Environmental Engineering Reference
In-Depth Information
Rainfall Intensity
Rainfall Duration
Human Influences
Influences
on
Runoff
Soil Composition
Vegetation Cover
Soil Moisture
Ground Slope
Figure 4.1
Influence on runoff.
standpoint, this is generally undesirable, because quick runoff does not pro-
vide enough time for the water to infiltrate into the ground and recharge
groundwater aquifers. Also, surface water that quickly runs off land causes
erosion and flooding problems. Probably the only good thing that can be
said about surface water that runs off quickly is that it usually does not have
enough contact time to increase in mineral content. Surface water that drains
slowly off land has all the opposite effects.
Drainage basins collect surface water and direct it on its gravitationally
influenced path to the ocean. The drainage basin is normally character-
ized as an area measured in square miles, acres, or sections. Obviously, if
a community is drawing water from a surface water source, the size of its
drainage basin is an important consideration. Surface water runoff, like the
flow of electricity, flows or follows the path of least resistance. Surface water
within the drainage basin normally flows toward one primary watercourse
(e.g., river, stream, brook, creek), unless some manmade distribution system
(canal or pipeline) diverts the flow. Surface water runoff from land surfaces
depends on several factors (see Figure 4.1 ):
Rainfall duration —Even a light, gentle rain, if it lasts long enough,
can, with time, saturate soil and allow runoff to take place.
Rainfall intensity —With increases in intensity, the surface of the soil
quickly becomes saturated. This saturated soil can hold no more
water; as more rain falls and water builds up on the surface, surface
runoff is produced.
Soil moisture —The amount of existing moisture in the soil has a defi-
nite impact on surface water runoff. Soil already wet or saturated
from a previous rain causes surface water runoff to occur sooner
than if the soil were dry. Surface water runoff from frozen soil can
be as high as 100% of the snow melt or rain runoff because frozen
ground is basically impervious.
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