Controlling Soil Erosion and Off-Site Impacts

The control of erosion on the field may or may not coincide with a desire to prevent off-site impacts such as down-valley flooding. With different aims, different approaches may be needed. In Western Europe, most recent concern has been about off-site impacts because of their high social and economic costs rather than the long-term loss of the soil resource associated with erosion on the field. Off-site impacts must be dealt with at a catchment scale. The challenge is to find mechanisms of incentivizing land managers in parts of the catchment where runoff and erosion are occurring, to invest in mitigation measures for the benefit of others. Thus, the emphasis has shifted to the problem of connectivity. This is especially apparent in arable-dominated landscapes where both anthropogenic features such as tracks, roads, and ditches enhance long-distance flow routes and large areas of the same crop give rise to the risk of runoff at the same time.1811

Controlling soil erosion is based on two principles: protecting the soil from runoff or wind by providing vegetative protection, and reducing the slope and thereby decreasing water velocity. Erosion generally takes place on bare soil (Figure 28.1), and therefore, the protection of a crop or crop debris at vulnerable times of the year is important. Most radical solutions such as minimum tillage and conservation tillage aim to have some degree of vegetation cover throughout the year and therefore drill the new crop through the remnants of the old, thus avoiding the bare ground inherent in tillage systems involving the plough.

The break-up of large fields into strips of different crops can also reduce the risk. Valley-bottom zones of concentrated flow may be grassed (grass waterways) to prevent erosion. Buffer strips of grass are widely used around arable fields in order to reduce runoff volumes and loss of soil from fields. Evans and Boardman show how return to grass of a small area interrupts valley-bottom runoff from large areas of winter cereals, thus preventing flooding of houses.!82,831

The desire to reduce slope gradients has traditionally been addressed by the construction of terraces. An added advantage is that a terrace allows for the cultivation of slopes that were too steep for agriculture. Terrace systems are known from the Middle Bronze Age on Crete1841 and from the Incas of Peru. Many systems around the Mediterranean are now abandoned, and erosion associated with their postabandonment state has been much researched.1851 Maintenance of terraces is therefore vital if they are to prevent erosion (Figure 28.11).

It is doubtful if any one conservation technique will be effective on its own. Most successful schemes involve several approaches. For example, the Melsterbeek catchment soil and water conservation scheme in Flanders uses grass waterways, buffer strips, and retention ponds (Figure 28.12). Successful schemes, such as this, must involve the local population of land owners and managers which generally requires financial support and technical assistance.I86,871 However, the scheme is successful in that it reduces the amount of damage caused by muddy flooding and the costs of mitigation measures are far less than the costs of damage.'871

In the Palouse wheat-growing area of Washington State, high erosion rates have persisted for over 100 years and are particularly associated with the practice of summer fallow. The costs of reducing erosion by conservation practices are well illustrated:

Applying various measures of conservation treatment to the land will affect the economy of the

basin. But erosion rates can be reduced by 40% in the low and high precipitation zones and 60%

in the intermediate precipitation zone—without decreasing farm income. The erosion rate can be

(A) Old abandoned terraces and recently rebuilt terraces, Mallorca, Spain (J. Boardman)

FIGURE 28.11 (A) Old abandoned terraces and recently rebuilt terraces, Mallorca, Spain (J. Boardman).

(B) Abandoned terraces, central Corsica, France (J. Boardman). (C) Abandoned and degraded terraces, Eritrea (J. Boardman).

Mitigation measures

FIGURE 28.12 Mitigation measures: grass buffers along field edges, the Melsterbeek catchment, Flanders (Belgium) (Karel Vandaele, Samenwerking Land 8t Water).

reduced 80% through maximum levels of conservation practices and retirement of 35,000 acres of steep, erodible land. Achieving this level of reduction would cost more than $29 million in reduced productivity and increased operating costs.1891

Soil erosion is often best addressed in a wide framework of natural resource improvements. The issue of sustaining the livelihoods of the farmers as well as introducing conservation measures is critical. Examples of successful approaches from West Africa, New Zealand, southern Brazil, northwest India, northern Cameroon, and Kenya are discussed by El Swaify et al.1881 There is also the well-known case of the Machakos District, Kenya.1891 Hudson discusses reasons for the failure of soil conservation projects.1901

Previously quoted extremely high erosion rates for the Loess Plateau, China, and associated high sediment yields for the Yellow River, appear to have been substantially reduced in recent years. Water- soil conservation practices are credited with 40% reduction in sediment loads and sediment trapping by major reservoirs with 30%, and most of the remaining is due to precipitation decrease.1911

We are left with the challenge of how to better organize agriculture and food production in a world of increasing population, climate change, and inequality. Soil erosion and its assessment play a part in this conundrum. Some form of changed or non-conventional agriculture is clearly needed. Among many suggestions, “smart intensification” offers some answers, but putting it into practice remains a major challenge.1761

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