Introduction

A set of twenty-four slides cannot, by itself, tell the whole story of soil erosion, and no attempt has been made to do so here. Rather, it is seen as providing examples of the problems which may be encountered when considering the subject and, it is hoped, will enable the teacher or lecturer to base the students’ work on practical situations. The following set of notes give a general overview of the subject.

Soil Erosion

Soil erosion is a major constraint to agricultural development in the Third World. The pressure for increased food production causes the expansion of agriculture into marginal lands and this often leads to accelerated soil erosion rates. The resulting high sediment loads in rivers and canals cause channel siltation and reduced water carrying capacities. Ultimately, sediment deposition in reservoirs can mean a severely curtailed operating life, with its consequent economic implications.

In recognition of these problems the International Development Group of HR Wallingford has, for a number of years, been involved in studies of the catchment erosion and sedimentation processes. During the course of these investigations, its staff have travelled to many countries and have accumulated a large collection of colour slides. The examples included in this pack have been selected to show the three phases of erosion which can broadly be categorised as the cause, effect and prevention. The number of slides chosen from any particular country merely reflects the photographic opportunities that were presented to our staff, and should not be taken to represent the relative magnitude of the problem in each country.

Soil erosion is a process which occurs, to a greater or lesser extent, in all countries of the world. Under natural conditions, rock is weathered and broken by successive sequences of wetting, drying, heating and cooling. Rainfall and wind, freeze and thaw act on the soil profile to break down soil aggregates and to detach and move soil particles. These processes happen completely independently of any intervention by man. Any sediment that is eroded may then find its way into the stream network, where it is moved downstream.

The ability of a river to transport material changes while it follows its course through the foothills, across the plain and into the sea. As the channel gradient (and thus the stream velocity) gradually reduces, large particles can no longer be supported by the flow and are deposited to form alluvial plains or deltas. Such deposition may prove advantageous to the indigenous peoples as was seen, for example, along the River Nile where annual flooding covered the fields with a layer of fertile silt. Scientists travelling with Napoleon in the early 18^th Century calculated that the annual rate of deposition was about 0.1mm, which was equivalent to 9.4 million tons. However, this same sediment created a major problem when the Aswan Dam was constructed in 1964 and the impounded reservoir retained about 98% of the total sediment load, equivalent to an annual deposition of 120 million tonnes.

A quantitative representation of the erosion problem is very difficult to obtain because of the problems associated with data collection, and the diversity of analytical methods used. However, the following table gives a general perspective, although it must be emphasised that the figures should be used for comparative purposes only.

The Erosion Process

The first stage of erosion is the detachment of material by water or wind. The detached sediment is then transported away from its original position, and may be deposited and in turn detached again. Thus, erosion of an individual soil particle can happen many times. In this slide pack we are just going to consider the water erosion process. Detachment and transport of sediment can happen in three ways :

1. the impact of rain drops on the soil breaks up major aggregations and splashes particles in all directions. On steep slopes more material is splashed downhill than up, resulting in a net downhill movement of soil.
2. as water runs over the soil surface it has power to pick up some of the particles that have been the subject of splash erosion. The water also has the capacity to detach particles from the soil surface. This capacity is enhanced by soil particles in the water, which abrade the soil surface. This process continues as flow accumulates into large channels, with soil particles being removed from the channel bed and sides.
3. on steep land, when soil is saturated, or when freeze and thaw processes have opened cracks along zones of weakness in the soil profile, the weight of soil is often sufficient to exceed the forces holding the soil in place. Under these circumstances, large masses may slip downhill, further losing their internal cohesion as they fall and becoming, at the bottom, loose heaps which can be easily be detached and transported again by the other two processes.

These processes result in three main erosion types :

1. Sheet erosion is a fairly uniform removal of soil particles over the whole soil surface. This is often the most difficult of erosion types to observe, as it can happen very slowly, maybe one or two millimetres of soil depth a year being lost from a field. However if the rate of soil removal is greater than the rate of soil formation, gradually the soil profile will be lost from the field, together with the nutrients that are contained in that soil.
2. Rill and gully erosion happens as water runs off an area in concentrated channels. Rills are small rivulets, of such a size that they can be worked over with farm machinery. Gullies are a much more serious problem, often being several metres deep and across. Gullies are perhaps on a footpath, or where water is being channelled to a ditch. Once erosion of this type starts, the gully will work its way upstream eroding soil at the top of the gully and moving it rapidly downstream.
3. Landslips are rapid mass movements of a whole soil profile downhill. Very large volumes of soil can be moved very quickly. Indeed in the Himalayas, whole villages have bee lost in this way in just a few minutes.

The physical factors that influence the natural rate of soil erosion are :

rainfall intensity and amount

soil type

land gradient

distance of overland flow

land use and vegetative cover

Accelerated Erosion

Increases in the world’s population and the subsequent demand to produce more food, means that there are few, if any, natural catchments left. The human race is everywhere engaged in activities that alter the conditions of the land surface. These activities can broadly be classified under three processes :

1. Increasing the exposure of the soil surface to direct impact by removing or reducing vegetative cover. This can be by large scale deforestation, shifting (or slash-and-burn- agriculture and over-grazing. In each case, the effect is to destroy a protective cover that may have taken decades to establish, and to replace it with inadequate cover that is unable to intercept the rainfall or provide resistance to overland flow.
2. Increasing the quantity and rate of surface run-off, or concentrating the existing quantities into narrower, more erosive streams. Any modification to the ‘natural’ ground cover as the result of agricultural activities will change the run-off system. The farmer’s inclination is to reshape the land in order to make the best use of the technology available to him. This may mean removing ridges and depressions that acted to reduce the rate of surface run-off and, on steeper slopes, may involve ploughing furrows across contours (ie up and down the slops) because it is easier and safer to do. New roads cut across a hillside will intersect existing flow lines and may concentrate the surface run-off in areas where the soils are not able to resist the increase erosive power.
3. Increasing the soil vulnerability by disturbing it. Ploughing or tilling are the usual means by which this is done and the result is a more mobile soil surface. The effects may be minimised by planting annual rather than short-term crops, thus reducing the frequency of disturbance, but the demand for a wide range of food varieties makes this very difficult to follow.

Increasing the availability of water for fishing or irrigation attracts permanent villages with their subsequent concentration of human activity. The adoption of a well-used footpath, forming a depression along which surface run off will be concentrated, creates idea conditions for the formation of a gully. The need for fuel wood around the settlement will be denuded of trees without any formal programme of reforestation. Some of this exposed soil may be used for growing vegetables and other land may be cleared for agriculture. But many forms of traditional agriculture provide insufficient ground cover to protect against erosion.

The loss of the soil surface is a visible indication of what may be a much more fundamental problem – a reduction in soil fertility. Under natural conditions the soil composition reaches an equilibrium level whereby decaying vegetation is removed and the processes of accelerated erosion are established, the natural nutrients are removed with the soil. If the farmer is to continue cultivating the same area he must make use of fertilisers if crop yields are to be maintained.

Prevention and Control of Erosion

There are two basic ways in which soil erosion may be controlled :

1. by minimising the effects of rainfall impact on the soil surface, and
2. by minimising the volume and/or velocity of run-off water

There are, however, several techniques that may be utilised in different parts of the catchment with the choice depending on conditions such as slope, soil type and crop cover. These techniques fall broadly into four categories; physical, vegetative, cultivation and recapture.

Physical Techniques

These methods involve reshaping the land surface in order to change the pattern of run-off. The first aim is to slow down the rate of overland flow, thus reducing the entrainment of surface particles. This is achieved by the construction of terraces (high walls, frequently of stone, supporting in-filled, flat areas) or bunds (low banks formed by ridging the soil). The second aim is to collect the flow into channels that are sufficiently protected to accept the flow without suffering damage. This second option may require the construction of weirs and drop structures to control the discharge velocity and thus limit erosion.

Vegetative Techniques

Plant growth serves two purposes, the leaves absorb rainfall energy and may reduce the size of drops impacting with the soil surface, and the root systems create a network of passages in the soil which increase infiltration and thus reduce run-off. To have the optimum effect, the greatest ground cover must be realised during periods of maximum rainfall. this may require a degree of intercropping or crop diversification if the twin aims of controlling erosion while maintaining crop yield are to be met. Vegetative residues may also be used to protect the soil by mulching – covering the soil surface with dead stalks and leaves.

Cultivation Techniques

The mobility of soils may be constrained by changing the method of cultivation and/or the equipment used in the tillage. The aim is to reduce the velocity of surface run-off and this may be achieved by reducing the effective gradient through contour ploughing and tied ridges. In some cases it may be possible to use ‘no-till’ techniques, where the ground is not disturbed by ploughing. This is often used in combination with mulching.

Recapture

This technique, while strictly not conservation, is one that has been used successfully in a number of places. The principle is to interrupt the flow of run-off water by building some form of dam or weir structure across the catchment outlet which will reduce the flow velocity and so allow the transported material to settle out of suspension. The deposited material can then either be dug out and returned to the fields from whence it came, or used for cultivation where it has settled. In the second case, another sediment trapping structure would then have to be constructed upstream of the first.

In many tropical and sub-tropical developing countries, soil erosion is a serious threat to agricultural production. There is a fair measure of agreement on the most appropriate erosion control techniques that could be employed to combat erosion in specific circumstances yet there is still considerable difficulty, in most developing countries, in securing active implementation of any erosion control policies on a substantial scale. One important reason for this resistance, or reluctance, seems to be the sparcity of the kind of data on which a quantitative analysis of conservation policies could be based.

The research that led to the production of this slidepack was carried out by HR Wallingford and was funded by the British Government’s Department for International Development.