Transcript Hydrologic Abstractions

Hydrologic Abstractions
Hydrologic Abstractions
Interception
 Evaporation
 Transpiration
 Depression
 Detention
 Infiltration
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INTERCEPTION
A portion of the rainfall is intercepted by
plant foliage, buildings, and other objects.
This water is not available for runoff.
 Interception typically removes about 0.5
mm during a single storm event. Values as
high as 1.5 mm have been reported.
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EVAPORATION
Evaporation is the change of the state of
water from liquid to vapor as a result of heat
addition.
 Evaporation from a body of water occurs
only if the surrounding air is not completely
saturated with water vapor, that is, if the
relative humidity is less than 100 percent.
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TRANSPIRATION
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Transpiration is the transfer of soil moisture from
the soil to the atmosphere by the action of
vegetation. Plants transpire water vapor through
their foliage.
Transpiration has minimal effect on individual
storms and is usually only taken into account in
long-term hydrologic budgets.
Evaporation and transpiration are commonly
lumped in one variable called evapotranspiration.
DEPRESSION STORAGE
Depression storage accounts for the water
that becomes ponded in land surface
irregularities.
 Depression storage depends on the land use
of the watershed and typically amounts to
0.5 to 8 mm during a single rain event. It is
inversely proportional to the watershed’s
slope
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DETENTION STORAGE
Detention storage is the volume of water
that moves as overland flow (sheet flow).
This volume is temporarily detained in
transit to the stream.
 Sheet flow storage depends on watershed
parameters including land use, vegetation,
slope, and rainfall intensity.
 Typical values range from 2.5 to 10 mm
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INFILTRATION
Infiltration takes place as part of the rain
percolates through the soil.
 The rate of infiltration depends on the soil
type, slope, vegetation, soil moisture
content, temperature, and the precipitation
intensity.
 Infiltration usually is the largest abstraction
and therefore has the most significant effect
on runoff.
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INFILTRATION
Infiltration rates generally decrease with
time as the rainfall proceeds and the soil
becomes saturated.
 Many infiltration models have been
developed from very simple to very
complex.
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Rainfall Intensity in/hr
Infiltration
Rainfall Excess
fi
-Index
Time in hours
Total Hydrologic Abstractions
The M-Index method lumps all of the
abstractions into an average infiltration rate.
 The Soil Conservation Service (SCS)
Curve-Number method allows for an initial
abstraction and then infiltration as a
function of soil type and land use.
 The Green-Ampt Infiltration equation
allows for the infiltration to become a
function of time.
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