2.3 Land degradation vis-à-vis land-use change
Over a considerable part of the globe land is produced by
partial consolidation of soil particles. Land degradation is the alteration and
spoiling of land, rendering it worthless for any useful purpose. Any human activity
that causes disintegration of these, triggers a procedure by which the fine soil
particles are lost through a series of processes namely, weathering, erosion and
transportation. This contributes to loss of lands quality and pushes it towards
land degradation. If land under natural use (say forest), is transferred to human
use (say excavation for any purpose or agriculture), or even left unused after
cutting out the green cover, the land gradually enters into the process of degradation.
The human activities which are instrumental in creating land degradation are those
concerned with disintegration of soil particles, exposing those to natural agents
of weathering erosion etc., and disfiguring the land surface. The major such activities
are deforestation (or damage to greenery) and excavation or disintegration of
land, while weathering, erosion and sedimentation are the effective processes.
2.3.1 Effects of deforestation
About 60,000 km2 of land all over the world is annually falling victim to the
process of desertification (Valdiya, 1987, pp.7).
Forests act as climatic stabilisers, inhibit flooding and sedimentation. Cooling
capacity of one full grown tree equals that of five average air conditioners operating
20 hours a day (Coates, 1981, pp. 512). Thus decrease in forest increases temperature
and disturbs the process that results rain. Tree roots provide extra shear strength
to the soils to aid in resisting gravity movement and surface erosion. This influence
may last upto 3 to 5 years after cutting the trees. The forest canopy acts to
dampen raindrop impacts, and the trees ground litter further reduces erosion potential.
The decaying vegetal mater enriches the nutrients in the topsoil horizon and increases
permeability to allow infiltration of water through it. Thus direct run-off is
minimised and sheet flooding is prevented.
Deforestation eliminates or reduces the number of benefits that forests produce.
Results of tree felling includes:
* increased erosion and soil loss
* increased downslope sedimentation with all its accompanying damages such as
more frequent flooding and silting of reservoirs
* upset of hydrologic regime with lowering of water table
* loss of soil nutrients
* destruction of wildlife habitats and
* reduction of aesthetic qualities
Deforestation has been shown (Coates, op.cit) to lower the minimum temperature
by 1° - 7°F in all seasons and to increase the maximum temperature by 1° - 3°F
in cold seasons. Humidity decreases by 2% to 25% and rainfall reductions by 1%
to 10% have also been documented in deforested areas.
Sediment yield from forests is generally only a fraction of what it is for other
land uses. Studies (Coates, op.cit.) have shown the following sediment yields
on an average from areas of alike climatic, geological and topographic conditions.
From forested areas @ 0.031 t/ha
From urban-suburban areas @ 0.31 t/ha to 0.63 t/ha
From farm land @ 0.78 t/ha to 3.1 t/ha
From land undergoing construction @ 16.5 t/ha to 31 t/ha
Another set of data given next shows soil yield from wooded areas which indicate
capacity of greencover to prevent soil loss.
with 20% greencover @ 0.252 t/ha
with 80% green cover @ 0.028 t/ha
That erosion increases with increase in slope is clear from the following data
of sediment production
forested lands with 10% slope produce on an average 0.001 t/ha. while the same
land with 20% slope produce on an average 0.004 t/ha. In 1969 the land-use pattern
of united states was
forest etc. 32%
cropland 21%
pasture 27%
urban use 8%
others (desert, swamp, tundras etc). 12%
It was reported in 1981 that the country was loosing about 4 billion (b) tons
(t) of soil annually compared to 3 bt in 1934 (Coates 1981, pp.507) and the country
started getting its forest cover declined.
Some studies on GLASOD has revealed that the globe has lost 22% of its green cover
(forest, cropland, pasture, etc.) since mid century, (Anon 2000, pp.3).