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Interception can be defined as that
segment of the gross precipitation
input which wets and adheres to
aboveground objects until it is
returned to the atmosphere through
Precipitation striking vegetation may be
retained on leaves or blades of grass, flow
down the stems of plants and become stem
flow, or fall off the leaves to become part of the
through fall. The modifying effect that a forest
canopy can have on rainfall intensity at the
ground (the through fall) can be put to
practical use in watershed management
The amount of water intercepted is a function of:
(1) the storm character,
(2) the species, age and density of prevailing plants and trees,
(3) the season of the year.
Usually about 10-20 percent of the precipitation that
falls during the growing season is intercepted and
returned to the hydrologic cycle by evaporation.
Water losses by interception are especially
pronounced under dense closed forest stands-as
much as 25 percent of the total annual precipitation
Additional information given in Table 3.1 includes some data on interception
measurements obtained in Maine from a mature spruce-fir stand, a moderately
well stocked white and gray birch stand, and an improved pasture.
It is important to recognize that forms of vegetation
other than trees can also intercept large quantities of
water. Grasses, crops, and shrubs often have leaf-area
to ground-area ratios that are similar to those
for forests. Intercepted amounts are about the same
as those for forests, but since some of these types of
vegetation exist only until harvest, their annual
impact on interception
is generally less than that of forested areas
Table3.2 summarizes some observations that have been made on
crops during growing seasons and on a variety of grasses
Factors that serve to determine interception losses
• Precipitation type,
• rainfall intensity and duration,
• atmospheric conditions
Snow interception , while highly visible, usually is not a
major loss since much of the intercepted snowfall is
eventually transmitted to the ground by wind action
and melt. Interception during rainfall events is
commonly greater than for snowfall events. In both
cases, wind velocity is an important factor.
The importance of interception in hydrologic
modeling is tied to the purpose of the model.
Estimates of loss to gross precipitation through
interception can be significant in annual or long-term
models, but for heavy rainfalls during
individual storm events, accounting for interception
may be unnecessary. It is important for the
modeler to assess carefully both the time frame of
the model and the volume of precipitation with
which one must deal.
Equation 3.1 Equation 3.2
Equations3 .1 and 3.2 can be used to estimate total interception
losses but for detailed analysis of individual storms, it is
necessary to deal with the areal variability of such losses.
General equations for estimating such losses are not available,
however. Most research has been related to particular species or
experimental plots strongly associated with a given locality. In
addition, the loss function varies with the storm's character. If
adequate experimental data are available, the nature of the
variance of interception versus time might be inferred.
Otherwise, common practice is to deduct the estimated volume
entirely from the initial period of the storm( initial abstraction).
Precipitation that reaches the ground may infiltrate, flow over
the surface, or become trapped in numerous small
depressions from which the only escape is evaporation or
infiltration. The nature of depressions as well as their size, is
largely a function of the original land form and local land-use
practices. Because of extreme variability in the nature of
depressions and the paucity of sufficient measurements, no
relation with enough specified parameters for all cases is
feasible. A rational model
can, however, be suggested.
Figure 3.3 illustrates a plot of this function versus the mass
overland flow and depression storage supply( P - F), where F is
the accumulated mass infiltration and P is the gross
precipitation. In the plot mean depths of 0.25 in. for turf and
0.0625 in. for pavements were assumed. Maximum depths were
0.50 and0 .125 in. respectively.
The figure also depicts the effect on estimated overland flow
supply rate, which is derived from the choice of the depression
storage model. Three models are shown in the figure: the first one
assumes that all depressions are full before over land flow begins.
For a turf area having depressions with a mean depth of 0.25 in.
The figure shows that for P - F values less than 0.25 in., there is
no overland flow supply, while for P - F values greater than 0.25
in., the overland flow supply is equal to i - f .
Depression storage deductions are usually made from
the first part of the storm as illustrated in Fig. 3.2. The
amount to be deducted is a function of topography,
ground cover, and extent and type of land development.
During major storms this loss is often considered to be
negligible. Some guidelines for estimating depression
storage losses have been developed based on studies of
experimental and other watershed.
Values for depression storage losses from intense storms
reported by Hicks are 0.20 in. for sand, 0 .15 in. for
loam and 0.10 in. for clay. Tholin and Kiefer have used
values of 0.25 in. in pervious urban areas and 0.0625 in.
for pavements. Studies of four small impervious
drainage areas by Viessman yielded the information
shown in Fig.3.4, where mean depression storage loss is
highly correlated with slope. This is easily understood
since a given depression will hold its maximum volume
if horizontally oriented
Using very limited data from a small, paved-street section, Turner
revised the curves shown in Fig. 3.5. Other sources of data related to
surface storage are available in the literature.