INTERCEPTION: Factors and Measurement

INTERCEPTION

Factors and Measurement

Ø  Laltanpuia

Ø  I Semester

Ø  Roll No. 22

Interception is the process in which water from precipitation is caught and store on a plant surface and eventually returned to the atmosphere without reaching the ground surface. On the other hand, Interception refers to precipitation that does not reach the soil, but is instead intercepted by the leaves and branches of plants and the forest floor. It occurs in the canopy (i.e. canopy interception), and in the forest floor. Because of evaporation, interception of liquid water generally leads to loss of that precipitation for the drainage basin, except for cases such as fog interception.

Once rain falls onto a vegetation canopy, it effectively partitions the water into separate modes of movement: throughfall, stemflow and interception loss.

Throughfall

This is the water that falls to the ground either directly, through gaps in the canopy, or indirectly, having dripped off leaves, stems or branches. The amount of direct throughfall is controlled by the canopy coverage for an area, a measure of which is the leaf area index (LAI). LAI is actually the ratio of leaf area to ground surface area and consequently has a value greater than one when there is more than one layer of leaf above the ground. When the LAI is less than one you would expect some direct throughfall to occur. When you shelter under a tree during a rainstorm you are trying to avoid the rainfall and direct throughfall. The greater the surface area of leaves above you, the more likely it is that you will avoid getting wet from direct throughfall.

The amount of indirect throughfall is also controlled by the LAI, in addition to the canopy storage capacity and the rainfall characteristics. Canopy storage capacity is the volume of water that can be held by the canopy before water starts dripping as indirect throughfall. The canopy storage capacity is controlled by the size of trees, plus the area and water-holding capacity of individual leaves. Rainfall characteristics are an important control on indirect throughfall as they dictate how quickly the canopy storage capacity is filled. Experience of standing under trees during a rainstorm should tell you that intensive rainfall quickly turns into indirect throughfall (i.e. you get wet!), whereas light showers frequently do not reach the ground surface at all. In reality canopy storage capacity is a rather nebulous concept. Canopy characteristics are constantly changing and it is rare for water on a canopy to fill up completely before creating indirect throughfall. This means that indirect throughfall occurs before the amount of rainfall equals the canopy storage capacity, making it difficult to gauge exactly what the storage capacity is.

Stemflow

Stemflow is the flow of intercepted water down the trunk or stem of a plant. Stemflow, along with throughfall, are responsible for the transferral of precipitation and nutrients from the canopy to the soil. In tropical rainforests, where this kind of flow can be substantial, erosion gullies can form at the base of the trunk. However, in more temperate climates stemflow levels are low and have little erosional power.

Stemflow is the rainfall that is intercepted by stems and branches and flows down the tree trunk into the soil. Although measurements of stemflow show that it is a small part of the hydrological cycle (normally 2–10 per cent of above canopy rainfall; Lee, 1980) it can have a much more significant role. Stemflow acts like a funnel, collecting water from a large area of canopy but delivering it to the soil in a much smaller area: the surface of the trunk at the base of a tree. This is most obvious for the deciduous oak-like tree, but it still applies for other structures (e.g. conifers) where the area of stemflow entry into the soil is far smaller than the canopy catchment area for rainfall. At the base of a tree it is possible for the water to rapidly enter the soil through flow along roots and other macropores surrounding the root structure. This can act as a rapid conduit of water sending a significant pulse into the soil water.

Interception loss

While water sits on the canopy, prior to indirect throughfall or stemflow, it is available for evaporation, referred to as interception loss. This is an evaporation process. The morphology of leaf and bark on a tree are important factors in the controlling quickly water drains towards the soil. If leaves are pointed upwards then there tends to be a rapid drainage of water towards the stem. Sometime this is a genetic strategy by a plant in order to harvest as much water as possible. Large broadleaved plant, such as oak (Quercus) tend to hold water well on their leaves while needled plants can hold less per leaf (although they normally have more leaves). Seasonal changes make a large difference within deciduous forests, with far greater interception losses when the trees have leaves than without. Durocher (1990) found that trees with smoother bark such as beech (Fagus) had higher rates of stemflow as the smoothness of bark tends to enhance drainage towards stemflow.

                Figure: Rainfall above and below a canopy. Indicated on the diagram are stemflow (white arrow on trunk); direct and indirect throughfall (lightly hatched arrow); and interception loss (upward facing darker arrow).

MEASUREMENT OF INTERCEPTION:

                The most common method accessing the amount of canopy interception is to measure the precipitation above and below a canopy and assume that the difference is from interception. Stated in this way it sounds a relatively simple task but in reality it is fraught with difficulty and error. Durocher (1990) provides a good example of the instrumentation necessary to measure canopy interception, in this case for a deciduous woodland plot.

               
Following is the measurement of Interception:

Throughfall:

Throughfall is the hardest part of the forest hydrological cycle to measure. This is because a forest canopy is normally variable in density and therefore, throughfall is spatially heterogeneous. One common method is to place numerous rain gauges on the forest floor in a random manner. If you are interested in a long-term study then it is reasonable to keep the throughfall gauges in fixed positions. In this way the throughfall catch should not be influenced by gauge position. To derive an average throughfall figure it is necessary to come up with a spatial average in the same manner as for areal rainfall estimates. To overcome the difficulty of a small sampling area (Rain gauge) measuring something notoriously variable (Throughfall), some investigators have used either troughs of plastic sheeting. Troughs collect over a greater area and have proved to be very effective. Plastic sheeting is the ultimate way of collecting throughfall over a large area, but has several inherent difficulties. The first is purely logistical in that it is difficult to install and maintain, particularly to make sure there are no rips. The second is that by having an impervious layer above the ground there is very little water or nothing entering the soil.

Stemflow:

                The normal method of measuring stemflow is to place collars around a tree trunk that capture all the water flowing down the trunk. On trees with smooth bark this may be relatively simple but is very difficult on rough bark such as found on many conifers. It is important that the collars are sealed to the tree so that no water can flow underneath and that they are large enough to hold all the water flowing down the trunk. The collars should be sloped to one side that the water can be collected or measured in a tipping-bucket rain gauge. Maintenance of the collars is very important as they easily clog up or become appropriate resting places for forest fauna such as Slugs!

Models for estimating canopy interception:

                As with evaporation, the main effort in estimating interception has been using numerical models. Regression models that link rainfall to interception loss based on a measured data set have been developed for many different types of vegetation canopy. Some of this models use logarithmic or exponential terms in the equations but they all rely on having regression coefficients based on vegetation type and climatic regime.

                A more detailed modelling approach in the Rutter model (Rutter et al., 1971, 1975) which calculates an hourly balance within a forest stand. The water balance is calculated taking into account the rate of throughfall, stemflow, interception loss through evaporation and canopy storage. In order to use the model a detailed knowledge of the canopy characteristics is required. In particular the canopy storage and drainage rates from throughfall are required to be known; the best method for deriving these is truth empirical measurement. The Rutter model treats the canopy as a single large leaf, although it has been adapted to provide a three dimensional canopy (e.g. Davie and Durocher, 1997) that can then be altered to allow for changes and growth in the canopy. At present, remote sensing techniques are not able to provide reasonable estimates of canopy interception. They do provide some use information that can be incorporated into canopy interception models but cannot provide the detailed difference between above-and below-canopy rainfall. In particular, satellites can give good information on the type of vegetation and its degree of cover. Particular care needs to be taken over the term ‘leaf area index’ when reading remote sensing literature. Analysis of remotely sensed images can provide a good indication of the percentage vegetation cover for an area, but this is not necessarily the same as leaf area index- although it is sometimes referred to as such. Leaf area index is the surface area of leaf cover above a define area divided by the surface area defined. As there are frequently layers of vegetation above the ground, the leaf area index frequently has a value higher than one. The percentage vegetation cover cannot exceed one (as a unitary percentage) as it does not consider the third dimension (height).

References:

Davie, Tim (2002): Fundamentals of Hydrology, Second Edition, Routledge

http://en.wikipedia.org/wiki/Interception_(water)

http://geography.geography-dictionary.org/Interception

http://en.wikipedia.org/wiki/Throughfall

http://en.wikipedia.org/wiki/Canopy_interception

http://en.wikipedia.org/wiki/Stemflow