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

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IMPACT OF SOCIAL DARWINISM ON GEOGRAPHY

IMPACT OF SOCIAL DARWINISM ON GEOGRAPHY

                                                                                                                                Laltanpuia

                                                                                                                                Roll No. – 22
                                                                                                                                M.Sc
                                                                                                                                Dept. Of Geography
                                                                                                                                Mizoram University

Social Darwinism is an ideology of society that seeks to apply biological concepts of Darwinism or of evolutionary theory to sociology and politics, often with the assumption that conflict between groups in society leads to social progress as superior groups out compete inferior ones. The term Darwinism had been coined by Thomas Henry Huxley in his April 1860 review of On the Origin of Species, and by the 1870s it was used to describe a range of concepts of evolutionism or development, without any specific commitment to Charles Darwin's own theory. The first use of the phrase "social Darwinism" was in Joseph Fisher's 1877 article on The History of Landholding in Ireland which was published in the Transactions of the Royal Historical Society. Fisher was commenting on how a system for borrowing livestock which had been called "tenure" had led to the false impression that the early Irish had already evolved or developed land tenure. Despite the fact that Social Darwinism bears Charles Darwin's name, it is also linked today with others, notably Herbert Spencer, Thomas Malthus, and Francis Galton, the founder of eugenics. In fact, Spencer was not described as a social Darwinist until the 1930s, long after his death. Darwin himself gave serious consideration to Galton's work, but considered the ideas of "hereditary improvement" impractical. Aware of weaknesses in his own family, Darwin was sure that families would naturally refuse such selection and wreck the scheme. He thought that even if compulsory registration was the only way to improve the human race, this illiberal idea would be unacceptable, and it would be better to publicize the "principle of inheritance" and let people decide for themselves.

Herbert Spencer, a 19th century philosopher, promoted the idea of Social Darwinism. Social Darwinism is an application of the theory of natural selection to social, political, and economic issues. In its simplest form, Social Darwinism follows the mantra of "the strong survive," including human issues. This theory was used to promote the idea that the white European race was superior to others, and therefore, destined to rule over them. At the time that Spencer began to promote Social Darwinism, the technology, economy, and government of the "White European" was advanced in comparison to that of other cultures. Looking at this apparent advantage, as well as the economic and military structures, some argued that natural selection was playing out, and that the race more suited to survival was winning. Some even extended this philosophy into a micro-economic issue, claiming that social welfare programs that helped the poor and disadvantaged were contrary to nature itself. Those who reject any and all forms of charity or governmental welfare often use arguments rooted in Social Darwinism.

The name Social Darwinism is a modern name given to the various theories of society that emerged in England and the United States in the 1870s, which, it is alleged, sought to apply biological concepts to sociology and politics. The term social Darwinism gained widespread currency when used in 1944 to oppose these earlier concepts. Today, because of the negative connotations of the theory of social Darwinism, especially after the atrocities of the Second World War (including the Holocaust), few people would describe themselves as Social Darwinist and the term is generally seen as pejorative. In The Descent of Man, and Selection in Relation to Sex of 1882, Darwin described how medical advances meant that the weaker were able to survive and have families, and as he commented on the effects of this, he cautioned that hard reason should not override sympathy and considered how other factors might reduce the effect.

At its worst, the implications of Social Darwinism were used as scientific justification for the Holocaust. The Nazis claimed that the murder of Jews in World War II was an example of cleaning out the inferior genetics. Many philosophers noted evolutionary echoes in Hitler's march to exterminate an entire race of people. Various other dictators and criminals have claimed the cause of Social Darwinism in carrying out their acts. Even without such actions, Social Darwinism has proven to be a false and dangerous philosophy. Scientists and evolutionists maintain that this interpretation is only loosely based on Darwin's theory of natural selection. They will admit to an obvious parallel between Darwin's theory of Natural Selection and Spencer's beliefs. In nature, the strong survive and those best suited to survival will out-live the weak. According to Social Darwinism, those with strength (economic, physical, technological) flourish and those without are destined for extinction.
It is important to note that Darwin did not extend his theories to a social or economic level, nor are any credible evolutionists subscribing to the theories of Social Darwinism. Herbert Spencer's philosophy is only loosely based on the premises of Darwin's work. However, according to evolutionary theory, nature is a "kill-or-be-killed" system. Those that cannot keep up are either left behind or cut off. If evolution, through chance, is solely responsible for life as we now know it, why should that process be countered? If "survival of the fittest" or "kill or be killed" cannot apply in what we define as "decent society," then, which is wrong, society or evolution? If neither, then how do we explain morality, charity, and compassion? Why drain resources from the strong to support the weak? Certainly, we should be charitable and help those in need. Though Darwin did not promote Social Darwinism, basic evolutionary theory raises some nagging questions.

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Rizzle Kicks

Back when I was younger, I wanted to be everything on the planet (on the, on the, on the planet) Now that I am older, it seems the ambition has vanished.

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Posted in: NEPNAWI