Conservation farming on steep lands - USAid

118 N. W. HUDSON
not to extrapo!ate by taking a practice that is successful under one set of
conditions and trying to use it in very different circumstances. For example.
a systeni that achieves 100 percent infiltration may prove satisfactory
with moderate rainfall acd sufficient storage capacity in the profile, but
it could be disastrous on shallow soils sitting precariously on bedrock in
high rainf:ill areas. Most of us have seen cases where ihe whole soil mantle
is stripped off by rnass movement of saturated soils.
A critical issue is how much runoff is likely to occur. And how often?
Surface infiltration is fairly easy to measure and to change, but this is not
the case for subsurface percolation and moisture storage in the profile. These
are not easy parameters to measure, and they ilre difficult to change. But
they too nlay affect surface runoff. We also need to know the amount of
rainfall and its intensities and frequencies. That information, then, can be
combined with soil data to get an indication of what the hydraulic situation
will be We can, of course, get the required information on surface
runoff by direct measurement, but this is a slow and expensive process.
It should be possible to predict runoff to the accuracy required for designing
soil conservation nieasures by nlodelling runoff from farni land. All
that is required is some bookkeeping on the inflows and outflows, somewhat
like flood routing. The mathematical models I have come zcross tend to
lose themselves in unnecessary complexities, but I once had a student work
on a simple physical model that I think is worth pursuing (Figure 1).
Clzoosi~tg tlte tactics. Once we know the hydrdulic situation, we can begin
thinking about what to do about it. Theoretically, sorting out the land use
strategy should come tirst. Moving out of food crops into tree crops or
commodities might be the best solution to an erosion problem, but that
is not the subject of this paper. We will try to look at that subject in another
part of this book, but I must just make the point that using land for what
it is suitable for is alwdys better than trying to overcome the problems after
using it unsuitably.
Looking at the nuts and bolts of runoff management, let us consider the
possible tactics. The first possibility is !o minimize runoff, or perhaps prevent
it entirely. In many cases, this will be ocr first choice, and there are
several examples of this approach in another part of this book. But minimizing
or eliminating runoff is not always possible, for instance, where rainfall
exceeds the absorption capacity, nor is it always desirable. Some crops
are adversely affected if they do not shed some surface runoff.
The second situation is where we must accept that some runoff is inevitable
and aim to control it so that it runs away with minimal damage.
This approach did vcry well in North America and was the mainstay of