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are particularly shallow. In cases where the water table is very high, it is often advisable
to avoid infiltration altogether.
Conventional Infiltration Concepts. Conventional approaches concentrate
on the infiltration capacity of a single end-of-pipe management facility such as a pond.
Infiltration potential elsewhere on the site is often discounted or only analyzed for its
effect on the flow of runoff into the facility. The conventional infiltration objective is to
concentrate flows in one area and then utilize the infiltration capacity of the natural soil
or conduits such as gravel. Natural groundwater flow patterns and recharge are often
not considered. Conventional approaches may result in the elimination of critical
volumes of flows to sensitive areas such as wetlands. Additionally, in many urban
areas, the high loads of fine sediments to centralized facilities and the impacts of
construction compaction can severely limit the infiltration capacity of the facility.
EVAPOTRANSPIRATION. Evapotranspiration is the loss of water from the
ground by evaporation and transpiration. Evaporation is the return of moisture to the
atmosphere from depressions, pond areas, or other surfaces. Transpiration is the
return of water to the atmosphere through plants; moisture is absorbed by the roots and
released through the leaves. The rate of evapotranspiration is dependent on air
temperature, humidity, wind speed, sunlight intensity, vegetation type, and soil
LID Evapotranspiration Concepts. LID designs use open areas and
vegetation to promote evapotranspiration. Larger areas used for evaporation, such as
ponds, should have a flow regime that controls mosquito breeding. LID designs should
not pond water for more than 72 hours as it may provide an opportunity for mosquitoes
to breed. By keeping surface areas small and shallow, water can quickly evaporate and
pollutants volatilize through plant uptake or evaporation.
LID designs also employ the capacity of vegetated areas to absorb, process,
volatilize, and treat non-point source pollution as well as atmospheric pollution.
Interception by leaves can significantly reduce the requirement for storage and
infiltration. A mature canopy can intercept a significant number of small-volume,
frequently occurring storms, absorbing precipitation into the plant leaves or evaporating
precipitation from the leaf surface.8 Additionally, uptake of soil moisture by plants helps
to maintain the soil's capacity to absorb rainfall.
Conventional Evaporation Concepts. Conventional stormwater
approaches are based on peak flow control over a short duration (usually 24 hours or
less). For these single event designs, the evaporation process is often discounted or