Geoscience Reference
In-Depth Information
Table 22.1
Some examples of indigenous soil and water conservation systems from African drylands.
Country (region)
Average annual ppt (mm)
Technique(s)
Major crops
Burkina Faso (Mossi)
400-700
Stone lines, stone terraces,
planting pits
Sorghum, millet
Chad (Ouddal)
250-650
Earth bunds
Sorghum, millet
Mali (Djenne-Sofara)
400
Pitting systems
Sorghum, millet
Niger (Ader Doutchi Maggia)
300-500
Stone lines, planting pits
Sorghum, millet
Somalia (Hiiraan)
150-300
Earth bunds
Sorghum, cowpeas
Sudan (West)
50-800
'Wadi agriculture', various
water-harvesting techniques
Sorghum, vegetables
Source:
after Critchley, Reij and Willcocks (1994).
techniques are well documented from deserts in Africa,
the Middle East, North and South America, and are still
widely used today (Gilbertson, 1986; Motsi, Chuma and
Mukamuri, 2004; Oweis and Hachum, 2006). Some con-
temporary examples from the drier parts of Africa are
shown in Table 22.1.
Terracing is a very ancient mechanical soil conservation
technique that has been employed for up to 5000 years in
Yemen, for example (Wilkinson, 1997). Terraces are com-
monly built on steep slopes and are designed to transform
them by creating a series of horizontal soil strips along the
slope contours. They intercept runoff, reducing its flow to
a nonerosive velocity, thus also acting to conserve water.
If well maintained, terraces are very effective, but they are
costly to construct and their physical dimensions act as a
constraint on the use of mechanised agriculture.
Check dams are constructed in a similar way to ter-
races and are designed to control erosion in normally dry
valleys. The dam impedes the flow of water generated by
the occasional rain storm, causing the deposition of sedi-
ment - which helps to build up a soil - and encouraging
the ponded water to soak into this alluvial fill. A small
check dam might support a single fruit tree, for example.
Larger dams may allow a cereal crop to be planted in a
small field. In Tunisia this type of small check dam and
its associated terraced area is called a 'jessr' (plural: jes-
sour). Jessour, which cover an area of about 4000 km
2
in
the south of the country, allow crops to be grown in an
environment that is otherwise too arid for agriculture. The
most common crops are olive trees and drought-resistant
annual grains with a short growing period, such as wheat
and barley (Schiettecatte
et al
., 2005).
More sophisticated systems for runoff farming or water
harvesting involve long stone bunds or walls that snake
across low-angle slopes to channel any storm-generated
overland flow towards a target portion of the valley floor.
area by the clearance of obstacles to overland flow such as
pebbles and boulders. Removing larger obstacles means
that soil pores quickly become clogged with fine mate-
rial when water flows across them, effectively sealing the
surface, reducing infiltration and thus generating greater
runoff (Evenari, Shanan and Tadmor, 1968).
A geographical review of these techniques by Bruins,
Evenari and Nessler (1986) indicates that some of the old-
est examples are found in the Middle East. In the Negev,
rainwater harvesting dates back at least 2000 years but a
well-developed runoff farming system at Jawa in Jordan
seems to be at least 5000 years old. There is also some
evidence to suggest that runoff was used for agriculture at
a site called Beidha in the Edom Mountains in southern
Jordan nearly 9000 years ago.
A similar range of techniques has also been employed
by traditionally nomadic people in the Karakum Desert in
Turkmenistan to secure water supplies for consumption by
their livestock and to produce some crops for themselves
and their animals (Fleskens
et al
., 2007). These methods
focus on flat or slightly sloping clay surfaces - 'takyrs' -
with little or no vegetation, which act as natural catchment
areas.
Several types of hand-dug basin that collect rainwa-
ter runoff provide temporary storage points for watering
livestock on takyr surfaces. In some cases dome-shaped
brick covers have been built over these small reservoirs
to reduce evaporation. Another technique is to increase
the productivity of small depressions in the takyr, where
water collects naturally, by digging trenches to channel a
greater flow of runoff. The depressions, or 'oytaks', have
a sandy topsoil and are used for hay production, but crops
such as melons and gourds are also grown. Typically, a
cultivated oytak of 10 m
2
requires at least 1000 m
2
of
catchment to provide it with adequate moisture.
Should any of these techniques designed to maximise
