Definition, Measurement and Factors Affecting Interception

Interception can be defined as thetemporary storage and prevention of precipitation from reaching the ground surface by natural and artificial objects.

Ward et al (2000) defined interception as that segment of the gross precipitation input, which wets and adheres to above ground objects until it is returned to the atmosphere through evaporation.

When precipitation falls into a vegetated surface, only a part may actually reach the ground beneath.

Depending upon the nature and density of the vegetation cover, a proportion of the rain may be intercepted by the leaves and stems of the vegetation canopy and temporarily stored on its surfaces.

Some or all of this water may be evaporated back into the atmosphere, and so take no part in the land-bound portion of the hydrological cycle; this is termed the interception loss. The remaining water which reaches the ground constitutes the net rainfall.

The bulk of this comprises through fall consisting of raindrops that fall through spaces in the vegetation canopy and water which drips from wet leaves, twigs and stems to the ground surface; a generally much smaller amount of water trickles along twigs and branches to run down the main stem or trunk to the ground as stem flow.

Interception process is important because it can help to reduce runoff and erosion problems. Secondly, the net rainfall beneath a vegetation canopy is generally less than the gross rainfall falling onto the top of the vegetation canopy. In some cases the interception loss may be quite large and can have a significant impact on the water balance.

Factors Affecting Interception

Several factors affect the magnitude of interception. Some of these factors are: vegetation type and density, wind system, duration and nature of rain.

Very dense vegetation cover is known to intercept more water than a sparse one. In addition, the general canopy density affects the rate and magnitude of interception. Also, the dryness and wetness of the vegetation is very important. Dry vegetation has higher interception storage – the ability of the vegetation surfaces to collect and retain falling precipitation, than wet vegetation, because of the differences in their surface tension.

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The wind system and the duration of the rain also affect interception. The winds affect the rate of water lost through evaporation.

Apart from creating turbulence within the whole system, the duration is important because the longer the period of rainfall, the more water that is likely to be intercepted.

The nature of the rainfall is also important because the ability of the vegetation to intercept water is higher during low intensity rainfall than during storms.

Measurement of Interception

The most common method of measuring interception loss (I) in the field is to compute the difference between the precipitation above the vegetation layer (P) and the net precipitation below the vegetation canopy, comprising the through fall (T) and stem flow (S), thus:

I = P – T – S (2)

Due to the difficulties of installing equipment underneath a vegetation canopy, this method has been used more for forest vegetation than for lower-order covers.

Through fall may be measured using funnel or trough gauges placed beneath the forest canopy, and stem flow may be collected by small gutters sealed around the circumference of the truck leading into a container.

In summary, the journey of precipitation from the clouds to the ground is not a straight one as has been highlighted from the above discussions. Part of it is intercepted by both natural and artificial objects.

Hence, the net rain that reaches the ground surface is less than the gross rainfall, which has implications on surface runoff and water balance.

The temporary storage and prevention of precipitation from reaching the ground surface by natural and artificial objects is called interception.

Interception accounts for the variation between the gross rainfall and the net rainfall that reaches the ground.

Factors that influence the magnitude of interception include vegetation type and density, wind system, duration and nature of rain.