The FAO (1979) has estimated that the world's agricultural production will need
to be increased by some sixty percent by the year 2000 if it is to keep pace
with the increase in population. The increase in production will need to come
from both an intensification of production on existing cultivated land and by
taking an estimated extra 200 million hectares into production.
Kovda (1977) has, however, estimated that 5 to 7 million hectares of land are
going out of cultivation each year due to soil degradation. If this estimate
is correct, and few would seriously question it, the extra 200 million hectares
of land to be taken into production will be necessary just to maintain the
present cultivated area of about 1.5 billion hectares and will not allow for an
increase in production.
Historians have often related the decline of civilisations to a decline in
fertility of their lands, but despite the lessons of history and more recently
some fifty years of intensive study the 'problem remains one of the most
Serious threatening the future of manking' Greenland (1977).
Although the problem is world wide, land degradation, mainly caused by soil
erosion, is especially severe in the Tropics where highly erodible soils and
highly erosive rainfall combine to magnify the risk. It is just these areas,
however, that lack the financial and technical resources necessary to combat
erosion by using the conventionally recommended techniques, such as terracing
and contour drains. In consequence a number of researchers, especially in
Africa, have been re-examining the wisdom of the conventional methods of dealing
with the problem. This examination was initiated by Hudson (1957) but has
gathered strength following the work of Lal, among others, at the International
Institute of Tropical Agriculture, Ibadan.
The conventional methods derive largely from the accepted practices of those
areas brought into cultivation by the 'classical' civilisations of both the old
world and the new, and the work carried out by organisations such as the United
States Soil Conservation Service. Work carried out in the 1940's and 1950's
was directed towards improving the conventional techniques and evaluating the
factors affecting the rate of erosion. This work culminated in the production
of the Universal Soil loss Equation (Wischmeier and Smith, 1960). Already,
however, Hudson (1977) was drawing attention to the important point that
although we could study and measure the effects of rainfall intensity, soil
erosivity and slope, we could not hope (or only at great cost) to affect
their influences on the rate of erosion.
In carrying out a study of erosion in St Catherine, Jamaica it was, therefore,
decided to evaluate the various land use types to determine which were having
the greatest and least effects on the rates of erosion and, on the basis of
this information to draw up a list of recommended land use types. It was
realised that the erodibility of the soils, the slopes and the rainfall would
all affect the observed rates of erosion for a given land use. The recommended
land uses were, therefore, made for specified soil/slope combinations and the
study was carried out in an area of fairly homogeneous rainfall.
It was decided that the methodology developed for this study should be both low
cost, and as simple as possible to apply. This would allow it to be adopted
by those countries lacking the resources in manpower and equipment to carry out
more sophisticated studies. It was, therefore, decided to use medium scale;
conventional black and white aerial photography, simple interpretation
techniques based on a photographic key, and basic transfer and analytic
techniques.
936
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