Soil erosion is one of the most destructive phenomena of land degradation. There is loss of not only water and plant nutrients but ultimately soil itself is lost, which, in turn, affects crop productivity. The soil finds its way into beds of streams and rivers, thus reducing their capacity to contain water which causes floods. We will consider the effects of land degradation one by one,
Surface Runoff and Floods
Most of the soil is lost through the surface runoff. Let us find out what surface runoff means. The precipitation or rainfall of the area, that is discharged from the area through stream channels and is thus lost without entering the soil, is called surface runoff. Surface runoff reduces or even prevents percolation of water into the ground. Runoff levels vary greatly from region to region and from soil to soil. In some humid regions, losses are as high as 50-60% of the annual rainfall. While annual runoff losses are much lower in semi-arid and arid regions, high rates of loss are not unusual during heavy storms, which are common in these regions. Runoff also increases sedimentation and flooding,
In India the core of the problem of flooding lies in the Indo-Gangetic basin accounting for 60Mha of cultivated land and 60% of the total flood prone area in the country. The basin also supports 40% of India's population and vital industries and provides mineral resources of the country. On the other hand, the Brahmaputra basin is critical due to the fact that the floods there are more frequent and seriously affect all developmental activities. It is interesting to note that the cause of floods in the Indo-Gangetic basin is not only the excessive rainfall and snowmelt on the hills but also the inability of the right bank tributaries of the Ganges to drain in time and the inability of the vast alkaline area to absorb and use the onsite rainfall.
Floods may basically be due to unusually heavy rainfall over short durations. However, many of the human interventions, that adversely affect the ability of land surface and soil profile to absorb, hold and utilise rain water and make its delayed release as sub-surface runoff or interflows, are also the main contributory factors.
Desertification is a problem generally associated with arid lands of the country. While water stress and drought are experienced over larger areas including arid deserts, semi-arid and even areas having moderate annual rainfall, both for productivity as well as sound environment, the problem of these two categories would have to be looked into critically. The problem of desertification is the diminution or degradation of biological potential of land which ultimately may lead to desert like conditions. The process results in widespread deterioration of ecosystems with the loss of plant and animal production.
The areas subject to wind erosion and arid conditions is estimated at about 38.73 Mha including 7.03 Mha of cold arid areas. The hot deserts and arid areas are located in 7 states, namely, Rajas than, Gujarat, Haryana, Punjab, Andhra Pradesh, Karnataka, and Maharashtra. The erodible nature of soils coupled with strong winds and decreased natural vegetal cover has set in more degradation within the area already subject to desertification, even though danger of extension of desert may not be there.
Loss of Nutrients and Land Productivity
The most significant effect of soil erosion is in the form of loss of soil as washoff. An estimated 6,000 Mt of soil is lost annually from the Indian subcontinent. Annual rate of soil loss into the sea through erosion is 5mm. This eroded material carries several valuable nutrients along With it, which are lost forever. Estimates about the quantum of such losses in terras of major plant nutrients (NPKJ is reported to be ranging from 5.37 to 8.4 Mt per year.
Several nutrients are lost during Hoods due to surface runoff and also due to leaching. In the regions where water percolation is high, the potentiality for leaching is also high. Soil properties also have a definite effect on nutrient-leaching losses. There is a greater nutrient loss in sandy soil than clay, because of higher rate of percolation and lower nutrient absorbing power of the sandy soil. Thus, in sandy soil, the nutrients in the top soil are lost due to wind erosion and also due to more rapid leaching.
Intensive land management under any of the primary production system, except multi-tiered forest, invariably leads to exhaustion of the land. Definite care is needed to put back the lost energy through organic sources. Soil erosion and land degradation through various means further affect the physical viability of the soil profile and the environment in terms of solid, water, air ratio as well as chemical and biological balances. These, in turn diminish the availability of nutrients and water to the plants and ultimately the productivity of the land.
A study of the potential population-supporting capacity of an area indicates that continuing soil erosion would render nearly 33% of the area insignificant in terms of productivity; while production in such eroded areas will fall by about 36%. Therefore, inspite of the increasing total productivity of the irrigated lands in 16 countries, including India, as studied, our country's overall production is likely to drop by about 12%.
When soil conservation is allowed to proceed in excess of permissible soil loss (here I2t/ha/year) over the years, the lands are relegated to lower productivity and finally to zero productivity. The land is classified into eight productivity classes on their capability of cultivation and need for conservation. The eight productivity classes are: I. Suitable for cultivation, II. Requires good soil management practice, III. Moderate conservation practice necessary, IV. Intensive conservation practice necessary, V, Unsuitable for cultivation, VI. No restrictions in use, VII. Moderate restrictions in use, and VIII. Severe restrictions in use.
Class I is flat or nearly flat land suited for cultivation but a few conservation practices are necessary. For II, III and IV lands artificial fertilisation will be required, but special measures of conservational management must be added. Class V, VI, VII are grazing or forestry lands with varying degrees of restrictions on use. The eighth class is suited for wildlife and recreation. Table 12.2 gives the relationship between the rate of soil erosion and long-term decline on land productivity.
Cost of Soil Erosion
Soil erosion is economically the most destructive phenomenon in the world. Not only is the productivity of the eroded land destroyed but the eroded soil reaches the productive land, which also loses its productivity. Expansion of the deserts is a prime example. The productivity of crop land lost annually is given in the following Table 12.3.
Thus a total of 7.84 Mha is estimated to be adversely affected in terms of productive uses. In some cases, the loss is irreversible while in others it is gradual and can be restored. However, we will try to illustrate it with some examples.
The productive land lost-due to advancement of ravines and gullies are mostly in command areas. At least 50% of these are highly productive crop land, each hectare costing between Rs. 10,000-20,000. If we take an average of Rs. 15,000 ha for half of 8,000 ha lost annually, the total amount lost will be 4,000X15,000=60 X 106 or Rs. 60 million /O4'year. Due to coal mining it is 500X104 or Rs.5 million/year.
The most severe damage is caused by floods, which on an average for the period between 1953 and 1988, is of the order of Rs.886 crore for all the losses, i.e., crop land, human lives and livestock and productivities, etc. The average crop land damage of 3.82 Mha/year at the rate of Rs. 1,000 Mha comes to an economic loss of Rs.3820 million or Rs.382 crore.
Sedimentation of various reservoirs is one of the most disastrous consequences of soil erosion. An estimated 480 Mt out of 5,344 Mt of the eroded materials ends up being deposited in various water reservoirs of India and drastically reduce their irrigation and hydro-power generation capacities. The annual cost of erosion in terms of reservoir sedimentation is estimated to be at least Rs. 10,000X100 Mha—m or Rs. 10,00,000 (Rs. 1 million/year). The eroded soil takes out the major plant nutrients such as NPK which is estimated to be between 5.37 to 8.4 Mt. Although, estimates vary widely, the loss almost equals the amount of fertilisers that is being applied throughout the country. Nearly 0.03kg/ha of nutrients are lost every year.
There are several other things lost during erosion and it is difficult to assess their economic value. These are:
i) Loss of fodder and forage production, decline in production of timber.
ii) Loss of natural species of flora and fauna and a consequent decrease in bio
iii) Loss of water resource points as evident by drying up of springs and nalas, and lowering of water level in wells.
iv) When the land is lost due to wind erosion or water erosion, apart from-physical, chemical and biological degradation, there is loss of employment opportunities. This leads to migration towards cities and towns, causing social problems related to unplanned urbanisation.
Thus, the costs of soil erosion include a wide variety of components of importance to man.