The Environmental and Social Costs of Improvement 27In Britain, a major cause of soil erosion has been the shift in recent years
towards the cultivation of winter cereals, driven by production-oriented policies.
The high price of wheat has encouraged winter cultivation in fragile environments
and this has led to a massive increase in soil erosion. It was long thought that water
erosion was not a problem for British agriculture. But, since the late 1960s, the
land sown to winter cereals has tripled, largely at the expense of grassland and spring
cereals. Erosion can be of the order of 30–95t/ha in fields where field boundaries and
hedges have been removed from critical positions. Bob Evans estimates that some
6200km^2 (4.4 per cent) of land in Britain is now at high risk and some 2100km^2 (1.5
per cent) at very high risk. Erosion is greatest when there is little vegetative cover,
such as during winter when winter cereals are being grown; when slopes are long,
such as in big fields; and when farmers cultivate up and down slopes, rather than
across the contour (Evans, 1990a, 1990b).
On the South Downs in England, for example, erosion was uncommon until
winter cereals were widely grown. In the late 1970s, only 5 per cent of these chalk
downs were under winter cereals, but this increased to 65 per cent by 1992. In the
past ten years, loss of soil accompanied by flooding has caused many incidents of
flooding of housing and farms, causing several hundred thousand pounds worth of
damage (Boardman, 1990, 1991; Boardman and Evans, 1991; Robinson and
Blackman, 1990).
To farmers, erosion reduces the biological productivity of soils and the capac-
ity to sustain productivity into the future. Although soil erosion is clearly costly to
economies as well as to farmers, it is difficult to calculate reliably the precise costs
(Eaton, 1993; Bishop, 1990). Studies in Mali, Malawi and Java suggest that the
costs to farmers are substantial, representing 3–14 per cent of gross agricultural
product (Table 1.4).
Off-site costs are also important. Soils are less able to retain water, which runs
off more readily into waterways carrying sediments. These block downstream irri-
gation canals, reservoirs and harbours. Reduced volume means both greater maxi-
mum flows and so more likelihood of floods, and reduced minimum flows in dry
seasons. The functional lifetime of reservoirs has declined in many countries. And
natural systems, particularly fisheries and coral reefs, are threatened by sediments
and agricultural pollutants.
In Java, sedimentation costs the economy US$25–90 million each year by
shortening the life of reservoirs, reducing hydroelectric output, and increasing
maintenance needs for dredging irrigation systems and harbours. In Thailand, a
steadily depleting capacity of 20 reservoirs costs US$0.3 million in forgone income
from reduced irrigation capacity and in lost hydroelectric capacity. Greater costs,
some $18 million annually, are incurred by the need to dredge 19 million m^3 of
sediment from the Chao Phraya River to keep the channel open to shipping. These
off-site costs can be substantially greater than the financial losses to farmers. At one
site in the UK, where soil loss reached 250t/ha, the estimated total loss to the farm-
ers was just £13,000, mostly due to lost seed and fertilizer, compared with £400,000
of damage to housing (Robinson and Blackman, 1990).