interception - hydrology

SAN RAMON, RAYMART ALFEREZ

BS in Civil Engineering

INTERCEPTION

• There are three basic locations of

water storage that occur in the

planetary water cycle. Water is

stored in the atmosphere; water is

stored on the surface of the earth,

and water stored in the ground.

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.

Additionally, interception of water on the ground surface during freezing and sub-freezing

conditions can be substantial. The interception of falling snow and ice on vegetation also

occurs. The highest level of interception occurs when it snows on conifer forests and hardwood

forests that have not yet lost their leaves.

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 tree

(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

Interception losses generally occur during the first part of a precipitation event and the

interception loss rate trends toward zero rather quickly.



Interception losses are described by the following equation (Horton reprinted by Viessman

1996):

Li = S + KEt

In the above equation, Li is the total volume of water intercepted, P = precipitation

e = base of natural logarithms

S is the interception storage, K is the ratio of the surface area of the leaves to the area of the entire canopy, E is the rate of evaporation during the precipitation event and t is time. This equation assumes that the precipitation is enough to satisfy the storage on the vegetation.

Interception can also be related back to the precipitation event with the following equation (Brooks 2003):

Li = Pg – Th – Sf In the above equation, Li is the canopy interception loss, Pg is the gross precipitation, Th is the throughfall Sf is the stemflow.

Assume that a rainfall event of intensity 1.25 cm/h falls over a uniformly forested

watershed of area 20 km2. If the only water losses during this event are those due to interception,

compute the volume of water that leaves the basin as storm runoff for a 1-hour and a 2-hour

rainfall. Interception volume as a function of precipitation volume P is given by,

(4)

Assume that K is 1.5, S is 0.2 cm and that the evaporation rate is zero.

a. Compute total precipitation volume:

P = (1.25 cm/h) (1 h) = 1.25 cm

P = (1.25 cm/h) (2 h) = 2.5 cm



Use equation 4 to obtain:

Li = (0.2 cm) (1 - exp(-1.25/0.2)) = 0.1996 cm

Li = (0.2 cm) (1 - exp(-2.5/0.2)) = 0.2 cm

Assuming that there is no change in basin storage, then the output of the basin is equal to:

Volume of Output = (P - Li)*Abasin = ((1.25 cm - 0.1996 cm)/100 cm/m ) (20 106 m

2) =

210,080.0 m3

Volume of Output = (P - Li)*Abasin = ((2.50 cm - 0.2 cm)/100 cm/m ) (20 106 m

2) =

460,000.0 m3

method 1

method 2



Stemflow is the portion of precipitation that is slowed by leaves and branches and then slowly

flows to the ground along the tree itself. Stemflow is not an abstraction itself; however,

stemflow can be abstracted through infiltration or depression storage.

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.

• As the Horton equation suggests, the total interception is dependent on the storm

duration, as longer duration storms allow more evaporation from the canopy during the

storm event. The intensity of the storm also plays a role in canopy interception

(Viessman 1996); however, there is debate as to whether intensity increases or

decreases interception storage in canopy (Keim 2003).

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 the modeler to assess carefully both the

time frame of the model and the volume of precipitation with which one must deal.

interception - hydrology

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