RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 6

Interception

Evapo-
transpiration

Reduced energy/
erosive force

Drip from foliage

Stemflow Stemflow

Evaporation from
surface

Evaporation from
surface

Temp surface ponding Temp surface ponding Infiltration into soil

Uptake by root system for
evapotranspiration

Percolation into groundwater

Figure 1-4 The perfect LID measure for stormwater management: a tree.

Of course, not all infiltrating rainfall follows an identical pathway of movement
in the subsurface, and the complex layering of the soil in different horizons, each
with a very different permeability, can make this journey lengthy and circuitous.
Where highly impervious layers exist in the soil mantle, infiltrating rain will
move across this surface, again following the energy gradient. The underlying
bedrock also influences the speed and direction of groundwater movement, in
both the unsaturated zone and deep below the water table. If the underlying
rock is comprised of soluble carbonates, it includes open solution channels or
subsurface flow pathways that formed hundreds of millions of years ago and now
provide an underground river network, carrying the rainfall many miles from the