Preface In this third edition the basic structure has been retained, and the main changes are bringing in ideas and experiences which have arisen since the second edition. For the benefit of readers who are familiar with the previous edition, and wish to know where the changes are: We have left out the discussion of wind erosion (old chapter 14) because that is now well covered in other texts. The detailed account of Land Capability Classification has been shortened (old chapter 9) because the trend is away from blueprint solutions and towards national or local planning procedures. The detailed discussion of the Universal Soil Loss Equation (old chapter 10) is replaced by a wider discussion of modelling in the new chapter 7. - The reduced emphasis on structures for the control of erosion is reflected in the new chapter 10 which replaces previous chapters 6 and 8. The material on hydraulic design is nowadays more appropriate in design manuals. - The increased importance of control through land husbandry results in a completely rewritten chapter 11 on biological control measures, and the questions of policy and implementation (old chapter 16) are discussed more fully in the new chapter 14. - All chapters have been revised with updated references. An important development during the last ten years has been new- thinking about soil conservation policies, and strategies for their implementation. That is not fully discussed here because it is the subject of Land Husbandry (Batsford 1992). Some of the topics in Land Husbandry have been introduced into this edition of Soil Conservation, and readers will forgive some duplication, which is necessary when trying to reach two different audiences. Soil Conservation was written as a teaching text, and has evidently proved acceptable for this purpose as it has been in print for 24 years, with ten printings, three translations, and three editions. Land Husbandry seeks to reach a wider audience including managers and decision makers in governments and development agencies, and practitioners in the many disciplines in the care and development of land resources.

1.1 The relation between man and the earth’s resources The balance between the demand which a community of plants or animals makes on its environment and the ability to satisfy those demands is not the static equilibrium of a laboratory balance with equal weights on either side. It is more like the unstable balancing of a circus acrobat - a series of swings and overcorrections, resulting in an oscillation on either side of the true balance point. Thus a species of wild life increases in number according to the availability of its food supplies, but then goes beyond the optimum number. The natural correcting factors of starvation or migration reduce the population, but there is an overcorrection, and soon the cycle starts again with increasing numbers. 1.1.1 The Malthusian Thesis In primitive societies the human population also oscillates about a mean as the limiting factors of starvation, disease, and war, maintain an uneasy balance against the natural tendency to increase. A serious imbalance arises when man learns how to modify the limiting factors, but allows the natural increase to go unchecked. The essence of the Malthusian Thesis is that the resulting instability is the inevitable order for mankind. The thesis is named after the English political economist Thomas Malthus who first gave it formal expression in MALTHUS (1966). In an age when significant improvements in health, hygiene, and the standard of living were just becoming possible, Malthus aroused great argument when he pointed out that these very improvements could lead to mi sery through overpopulation. The key point of Malthus’s argument is that population increases in what he called geometric ratio, what today we would call an exponential curve, because whenever there is an increase in population there are more people to produce the next generation, so the rate of population growth accelerates - in modelling terms a positive feedback loop. On the other hand, when we increase food production there is no feed-back loop, the best we can hope for is for a constant linear increase. The natural sequence is therefore for food demand to increase faster than food supply, leading at some point to food shortage.

2.1 Geological and accelerated erosion Erosion has always taken place, and always will. The surface of the earth is constantly changing, with mountains rising, valleys being cut deeper and wider, the coast line receding here and advancing there. The physical pattern of the surface of the earth which we see today is not the result of some single cataclysmic sculpturing but the result of changes so infinitely slow that only after centuries is the effect noticeable. Erosion is simply one of the aspects of the constant process of change. It is fundamental to the formation of alluvial soils and sedimentary rocks. Man’s activities seldom slow down or halt the process but frequently speed it up. We usually refer to geological erosion or normal erosion, or natural erosion when we mean that which results only from the forces of nature, and to accelerated erosion when the process is influenced by man. When trying to predict the probable severity of man-made erosion it helps to consider the rate of geological erosion. If the conditions of climate and topography are such that geological erosion is quicker than usual, then these sameronditions will also lead to particularly severe accelerated erosion. Extreme examples of both kinds of erosion are shown in plates 2.1 and 2.2.

3.1 Physical characteristics There is obviously an association between the amount of rainfall and the amount of soil erosion, i.e. more rain goes with more erosion, and less rain with less erosion, but in statistical terms the correlation between the two is poor. The same total quantity of rain can on different occasions result in widely differing amounts of erosion, and so other more specific measures are required to describe the ability of rainfall to cause erosion. In this chapter we shall establish the known facts about the main physical properties of rainfall, and then in chapter 4 show how these properties are related to the erosive power of rainfall. 3.1.1 Quantity of rainfall Any measure of rainfall amount is a sample, and so associated with the inevitable problems of sampling, namely, 4is the sample representative of the whole?’ and ‘is the sample measured accurately?’ On both counts the data of rainfall which are usually available are of very doubtful reliability. Considering firstly the size of the sample measured, a very intensive network of rain gauges such as that established in an experimental catchment might be as dense as one gauge per 25 hectares, but this corresponds (with a 125 mm diameter gauge) to a sample of about one in 20 million, which would for any other scientific measurement be considered totally inadequate. The density of gauges in a country which considers itself very well provided might be one gauge per 25 square kilometres - a sample of one in 2 000 million, and in an undeveloped country the density of one gauge per 2 500 square kilometres is a sample of 1 in 200 000 000 000! Secondly, the siting of this remarkably small number of gauges is usually arranged primarily on convenience, eg where they can be easily read and contained, with little or no consideration of whether the site is representative of the whole. Thirdly, the accuracy of the measurement is usually unknown and studies have shown that variations in the shape and size of the instrument, and in the height at which it is mounted, and the way it is shielded from the wind, can seriously affect the recorded amount of rainfall. Clearly, even the apparently straightforward question of how much rain falls is not answered as precisely as might be imagined from a survey of rainfall tables and maps.

4.1 Defining erosivity and erodibility Rainfall erosion is the interaction of two items - the rain and the soil. The amount of erosion which occurs in any given circumstances will be influenced by both, and our study of the processes of soil erosion is simplified by considering the two aspects separately. We know from observation that one storm can cause more erosion than another on the same land, and we also know that the same storm will cause more erosion on one field than on another. The effect of the rain is called erosivity and the effect of the soil is called erodibility. The way they differ is best explained by examples. Let us imagine an experiment station which has over its whole area exactly the same soil type and slope. Assume also that the whole of the station is covered by a completely uniform crop. If the amount of erosion taking place during each storm were measured, the amount would be influenced only by the nature of the rain. Comparing the effect of one storm with that of another storm would provide a relative measure of the power of each storm to cause erosion, and that is what is meant by erosivity. This will enable us to say that on this imaginary experiment station, storm A caused a soil loss of X kilograms per hectare and storm B caused a soil loss of Y kilograms per hectare. If Y is half of X, then the erosivity of storm B is half of the erosivity of storm A.

5.1 Definitions The erodibility of soil is its vulnerability or susceptibility to erosion, that is the reciprocal of its resistance to erosion. A soil with a high erodibility will suffer more erosion than a soil with low erodibility if both are exposed to the same rainfall. Whereas the erosivity of rainfall was shown in chapter 4 to be a fairly straightforward measure of the rain’s physical properties, the assessment of erodibility is more complicated because it depends upon many variables. There are alternative uses of the term erodibility. In the widest sense it can be used to include all the variables which effect erosion except the erosivity of rain, as in the first line of figure 2.2. It is also used in the narrower sense as a measure only of the effect of physical characteristics of the soil, as in the erodibility index K of the Universal Soil Loss Equation, where the effects of land management and crop management are assessed separately, as shown in the last line of figure 2.2. 5.2 Factors influencing erodibility In this chapter we will look at the three broad groups of factors which affect erodibility. First, there are the physical features of the soil, including the chemical and physical composition. Second, there are the topographic features, such as the slope of the land. Third, there is the management of the land, i.e. how it is used.

6.1 Quantities and rates of runoff Before a start can be made on the design of the channels and ditches and other works which can deal with surface runoff we need to know the probable quantity of water. If the object is to impound or store the runoff then it may be sufficient to know the total volume of water to be expected, but the usual conservation problem is conveying water from one place to another, and in that case the rate of runoff is more important, and particularly the maximum rate at which runoff is likely to occur. That is the flow which a channel must be designed to accommodate. In a hypothetical catchment area* with an impervious surface and no losses the maximum rate of runoff would be directly proportional to the rate of rainfall. In natural catchments there are other factors: some of the rain is intercepted by vegetation, some infiltrates into the soil, some starts moving over the surface but is trapped in depressions, and some is lost by evaporation. Estimates of rates of surface runoff therefore depend on two processes: estimating the rate of rainfall, and estimating how much of the rainfall becomes runoff. There are several empirical methods for doing both and for combining the two factors, but because they are empirical two points must be stressed. Firstly, as with all empirical solutions, data, tables, and formulae developed in one situation should not be applied in different conditions without careful checks on whether the method is valid in the new situation. Secondly, each of the three main methods which will be

7.1 The purpose and type of models Sometimes the simplest way to estimate the effect of a physical process is to use an established equation, or formula, or charts and diagrams based on existing knowledge. The current fashion is to call such aids and solutions models, and they tend to become increasingly complicated and consequently dependent on computers to handle the mathematical processes. It is important to identify the exact objective and purpose of models which are designed to estimate soil erosion. Two main divisions are models for prediction and models to assist with explanation. If the purpose is to predict amounts of erosion under alternative management practices then empirical models such as the USLE are efficient and effective. For an understanding of the processes of erosion it is necessary to turn to physically-based or deterministic models. The two objectives should not be cross-threaded onto the wrong kind of model, i.e. the USLE should not be seen as a vehicle for research studies on the mechanics of erosion. Neither should sophisticated simulation models be used as the basis for evaluating conservation practices which are better designed on the basis of experience, and evaluated by direct testing in the field. The different kinds of models need a short explanation: An empirical solution or model is one based on observation or experiment, and not derived from theory. It fits observed facts, and allows us to predict what will happen in certain circumstances, because we know what has happened before in those circumstances. The reliability of such methods depends on the database of experience: we might say This will certainly happen’ (because it always does), or “Will probably happen’ (because it usually does), or ‘it may happen’ (because it sometimes does). An empirical solution may be a simple approximate relationship or a complex multiple regression equation.

8.1 The purpose of erosion research The need for research A soil conservation programme needs to be based on factual information about rates and quantities of erosion. For example some land obviously needs to be protected by mechanical works such as channel terraces, some land equally obviously does not need any protection, and a great deal of land lies somewhere between the two extremes. At some point the terraces become unnecessary or uneconomic, but a logical decision on where to draw the line can only be made if data are available on how much soil erosion is occurring and how much it would be reduced by terraces. Similarly, the effect of channel terraces should be compared with the effect of alternative measures, and the effect should also be measured on the different soils and crops which are likely to be encountered. Nowadays there are excellent opportunities for research workers to exchange the results of their work. The national and iniernational journals are packed with accounts and results of research reports in their proceedings (as listed in Further reading). Some of the reported research results can be usefully applied in countries other than where the research was done, but care is required in any transfer of technology. If the local conditions of soil, climate, or land use, are different, the results may not apply. However, research in other situations can often serve as a pilot trial to show which factors should be studied locally. Even when local conditions are so different that the results do not apply at all, the techniques and apparatus developed by other workers can still be helpful.

9.1 Planning land use Modern techniques such as mechanization, better crop varieties, and the scientific use of fertilizers can transform agriculture, but before they can be effective the use of the land has to be right. No techniques will make it possible to grow a good crop if the soil conditions are unsuitable for that crop, and no conservation works can prevent erosion when the basic cause is trying to grow crops on land which is really unsuitable for arable farming (plate 9.1). The object of land use planning is to determine the characteristics of land, its possibilities, and its problems, and try to match these to a form of land use which will be sustainable and economically productive. 9.7.1 Some planning principles An often quoted definition is that ‘planning is the conscious process of selecting and developing the best course of action to accomplish an objective’. In the case of Land Use Planning the objective is the efficient intensive use of land resources. It is difficult to imagine any field of human activity where some form of planning is not essential to achieve the objective. Consider going on holiday. Certainly it is possible to come home from work one evening, throw some clothes into a suitcase, go to the railway station or airport and buy a ticket to the first holiday resort which comes to mind. Some fortunate people may indeed enjoy so unplanned a holiday but for most of us it would be disappointing to find oneself in a mountain holiday resort without climbing boots, or on a sunny beach with only winter clothing in the suit case. No, one is more likely to achieve the objective of a happy holiday by a planned approach and this means going through a logical sequence of steps, i.e.

10.1 Different practices for different purposes There are so many different practices used in erosion control that to describe them needs some form of grouping. For a long time mechanical protection was used to describe all the methods which involved earth moving, such as digging drains, building banks, levelling sloping land, and so on. Anything else was lumped together under biological methods for want of a better title. This is appropriate for large mechanized farms where the approach is to use machines to do the earth-moving and follow up with improved farming methods. But this division does not suit the concept of erosion control through better land husbandry supported by mechanical protection, and the division is artificial when we start talking about progressive terracing using grass strips or live hedges. An approach more suited to today’s thinking is to group methods and techniques according to their main objective. All have an element of saving soil loss, but an important difference is the handling of surface runoff. This chapter identifies three kinds of water management objective and two types of soil management on sloping land.

11.1 The farmer’s point of view In the last chapter possible conservation practices were reviewed in groups according to their purpose in terms of influencing runoff or soil loss. Now, looking at ways of limiting soil degradation by improved land husbandry, the primary consideration is to look at the alternatives from the farmer’s point of view. In fact we need to be very wary of manual-type recommendations. A better approach is to offer a menu of alternatives, and the role of the professional is to help the farmer choose what is appropriate for his or her circumstances. A recent review of technologies acceptable to resource-poor farmers points out that combining practices into a farming system must take account not only of the physical factors such as soil and climate, but equally the available resource inputs, especially cash and labour, and the farmer’s objectives (STOCKING 1993). The objectives of a commercial farmer are usually to maximize yield and income, but the subsistence farmer is likely to be more interested in improving food security by reducing the risk of failure, or improving the return on inputs of seed, fertilizer and labour, or improving the quality of life by reducing drudgery, particularly relevant in the case of women farmers. We need a new approach to the farmer when recommending changes to farming practice. Instead of recommending practices because they will reduce erosion, we should show how they can lead to increased production. For example, practices which lead to better soil structure, more organic matter, or more moisture-holding capacity should be put forward because they improve yield, not because they reduce the soil erodibility. Increasing infiltration from better ground cover or the use of contour farming or ridging can result in better crops as well as reducing soil loss. Improved crop management could include more use of fertilizer and manures or crop rotations. These practices will make more sense to the farmer if they are recommended as leading to better production, which he can instinctively appreciate as desirable, rather than because they will reduce soil erosion which is likely to be a less important objective.

12.1 The nature of gully erosion Gully erosion is spectacular and widespread, and so it is often used by conservationists as a characteristic symptom of erosion. As a result there is a danger of its importance being over-emphasized. Certainly it is most important as a source of sediment in streams, but in terms of damage to agricultural land or reduction of agricultural production, it is usually not very important for the simple reason that most land subject to severe gully erosion is of little agricultural significance. The really spectacular erosion so popular in non-technical reports shows large areas cut up by deep interconnecting gullies, but this situation is usually found in semi-arid climates unsuitable for any serious agriculture, or on soils which have such adverse chemical or physical properties that their potential production is very low. Added to this is the fact that gully control is always difficult and expensive, so the cost of reclamation usually exceeds the value of the land. Certainly in the sense of reducing the amount of eroded soil choking the dams and rivers it may be highly desirable to do something about gully erosion, but it is very much a case of prevention being better than cure. Limited resources in manpower, money and materials can usually be better employed in preventing future gullies than in curing existing ones. The worldwide occurrence and recognition is well demonstrated in the variety of names: in English and American - gully, in Egypt - wadi, in South Africa - donga, in French-speaking lands - ravine, in India - nulla, in South America - carcava, or arroyo, and many others. The definition is the same in any case, a steep-sided eroding watercourse which is subject to intermittent flash floods. They are more common in semi-arid climates with infertile soils and sparse vegetation, but some spectacular examples are found in tropical forests in deep soils with dense vegetation.

Damage to arable land tends to attract most attention but degradation of non-arable land can also have serious effects, whether economic, or social, or on the food supply. In dry seasons, loss of grazing capacity is a common and serious problem, forest land may suffer through over-exploitation, roads can cause much damage particularly on steep land. In this chapter we will look at the more important erosion problems on non-arable land. 13.1 Erosion on grazing land On well-managed high-yielding pasture it is unlikely that there will be any erosion, for the ground will have a good uniform cover. But land which is not arable because it is too poor, too infertile, too stony, or too arid, and is only described as grazing land because it is unfit for more intensive use, is a very different matter. Again, erosion control can be directly and precisely equated to improved management - anything leading to a better water balance and a better ground cover will lead to reduced erosion.

14.1 Changes in policy and strategy 14.1.1 Policy changes In the discussion on land use planning we suggested that the ideal pattern of development is to establish a long-term national or regional policy, which leads to medium-term strategies, and short-term tactical operations. In the real world a curious situation is arising in that the adoption of the new land husbandry approach is growing so fast that the practitioners have moved ahead of the politicians and opinion formers, with the result that the implementation of strategies by Non-Government Organisations, aid agencies, and senior field officers, is helping to shape new policies. There is always the problem that national policies ought to be long term, but are created by politicians usually working within a short time frame to the next election. Fortunately, the land husbandry approach allows planners at all levels from farmers to governments to appreciate the long-term issues. The new approach was mentioned in section 9.2.1 in connection with land use planning. It is summed up in the slogan From soil conservation to land husbandry, and two reviews using that title list the main points as the need for:

Appendix 1 Some conversion factors from metric or SI units to Imperial units