Conservation farming on steep lands - USAid

CONSERVATION PRACTICES AND RUNOFF WATER DISPOSAL 121
red clay loams in Brazil and Indir?, but on the black clay soils of India
it has proven counterproductive because it tloods the crop.
Large trenches to store surface runoff are sometimes used in the early
stages of forest establishment as a temporary lneasure until the vegetative
cover takes over. They are usually built without my gradieiit and with cros:;-
ties every 10 to 15 meters to restrict water movement in the channel.
Errcrcitr: in stages. The labor required to construct bench terraces is
c1;nsiderabie. It can be reduced in total and spread over a longer period
of time Lj using downhill niovement of soil to help level the terraces. This
is sometimes described as using downhill erosion to form the terraces, but
in tnost cases the downhill movenient of soil resulting from cultivation has
more effect.
Maher (8) described the use of this systeni in Puerto Rico in the 1940s,
where masonry walls were buiit up in several stages over a number of years
(12) (Figure 2a). A rriore recent application of the same principle occurred
in Venezuela (15)(Figure 2b). Where stone is not available, the effect can
be achieved by earth banks, as in the fanya juu system in Kmya (13). The
name means to throw uphill because the soil to form the bank is exc~vated
froin the downhili side and thrown up to form the bank. Vegetation is planted
on the edge of the bank and the downhill bce to stabilize the bank and
to increase deposition on the uphill side. Periodically, the process of throwing
up more soil onto the bank is repeated.
A useful variation that speeds up the process and sprcads the labor requirement
is to put in fanya juu terraces in two stages (Figure 2c). First,
terraces are put in with a vertical i~itcrval of 2 meters; then, at a later stage,
additional lines are put in-between. In Kenya, use of this method has resulted
in nearly level terraces in as little as 7 years.
The width and spacing of bench terraces is determined by a few simple
factors. The width is usually dictated by the method of cultivation. Oxen
and tractors, for example, need a minimum width to turn.
On the other hand, the volume of earth moved increases with the width
of the terrace since:
where C is the cross-sectioned area (m2), W is the bench width (m), and
Hr is the heigl.1.i of the riser (m) and is the vertical interval plus the change
in elevation across the terrace if there is a reverse slope (Figure 3).
The possible width of the terrace without excavating into subsoil or rock
is a function of both soil depth and land slope (Figure 4). A simple design
method suggested by Hurni (7) is to use a vertical interval of I meter for