File:Tree trench schematic with interception.png

File:Tree trench schematic with interception.png

Stormwater stored beneath the underdrain will infiltrate into the underlying soil. Credit is given for the volume of water stored between saturation and field capacity of the media. Above an underdrain, water stored between saturation and field capacity of the media drains to the underdrain and is returned to the stormwater conveyance system. It therefore receives no credit.

Water held between field capacity and the wilting point of the media is available for uptake by the tree. This includes the entire thickness of the media regardless of the presence of an underdrain. Water taken up by the tree comprises a stormwater credit. Precipitation may also be intercepted by the tree and either evaporate or be slowly released such that it does not runoff into the conveyance system. This water therefore also comprises a credit. For specific calculations and information on soil properties, see the discussion in the section on calculating credits for tree BMPs.