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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

evaporation

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

schemes.

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

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.

Factors that serve to determine interception losses• Precipitation type,• rainfall intensity and duration,• wind• atmospheric conditions

affecting evaporation

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 themodeler to assess carefully both the time frame of the model and the volume of precipitation with

which one must deal.

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).

Equation 3.1 Equation 3.2

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 generalized

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.