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Soil erosion and population density

Accelerated erosion and excessive runoff are connected with a kind of development that throws the balance of the countryside out of kilter: clearance of fragile zones, denudation and compaction of soil through overgrazing, exhaustion of soil through intensive cropping without compensation from applications of organic matter and nutrients. If it is true that human activity increases erosion risks through ill-judged farming methods, there is hope that the present trend can be reversed: by improving infiltration to produce more biomass, and increasing plant cover to return more organic residue to the soil, thereby reducing the runoff, erosion and drainage that soon deplete tropical soils. In this context, soil conservation is not the land-use planner's main aim, but simply one component of a technological package to make possible the intensification of agricultural production vital to meeting this century's major challenge: to double production every ten years to keep pace with population growth in tropical countries.

Some writers claim that erosion increases as a function of population density (Figure 4). It is true that in a given agrarian system, if the population passes a certain threshold, land starts to run short, and soil restoration mechanisms seize up (Pieri 1989). For example, in Sudano-Sahelian zones, when the population exceeds 20-40 inhabitants/km², the fallow period is shortened to the point of ineffectiveness, and one speaks of a densely populated degraded area when the population reaches about 100 inh/km². Adults then have to migrate during the dry season to find supplementary resources in order to ensure their families' survival (e.g. Burkina Faso).

Interestingly enough, in other more humid tropical zones - with two cropping seasons or richer, volcanic soils (Java, for example) - the term high density is not used until the population goes beyond 250-750/km². The cases of Rwanda and Burundi are particularly striking: despite very acid soils and slopes of over 30-80%, families manage better on a single hectare than in the Sahel, so long as they intensify their production systems, practice intercropping, plant trees, stable stock, quickly recycle all wastes, and stop the bleeding of nutrients through erosion and drainage.

It may then be concluded that the environment becomes degraded as population density grows, until it reaches a certain level after which farmers are obliged to change their production systems. This is what has happened in Sudano-Sahelian zones with the prolonged drought of the past 20 years (the population is scarcely growing any more because of emigration). Farmers in Yatenga are willing to invest 30 to 100 days a year to install erosion control structures allowing them to manage water and soil fertility on their plots: stone bunds, ponds, rows of trees or grass strips, re-establishing pastures and treed paddocks on cultivated blocks (Roose and Rodriguez 1990; Roose, Dugue and Rodriguez 1992).

Traditional erosion control strategies

For seven thousand years, humanity has left records of the battle with erosion, soil degradation and runoff, trying to improve soil fertility and water management (Lowdermilk 1953), and it can be seen that traditional methods are closely bound up with social and economic conditions.

Shifting cultivation, the oldest strategy, has been used on every continent wherever and whenever population was less dense (20-40/km² depending on soil richness and rainfall). After clearing and burning, crops are grown on the ashes, and the land is then abandoned when it no longer yields enough return for the work (invasion of weeds and loss of the most easily assimilated nutrients). A considerable reserve of land (about 20 times the cultivated area) is required for the system to remain in balance: if demographic pressure increases, the fallow period is shortened, leading to steady soil degradation. These strategies are well suited to sparsely populated areas with deep soils and annual rainfall of over 600 mm.

By contrast, bench terracing or irrigated Mediterranean terraces coincide with a dense population and a shortage of land for cultivation (especially in mountain areas) and occur where labour is cheap. Such strategies require 600 to 1200 days' work per hectare to build and maintain erosion control structures, plus an enormous effort to restore soil fertility, and are accepted by farmers only where they have no other alternative for survival or for the production of profitable crops. This happened in the case of the Kirdis of northern Cameroon as they held out against the ascendancy of Islam, or the Incas of Peru in the Machu Picchu region, who built remarkable bench terraces in the 15th century as a defence against incursions by peoples from the Amazon Basin and then by the Spanish (Guide Bleu du Pérou, Hachette, pp. 246-247).

Ridges, intercropping and agroforestry. In the humid, volcanic forest zones of southwestern Cameroon, despite dense population (150-600/km²), the Bamiléké have succeeded in establishing a reasonable balance by combining intercropping, which covers large ridges throughout the year, with various systems of agroforestry.

Stone lines and low walls combined with fertility maintenance through use of organic manure. Like various other ethnic groups in Africa, the Dogon of Mali took refuge in the sandstone cliffs of Bandiagara in former days to resist Moslem influence, and had to develop a whole set of conservation practices in order to survive:

• small fields surrounded by sandstone blocks to trap sand in the dry season and runoff during the rains;

• low stone walls and bringing sandy earth up from the plain to create soil on sandstone slabs that act as microcatchments to harvest water;

• honeycomb constructions for onion production, watered with calabashes;

• mulching and composting with crop residues, domestic waste and animal manure in order to maintain household gardens in arid, sandy conditions.

Bocage or the close association of cropping, animal husbandry and arboriculture. Europe has already experienced several erosion crises, the most well-known in the Middle Ages, when population pressure forced abandonment of the natural fallow period. Tilling the soil and ploughing in dung were introduced with a view to restoring the chemical and physical fertility of soil more quickly. Stock farming was combined with cropping, and the countryside was partitioned by a series of thickets, small fields and meadows surrounded by hedges.

Nowadays, however, the mechanization and industrialization of agriculture, the economic crisis and the breakdown of traditional societies are forcing the abandonment of these methods, which geographers and anthropologists have described in glowing terms but which are viewed askance by "modern" soil conservation experts, who consider them inadequate to solve the problems of large-scale watershed management (Critchley, Reij and Seznec 1992).

Such positions certainly require re-examination, and although there is no wish to idealize traditional methods, analysis should be devoted to their spatial distribution, operating conditions, effectiveness, cost, and present vitality; above all, ways of improving them must be developed (see the proceedings of the European Community meeting in Crete, 1993).

Modern strategies for developing rural water infrastructures

More recently, various modern erosion control strategies have been developed, basically to improve the land, reshape it (terracing), and provide hydroagricultural infrastructures. Priority was given to mechanical means of water management.

Rehabilitation of mountainous land (RML) began in France in 1850, then spread to European mountain areas, where forestry departments sought to protect fertile plains and communication routes from torrent-generated damage by buying up degraded mountain land, reestablishing plant cover, and controlling torrents through civil and biological engineering techniques. They had to deal with a crisis in which upland small farmers could no longer survive without pasturing their herds on common lands, which then became degraded through overgrazing (Lilin 1986).

Soil and water conservation (SWC) on cultivated land in the United States has been the province of agronomists since 1930. The rapid expansion of industrial crops offering little cover such as cotton, groundnut, tobacco and maize - in the Great Plains had unleashed cataclysmic wind erosion, such as the dustbowl effect, when the sky was darkened even at midday, and water erosion as well. By 1930, during the Great Depression, 20% of arable land had been degraded. Public opinion forced the government to act. Under the impetus of Bennett (1939) the Soil Conservation Service established soil conservation districts, providing advice and assistance to farmers wanting technical and financial help to manage their land. Agronomists and hydrologists at headquarters carried out studies and drew up projects.

Today there are still two conflicting schools of thought in erosion control:

• one school follows Bennett in arguing that gullying is what causes the most spectacular transport of solids; since gullying is a result of runoff energy, which is a function of its squared mass and speed (Runoff energy = 1/2 m²), erosion control concentrates on mechanical means of reducing runoff speed and its erosive force (diversion bunds), weirs and grass spillways) without reducing the mass of runoff on fields;

• the other school follows Ellison's work (1944) on rain splash, and that of Wischmeier's team, arguing that runoff develops following degradation of the surface structure from the impact of raindrops; erosion control here centres on the fields, concentrating on plant cover, cropping techniques and a minimum of structures.

These two approaches have been identified in France on large-scale holdings:

• one on slaking loamy soils, especially in winter (closed soil with little cover);

• the other on the same land during spring storms, on seed beds and especially on sandy soils (around the River Sarthe or in south-western France).

Analysis of the dynamics of erosion and runoff (caused by saturation or the condition of the slaking surface) helps assess the relative importance of areolar and linear erosion and determine the implications for erosion control strategies (comm. from Lilin, 1991).

Soil protection and restoration (SPR) [Plates 8 and 9] developed in Algeria, then spread around the Mediterranean basin between 1940 and 1960 in an attempt to deal with serious sedimentation problems in reservoirs and the degradation of roads and land. The primary objectives were those of protecting land degraded by overgrazing and clearing, and restoring its infiltration potential by planting trees, considered the best way of improving soil. Major mechanized resources and an abundant local labour force were mobilized to control sheet runoff on cultivated land (various kinds of bunds, Monjauze embankments, etc.), in order to reforest degraded land and set up zones of intensive farming (Plantié 1961, Putod 1960, Monjauze 1962, Gréco 1979).

The foresters' main concern was with agricultural regeneration, which took place within the framework of the "rural renewal" (Monjauze 1962). For them the SPR concept was more important than it was for the advocates of RML.

However, this operation developed in an authoritarian political context (the Algerian war) and the social goal of fighting unemployment rapidly became a priority (ditch-digging) while other resources were blocked by the political situation (comm. from Mura, 1991).

All these measures have not been in vain, as some critics would maintain, for degradation of the countryside would certainly have been even worse without them. However, people seriously began to doubt the validity of the whole SWC approach after an American study revealed that erosion had in fact hardly affected the productivity of deep soil. It has been shown in many cases that soil is a renewable resource, although the cost of restoring it is often prohibitive in view of the available economic resources. Nonetheless, there are cases - in Burkina Faso, Rwanda and Haiti - where demographic pressure and pressure on land have led to the restoration of degraded land in record time (one year).

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