1. Physical Factors

In general, the rate of soil erosion can be broadly taken to be a function of the amount and velocity of surface run off, considered in relation to certain physio-chemical properties inherent in the soil, together with the degree of friction offered by the vegetal covering over the land, Thus, the criteria in the study of the rate of soil erosion can be enumerated as follows.

(1) Climatic condition: amount, intensity and duration of rainfall. (2) Topography of the land, especially the degree and regularity of land-slope. (3) Drainage area, as determined by the size and shape of watershed is also the hydraulic characteristic of the catchment basin. (4) Physical and chemical properties of soil, determining the degree of resistance. (5) Nature and extent of vegetative covering, including the nature of cultivated crops.

1. Since run-off may proceed without erosion but in erosion without run-off, the nature of surface run-off may be taken as the supreme condition determining the rate of soil erosion. Surface run-off on the other hand, is a direct resultant of the climatic conditions. Thus, whether it is natural or man-induced, erosion finds different expressions from climate to climate, as to both the forms it assumes and the rate at which it occurs.

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Thus, in humid and tropical climates, excessive precipitation, high humidity and very low evaporation tend to promote decomposition of rocks and gully erosion on extensive scale; high temperature and heavy rainfall will induce heavy sheet erosion; desiccation and even semi-aridity together with occasional high winds would result in devastating wind erosion.

As to individual climatic factors, the precipitation comes first, since the source of all kinds of run-off lies in the precipitation. The amount of precipitation determines the volume of sub-soil flow, as also that of surface run-off that erodes the land. The nature of soil erosion and the amount of soil loss are in direct functional relationship with the amount, intensity and the duration of rainfall.

2. The topographic nature of the land, especially the degree of land slope has a profound influence on the rate of soil erosion. As the velocity of the run-off, running from the upper reaches of the watershed down to the point of egress is hydro-dynamically dependent upon the declivity of the land, it is obvious that the greater the slope of the land, the greater the velocity of run-off, the less the time available for sub-soil absorption, and greater the surface run-off will be in volume with increasing ravages of soil erosion.

3. The size and shape of watersheds together with the presence or absence of well- defined drainage channels determine, inter alia, the degree of soil erosion.

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4. With the increasing concern for the seriousness of soil erosion, laboratory investigations were carried out so as to assess the erosive nature of different soils. It was then noticed by the workers, that apart from the effect of declivity, slope-length and precipitation, certain other physico-chemical properties inherent in the soil affect greatly the rate of soil losses.

The texture of soil, in combination with its structure, determining the absorbing capacity of soil, is an important physical property in this connection.

In general, soils of coarse texture, being invariably associated with a single-grain structure involve high permeability and are less resistant to erosive force of run-off than fine-textured soils.

On the other hand, fine-textured soils of high colloidal content, being usually associated with granular or crumb-structure are very resistant to erosion, since very high velocity of run-off is required to dislodge the aggregated particles. Single-grained or deflocculated structure yields to erosion very easily, since the lighter individual particles are easily dispersed and dislodged by run-off water.

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5. The entire study of soil erosion would be perhaps incomplete without a proper recognition of the role of plant life in soil erosion. The importance of plant life lies in the fact that it affords the only natural protective cover over the soil profile against the erosive force of water and wind, and any indiscreet tampering with this natural floral cover with a conscious or unconscious mind, would result in the full fury of soil erosion and floods. Natural vegetative cover, such as forests, shrubs, grasses protect

the land surface from the erosive attack of rain and winds in the following ways: (a) Ground vegetation reduces the volume of precipitation by direct dispersion, interception and evaporation of rain drops falling on leaves and foliages, (b) It protects the soil from the violent impact of raindrops which tend to loosen the soil particles, (c) It increases the friction with the flow of run-off, thereby reducing the volume and velocity of run-off and lengthening the ‘time of concentration’ of rainfall, with consequent decrease in soil loss, (d) The knitting and binding effects of the intricate root systems make the soil resistant to erosion, (e) Decay of penetrating root systems have tubular cavities promoting infiltration rate, thereby reducing off-flowage. (f) Transpiration through the tissues and leaves of vast quantity of moisture from sub­soil layers.-(g) Abundant supply of organic matter from leaf-fall and litter improves soil structure, thereby greatly increasing the absorptive capacity of soil and reducing the surface run-off. The combined effect of all these phenomena is reflected in the very small volume and velocity of run-off, and insignificant amount of soil loss resulting from a given amount of rainfall in a forested area.

Prolonged investigations in the cultivated areas have revealed a number of facts of high significance, which are generally applicable to all parts of the earth.

(A) In the first place, it has been definitely observed that different crops are associated with different amounts of water and soil losses.

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Three facts can be taken to be responsible for this disparity of soil losses under different crops. Firstly, the varying degree of immunity afforded by the different crop-roots against the erosive forces; secondly, the varying lengths of period during which the land is left bare under different crops; and thirdly, the climatic environments favourable for raising different crops. Naturally, a crop, which keeps its fields exposed through greater number of days, is liable to be associated with very serious loss in soil. So, also heavy showers during the season when the land is under crop-stubles or young immature crops will result in devastating soil losses.

(B) In the second place, it has been observed that rising of a crop in rotation with others definitely minimizes the soil and water losses, while continuous cultivation of same crop in the same field is associated with serious soil erosion.3

Obviously, the protective covering throughout the greater length of time during a year, particularly during the seasons of intense rainfall, greater supply of organic matter through the stubles, roots and residues, partly improving the structure of soil and facilitating infiltration, account for the strikingly reduced soil and water losses under rotative cropping. Of course, the nature of rotation and the kinds of crops to be included in the cycle depend mainly on the climatic environment and nature of the soil.

(C) Lastly, the prevailing technique of cropping also has an important bearing on the soil and water loss, especially in hilly and rolling types of topography. Cultivation of row crops along the direction of slope, instead of along the contours, is invariably followed by tremendous loss of soil.

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(D) The best examples of this to be found in the steep slopes of Himalayan foot hills, Assam hills and in the Nilgiris, where abnormally high and rapid loss of soils from the potato fields is definitely due to the cultivation of potatoes in rows running up and down hill, instead of along the contours. In fact, much of the excessive loss of topsoil in the cultivated slopes of India is due to the faulty methods of raising crops without least regard to the prevailing slopes and to the absence of proper terracing and bunding.

(E) It has been proved experimentally that raising a row crop along the contour lines or growing a clean-tilled crop in strips alternating with a close growing crop, is particularly efficacious in minimizing soil losses from steep cultivated slopes.

In conclusion, it should be clearly conceded that the degree and extent of soil erosion is not due to a single factor, nor that all the factors work always in conjunction. As a matter of fact, the degree of soil erosion may be taken as a function of the amount and intensity of rainfall, slope and area of ground, physical and chemical properties of soils and the extent and nature of vegetative cover.

Economic Factors

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In addition to all these conditions under the specific climate, edaphic and vegetative environments, certain other economic factors, arising out of the traditional outlook, social economy and the economic structure of the community have some direct bearing with the soil erosion of the land. Of these, the system of farming, size of the farmer, tenancy, tenant-landlord relationship, heavy debt burden, lack of supplementary enterprises, etc. are too significant facts to be overlooked.