Soil erosion is the removal of top soil from its resting place by responsible to accelerate soil erosion. arious-physical agencies like wind, water and ice. Several biotic factors id in loosening the top soil for the action of physical agencies. Therefore, depending upon the factors involved, there are two types of erosion- geological and accelerated.
Geological or Normal Erosion : The top soil removed by physical agencies of water or wind under normal conditions of physical, biological and hydrological equilibria. The rate of soil erosion is slow. An equilibirum is maintained between the removal of the topsoil and the formation of new soil from below through weathering. The geological or nomal erosion often produced a wavy or undulating surface with alternate ridges and depressions.
Accelerated Erosion : The removal of top soil occur at a much faster rate as compared to the formation of new soil from below. It result in the permanent loss of fertile part of the soil. Accelerated erosion occurs due to both physical and biotic factors. Amongst the most important physical factors are excessive rainfall and drought. Earth quakes and land slides also promote erosion. the biotic factors are mostly created by human interference. The various factors and processes which accelerate erosion are as follows :
(1) Heavy Rainfall : In India heavy rainfall occurs during the monsoons. It may be as high as 15 cm/day. Such an amount of rainfall cannot be absorbed by normal soils. The excess rain water accumulates on the soil. It puddles the soil and fills up the soil pores, so that soil permeability is further reduced. The beating of rain drops against the wet and clogged soils loosens the soilparticles from the upper part. They get suspended in water and are lost as the water flows along the depressions. On the slopes the excess rainfall is even more dangerous. As it passes along the slope, it gathers momentum and develops a high cutting and carying capacity. it is estimated that in India about 2,60,000 million tons of soil are annually displaced and dispersed in irigation channels, reservoirs, river beds and sea.
(2) Aridity : It is a major factor in the loss of vegetation and loosening of the top soil. According to Legris and Meher Homji (1975) Rajasthan has' suffered 5 arid phases since stone age. Each arid phase destroyed the previous mesophytic vegetation and laid the area bare for growth of sparse sclerophyllous vegetation and action of wind.
(3) Overgrazing : India has a large population of grazing animals (over 360 million). Grazing is intense on the outskirts of villages and the edges of forests. The migratory herdsmen take the animals to mountain tops during summer and bring them to foothills during winter. Overgrazing destroys the palatable specles, trampling kills many herbs and seedlings. It also causes compaction of soil, thus decreasing its porosity and permeability.
(4) Human Interfer ice: It is the major biotic factor since human activity influence the rest of biosphere. For example, excessive hunting of carnivorous animals as increased the population of herbivorous animals in
the forests of Madhya Pradesh. They have not only damaged the forests
but have also spoiled crops in the nearby villages.
Scraping and removing of litter from the forest floor are two human activities which lay the ground bare and expose it to agencies of erosion. Lopping and felling of trees in excess of natural regeneration destroy forest structure. Forests have been cleared in the past for producing
agricultural land and bulding of new industrial and residential complex.
Forest clearing through felling and buning has been carried out since the
beginning of agriculture by man. Mahabharata mentions the clearing of
Khandav forest situated between Jamuna and Gangas. Shifting or swidden
agriculture (slash and burn method) consisted of clearing forest and sowing
crops in the cleared areas for a few years. The practice is still in vogue in
parts of Assam (Pathak et.al, 1978). The cradles of the past civilization
have turned into modern day deserts because of over exploitation of natural resources and imigation channels they built for obtained water. The modem Iraq (then Mesopotamia) contains remains of several ancient
canals. Two of them are the largest of all time (Willcocks, 1911). Indus Valley Civilization and Sahara Civilizatiof had well irigated lands. However, irigation systems could not be maintained with the change of politfcal power. The system which was buill up lor increasing the productivity of the land, turmed the same into the most unproductive area.
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