a 1 square centimeter leaf transpires only about 1/5 as much as 1 square centimeter of pan surface.

It is recommended that calculations be based over a three day period.

To determine the credit, compare the volume of water available for ET to the theoretical ET over a 3 day period. The credit is the smaller of these two values.