Global Extent of Soil Erosion

Soil erosion costs the US economy between US$30 billion (Uri & Lewis 1998) and US$44 billionn(Pimental et al. 1993) annually. The annual cost in the UK is estimated at £90 million (Environment Agency 2002). In Indonesia, the cost is US$400 million per year in Java alone (Magrath & Arens 1989). These costs result from the effects of erosion both on- and off-site.
On-site effects are particularly important on agricultural land where the redistribution of soil
within a field, the loss of soil from a field, the breakdown of soil structure and the decline in
organic matter and nutrient result in a reduction of cultivable soil depth and a decline in soil fertility. Erosion also reduces available soil moisture, resulting in more drought-prone conditions.
The net effect is a loss of productivity, which restricts what can be grown and results in increased
expenditure on fertilizers to maintain yields. If fertilizers were used to compensate for loss of fertility arising from erosion in Zimbabwe, the cost would be equivalent to US$1500 million per year (Stocking 1986), a substantial hidden cost to that country’s economy. The loss of soil fertility through erosion ultimately leads to the abandonment of land, with consequences for food production and food security and a substantial decline in land value.

The on-site costs of erosion are necessarily borne by the farmer, although they may be
passed on in part to the community in terms of higher food prices as yields decline or land
goes out of production. The farmer bears little of the off-site costs, which fall on local authorities for road clearance and maintenance, insurance companies and all the land holders in the
local community affected by sedimentation and flooding. Off-site costs can be considerable.
Erosive runoff from arable land in four catchments in the South Downs, England, in October
1987 caused damage equivalent to £660,000 (Robinson & Blackman 1990). Sedimentation ponds to trap sediment and runoff generated from arable land in an area of 5516 km2 in central Belgium cost €38 million to construct and €1.5 million annually to maintain (Verstraeten & Poesen 1999).

Table: Annual rates of soil erosion in selected countries (ton per hectare)

Countries	Natural Soil	Cultivated Soil	Bare Soil
China	0.1 - 2	150 - 200	280 - 360
USA	0.03 - 3	5 - 170	4 - 9
Australia	0.0 - 64	0.1 - 150	44 - 87
Ivory Coast	0.03 – 0.2	0.1 - 90	10 - 750
Nigeria	0.5 - 1	0.1 - 35	3 - 150
India	0.5 - 5	0.3 - 40	10 - 185
Ethiopia	1 - 5	8 - 42	5 - 70
Belgium	0.1 – 0.5	3 - 30	7 - 82
United Kingdom	0.1 – 0.5	0.1 - 20	10 - 200

[Sources: Browning et al. (1948), Roose (1971), Fournier (1972), Lal (1976), Bollinne (1978), Jiang et al. (1981), Singh et al. (1981), Morgan (1985a), Boardman (1990), Edwards (1993), Hurni (1993)]

The rate of soil loss is normally expressed in units of mass or volume per unit area per unit
of time. Under natural conditions, annual rates are of the order of 0.0045 t ha-1 for areas of moderate relief and 0.45 t on per hectare for steep relief. For comparison, rates from agricultural land are in the range of 45–450 t ha-1 (Young 1969). These differences have encouraged many researchers and practitioners to distinguish between ‘natural’ and ‘accelerated’ erosion, the latter being the result of human impact on the landscape. In practice, such a distinction is often unhelpful because it leads to a view that all unacceptably high rates of erosion must be accelerated, whereas the rates are actually dependent on local conditions. So-called accelerated rates of erosion in lowland England may, in fact, be an order of magnitude lower than the natural rates recorded in the Himalaya, Karakoram or Andes. Theoretically, whether or not a rate of soil loss is severe may be judged relative to the rate of soil formation. If soil properties such as nutrient status, texture and thickness remain unchanged through time, it can usually be assumed that the rate of erosion balances the rate of soil formation. More practically, severity is better judged in relation to the damage caused and the costs of its amelioration.