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Part one: Erosion control strategies and the concept of land husbandry

Chapter 1. Definitions: words conceal a philosophy
Chapter 2. History of erosion control strategies
Chapter 3. Some social and economic aspects of erosion

In the Sudano-Sahelian savannah (400-700 mm rainfall) human-induced land degradation as a consequence of intensive tillage cropping. Impervious crusting soils are being rehabilitated by traditional "zaï" techniques (pitting and manuring).

Chapter 1. Definitions: words conceal a philosophy

Erosion
Soil loss tolerance
Erosion varies according to place: different agents, two perspectives
Erosion varies according to time
Soil degradation
Factors in the water balance

The problems of environmental degradation are closely bound up with the development of populations and civilizations. They are of equal interest to agriculturists, foresters, geographers, hydrologists and sedimentologists as well as to social economists. However, each discipline has developed its own specialized language, so that the same words can have different meanings to different experts.

It is therefore necessary to specify the meaning of words and the meanings given them by the various specialists who enter the picture at different points in time and space in pursuit of their own goals. This is vital for the design of more effective erosion control projects.

Erosion

"Erosion" comes from erodere, a Latin verb meaning "to gnaw. " Erosion gnaws away at the earth like a dog at a bone. This has given rise to the pessimistic view of some writers who see erosion as a leprosy gnawing away the earth until only a whitened skeleton is left. The chalky mountains around the Mediterranean well illustrate this stripping away of the flesh of mountains as the trees are cut down and the sparse vegetation burned (e.g. Greece). In reality, this is a natural process which indeed wears down all mountains (also referred to by the English school as the denudation rate, which is the lowering rate of the soil level); however, at the same time erosion enriches valleys and forms the rich plains that feed a large part of humanity. It is therefore not necessarily desirable to stop all erosion, but rather to reduce it to an acceptable or tolerable level.

Soil loss tolerance

In terms of erosion, tolerance was first defined as soil loss balanced by soil formation through weathering of rocks. This can vary from 1 to 12 t/ha/yr, according to climate, type of rock and soil depth. However, it was very quickly realized that the productivity of the humiferous horizons, rich in biogenic elements, is far greater than that of alterites, weathered rocks which are more or less sterile. Moreover, this approach ignores the importance of the selective erosion of the nutrients and colloids that are what make soils fertile. Tolerance was then defined as erosion that does not lead to any appreciable reduction in soil productivity. Here too, however, there were considerable problems. There is still not enough known about the loss of productivity of different types of soil in relation to erosion; and in the case of some deep soils on loess, high soil losses on slopes entail only a small drop in soil productivity, but do lead to unacceptable damage downstream in terms of pollution of fresh water and siltation of dams.

FIGURE 1 Variety of erosion problems in different places: diversity of perspectives and agents

Three aspects must therefore be considered: speed of soil rehabilitation; maintenance of soil productivity given equal inputs; and respect for the environment in terms of water quality, especially runoff sediments (Stocking 1978, Mannering 1981).

Erosion varies according to place: different agents, two perspectives

Erosion is the result of several processes and can be divided into three phases: loosening of particles, solid transport, and sedimentation. Whatever the scale of study - a square metre or a watershed of hundreds of thousands of square kilometres - these three phases are always found, although they will differ in intensity, with the agents of erosion differing according to the predominant phase.

In mountainous country, when plant cover is destroyed, gullying, torrents and landslides carry away much solid matter, causing widespread damage to communication networks. Public works engineers and foresters then come in to maintain lines of communication, replant rangelands and ski runs, reforest denuded slopes and control torrents. Rural populations are primarily concerned with managing water and nutrients on pastures or irrigated terraces rather than combating erosion (see the Cévennes and the irrigated Alpine grasslands in France).

In the foothills where slopes are still steep, erosion damage comes from gullying by torrents, which transport huge amounts of sediment load, and to a lesser degree from vegetation degradation through overgrazing or fires and "pirate" (unplanned, unsupervised) farming. Here again, foresters will try to solve the problem of dam siltation through rehabilitation of mountainous land (RML) and soil protection and restoration (SPR).

Lastly, in the plains, the most frequent problems are siltation of canals, rivers and ports, flooding of major riverbeds, muddy colluvial deposits in residential areas (ill-advisedly built downhill from land that, though it should not be, is under mechanized cultivation), and water pollution (fine suspended sediment [washload] or toxic discharges from farming or industry).

As Figure 1 shows, the parties to soil degradation and the departments engaged in erosion control vary, as do their goals and strategies. The wide range of forms of erosion in different places is matched by a similar variety of agents of erosion control and interests at stake.

On farms and hillsides, those who manage the land, i.e. farmers, agronomists, soil scientists and geomorphologists, speak of erosion or soil loss (sediment yield). In speaking of rivers, hydrologists and sedimentologists talk about sediment delivery, or suspended load (clay, silt and organic matter in suspension - i.e. the washload), and bedload (coarse sand and gravel). There can be considerable differences - arising from the so-called "sediment ratio" - between hillside erosion and sediment delivery in a river. What happens is that some heavier sediment is deposited, if only temporarily, at the foot of slopes and in valleys, providing nutrients to colluvial and alluvial soils and not reaching the sea or a dam reservoir until much later, so that the sediment ratio is less than 1. Specific washload (t/km²/yr) decreases as the watershed increases in size. For example, on loesses in Brabant, Belgium, Bolline (1982) recorded particle detachment due to splash erosion at a rate of about 130 t/ha/yr under a rotation of beet and wheat. Soil loss from the foot of 25-metre-long plots was no more than 30 t/ha/yr, and sediment transport in the nearby river barely 0.13 t/ha/yr. In France, some experiences (Boiffin, Papy and Peyre. pers. comm., 1990) have shown that erosion on the slaking loamy soils of the Paris basin is worrying only when conditions favouring runoff concentration occur together: soil sealed by slaking crusts, scanty plant cover, extended rainy period, large plots where land consolidation has eliminated runoff management structures.

By contrast, in mountains or wherever drainage slopes are steep (e.g. the Mediterranean region), the erosive energy of runoff is higher than that of rain. Soil loss from cultivated fields may be small (0.1 to 15 t/ha/yr - Heusch 1970, Arabi and Roose 1989), while sediment transport exceeds 100 to 200 t/ha/yr in gullies and wadis (Olivry, pers. comm., 1989; Buffalo, pers. comm., 1990). In this case, the larger the catchment area, the more abundant and fast-moving is the concentrated runoff, the greater are peak discharges, and the more runoff gnaws away at the bed and sides of wadis, causing gullying and landslips on low terraces. In this last case, the sediment ratio can be higher than 1 and specific erosion can increase with the size of the catchment area (Heusch, pers. comm., 1973).

Erosion varies according to time

• Normal or geological erosion (morphogenesis) is generally defined as the process that slowly shapes hillsides (0.1 to 1 t/ha/yr), allowing the formation of soil cover from the weathering of rocks and from alluvial and colluvial deposits (pedogenesis). A terrain is described as stable when pedogenesis (speed of rock weathering) and morphogenesis (erosion, denudation) are in balance.

• However, geological erosion is not always gradual. In zones subject to paroxysmic orogenic upthrust, the sediment transport rate can reach 50 t/ha/yr (Indonesia, Nepal, the Bolivian Andes) and up to 100 t/ha/yr in the Himalayas which are rising by 1 cm every year. Likewise, in cyclone-prone tropical zones, morphogenesis is currently very pronounced, especially where plant cover has been degraded (communication from Heusch 1991). Geological erosion can also occur suddenly and catastrophically following rare events - a series of rainstorms which soak the ground, or during seismic or volcanic activity. An example would be the memorable mud flows in Colombia which wiped a village of 25000 inhabitants (Nevado del Ruiz) from the map in a single night in 1988. At the Telman dam in southern Tunisia, Bourges et al. (1979) have recorded annual average runoff of 14% to 25% of rainfall and soil loss of 8.2 t/ha/yr, but on 12 December 1978 there was a once-in-a-century rainfall of 250 mm in 26 hours, resulting in 80% runoff and soil loss of 39 t/ha in a single day. Such catastrophic phenomena are not rare on the geological timescale. Flotte (pers. comm., 1984) has described the torrential lava flow at Mechtras in Great Kabylia (Algeria) of about 150 million m³, covering 18 km², 7 km in length, on a 6.8% slope. These catastrophic movements, involving large volumes of mixed material and spreading over several kilometres in a very short time, often depend on climatic factors different from those known today. However, such masses could always be set in motion again if the required climatic factors coincided (exceptionally heavy rain after soil freezing or emission of steam from volcanoes or earth tremors), or after poorly-planned "management" has unbalanced slope equilibrium.

It is very difficult to control these two types of geological erosion, for the necessary means are expensive and not always effective. In France, the Major Risks Department of the Finance Ministry (la Delegation aux Risques du Ministère des finances) will declare a state of natural disaster and require insurance companies to reimburse the damage, so the costs are passed on to the whole community of insurance-holders.

• Erosion accelerated by human activities, following careless exploitation of the environment, is 100 to 1000 times faster than normal erosion. It takes a soil loss of 12-15 t/ha/yr, i.e., 1 mm/yr or 1 m/1000 years, to exceed the rock weathering rate (20 to 100000 years to weather a metre of granite in high-rainfall tropical conditions, according to Leneuf 1965). The arable layer loses particles through selective erosion ("soil skeletonization") and gets thinner (scouring), while runoff increases (20 to 50 times more runoff under crops than under forests), resulting in peak flows further downstream that are highly prejudicial to the hydrographic network (Roose 1973).

Definitions must still be given of the suspended load (the weight of particles in suspension in water), the capacity of a fluid (the mass of particles it can transport) and the competence of a fluid (the largest size of particle it can transport in relation to its speed).

FIGURE 2 Nature of problems: imbalances in the "managed" environment lead to soil degradation, then erosion speeds up the process

Soil degradation

[Plate 3]

There are also a number of causes of soil degradation: salinization, waterlogging, compaction through mechanization, mineralization of organic matter, and skeletonization through selective erosion. In the humid tropics, although erosion comprises three phases (detachment, transport and sedimentation), degradation of cropland affects only the destabilization of the soil structure and soil macroporosity but not particle transport over long distances. Basically it comes from two processes:

• mineralization of organic matter in the soil (more active in a hot, humid climate) and mineral uptake by crops (uncompensated by applications of manure), leading to a reduction in the activity of the micro- and mesofauna responsible for macroporosity;

• skeletonization or relative increase of sand or gravel in the surface horizons through selective erosion of fine particles, organic matter or nutrients as a consequence of rain splash, which compacts the soil, breaks up clods, and carries off particles which form thin slaked surfaces and sedimentation crusts in the vicinity, which then encourage runoff.

An example of the degradation chain for tropical soil is given in Figure 2.

Under tropical forests, soil is very well protected from sun and rain energy by the canopy (850 t/ha of biomass), which tempers temperature fluctuations, and also by the under-storey and especially the litter (8-15 t/ha/yr of organic matter recycled throughout the year) which feeds the mesofauna and quickly recycles nutrients (turnover). Roots are plentiful in the topsoil and up to the litter, reducing nutrient loss through drainage and runoff. A small number of roots penetrate to a great depth, taking up water and nutrients when the topsoil is dry. Scanty runoff (1-2%), 50% evapotranspiration and a similar amount of drainage result in the formation of deep homogenous soils, more acid on the surface than at depth. The vigour of forests (with the largest trees dominating at heights of over 35 m) may be misleading as to the fertility of the (ferralitic) soils on which they grow. Tropical forests in fact are continuously recycling their residues and recovering (from deep below) nutrients leached by drainage water or released from deep weathering of rocks and minerals, in a process described as biological upwelling (Roose 1980b).

Savannah is much less efficient in counterbalancing variations in energy. The biomass (50-150 t/ha) is much smaller, and the litter (0-5 t/ha/yr) is burnt off by frequent bush fires, leaving the soil bare to face the first brief but very violent storms. Runoff is therefore much greater than under forest, especially when there are late fires (Roose 1979).

The hotter and drier the climate, the more termites there are and the fewer earthworms, but termite tunnelling and turning under of organic matter (below the fire zone) are less beneficial than the activity of earthworms (Roose 1975). Evapotranspiration and runoff being stronger (because of slaking crusts) and rainfall less plentiful, the wetting front does not penetrate so far into the soil and deposits fine particles detached from the surface and iron compounds containing organic matter. These are the leached tropical ferruginous soils. Horizons vary more widely, and the soil is less homogenous. Roots regularly penetrate to the accumulation horizon, though not as deeply as under forest.

FIGURE 3 Water balance in the rainforest of Côte d'Ivoire and the savannahs of Burkina Faso (from Roose 1980a)

How does the situation develop under cultivation following clearance of forest or savannah?

In terms of plant cover, there is a simplification of the ecosystem (under forest there are more than 200 species of trees per hectare, fewer than 25 under savannah, and at best 2 to 4 species with mixed cropping). The biomass (0-5 t/ha) decreases, as does rooting, often hampered by cropping techniques (slaking crusts and deep tillage). Soil cover is reduced in time (4-6 month cycle) and provides poor protection from the sun's rays (higher temperatures are reached) and rain splash (slaking crust formation and heavy runoff).

At the level of the soil, the climate is hotter and drier under cultivation and energy less buffered than under forest:

• litter is much reduced, except where there are cover plants;

• levels of organic matter and micro- and mesofauna activity fall;

• macroporosity breaks down after a few years, and infiltration capacity decreases;

• soil becomes more compacted and spatial discontinuities develop: thin slaked surface and plough pan.

It is thus clear that cropping on cleared forest land is a real disaster, compromising the whole balance of the soil system. Nutrients are lost faster, compensatory deposits decrease, and the physical and chemical fertility of the land collapses after a few years' intensive cropping. There have been many instances of the failure of "modern" cropping, such as that of the Compagnie Générale des Oléagineux Tropicaux in Casamance during the 1950s.

Runoff and erosion are thus very clear alarm signals that the cropping system is out of balance with the environment and that soil fertility must be restored, either by a long period of forest fallow (20-30 years) or by robust measures to re-establish macroporosity (tillage), organic matter, the fermented biomass needed to revive it (manure or compost), and the dressing to strengthen its structure and improve pH. What still in fact has to be done is to work out better modern systems of clearing land and intensive production systems more capable of sustainable and balanced production than the traditional ones now in place.

Factors in the water balance

Rainfall and ephemeral inputs (dew, mist: a few dozen to 150 mm per year) vary greatly according to altitude, distance from the sea and orientation of hillsides to moist rain-bearing winds.

The different terms of the water balance must be defined (Figure 3):

Rain = Runoff + Drainage + Actual evapotranspiration ± Var. stored groundwater

Surface runoff is the excess rain which does not filter down into the soil, running along the surface, forming rivulets and quickly joining up with the river where it causes high peak floods in a relatively short time (response time about half an hour in a 1 km² basin).

Subsurface flow or interflow is slower, for it moves through the top horizons of the soil, which are often much more porous than the deeper mineral horizons (response time of several hours in a 1 km² basin).

Lastly, temporary and permanent water tables hold back the base flow of rivers because of much slower discharge (response time of several days for a basin of several km², or even some months for the largest basins).

Chapter 2. History of erosion control strategies

Soil erosion and population density
Traditional erosion control strategies
Modern strategies for developing rural water infrastructures
Land husbandry

Erosion is an old problem. From the time land emerges from the seas it is lashed by the forces of wind, waves and rain. In response, people try to counter the negative effects of these agents of erosion.

The development of agricultural production involves an increased risk of land degradation:

• either by expansion to new land which turns out to be fragile and becomes exhausted after a few years' farming, through mineralization of organic matter and removal of nutrients without adequate replenishment,

• or by intensification and the wrong use of inputs:

• intensive mineral fertilization can lead to soil acidification and water pollution (particularly if inputs are out of balance with crop requirements and soil storage capacity);

• irrigation reduces soil structure stability or results in salinization (in arid conditions);

• mechanization, especially motorization, speeds up the mineralization of organic matter in soil, the degradation of soil structure and the compaction of deep horizons, accentuating the soil's response to wetting (a sharp drop in infiltration rate at the ploughing depth, even when there is no real ploughing pan).

While there is an increased risk of soil degradation when land is put under cultivation, rural societies do their best to gradually build up techniques that will allow the long-term preservation of soil productivity (organic or lime dressing, drainage, multicropping). However, when new needs emerge too fast, a crisis will arise to which rural society cannot respond in time. And here the State must step in to help overcome the crisis by technical assistance (technical guidelines) and financial support (subsidies).

Soil degradation through erosion, acidification or salinization is probably one cause of the decline of ancient civilizations, in that population concentration in countryside and town led to excessive economic pressure on production from the countryside (e.g. 12th-century France, Egypt today). Where fields are no longer left fallow, soil degradation soon sets in, with nothing to compensate for what crops take from the soil or for losses from erosion or drainage.

FIGURE 4 Relationship between population density, erosion, cropping, stock-raising and fertility management

As early as 1944, the geographer Harroy had clearly realized why "Africa is a dying land": it was dying as a result of the destabilizing methods of colonial systems which intensified soil use, hastened removal of assimilable nutrients and mineralization of organic matter, and pushed the indigenous people on to the poorest and most fragile land, reducing the length of fallow periods. He advocated a three-pronged policy:

• full protection of national parks in order to protect natural ecosystems;

• terrace-type erosion control structures such as bench terraces or infiltration (blind) ditches;

• research on balanced cropping and production systems combining animal husbandry, forestry and agriculture (agroforestry).

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