Agriculture Reference
In-Depth Information
In the Negev desert of Israel, once-abandoned systems
of small catchment runoff farms have been reconstructed
and made to produce crop yields equivalent to those of
irrigated farms in the same region (Evenari, et al., 1961).
The farm unit consists of catchment areas for rainfall on
the slopes of the watershed surrounding flattened drainage
channels where runoff is collected. Low rock walls chan-
nel rain runoff down into the small flood plain of the
channels. This system can collect 20 to 40% of the rainfall
that occurs, and removing loose rock from the soil surface
on the hillsides can increase runoff collection to as much
as 60%. Small rock checkdams in the larger channels at
the bottom of the slopes concentrate runoff to a depth
sufficient to allow water to penetrate to approximately 2
m into the soil, after which the soil dries and leaves a crust
relatively impervious to evaporative water loss. As each
check dam fills, it spills over into others below, watering
a complex system of floodplain farm plots. Crop yields
of grains such as barley and wheat, and fruits such as
almonds, apricots, and grapes, are quite respectable for
such an arid region. Rather than attempt to create large
reservoirs of water that would mostly evaporate in such a
climate (and accumulate nutrient-rich sediments), both
water and nutrient-rich sediments are stored on-site in the
water harvest system (Figure 6.6).
A similar system still is used in the arid American
Southwest, where native American groups such as the
Hopi and Papago have been practicing a form of water
harvesting for many centuries (see the accompanying
case study). The flow from heavy convective rainfall in
the mountains during the summer is diverted over allu-
vial fans as a shallow sheet of runoff, rather than being
allowed to concentrate in a stream channel. This sheet
of water then “irrigates” annual crops of corn, beans,
squash, and other local crops. The upper watershed is
not manipulated as in the Negev system, but similar
manipulation of runoff on the floodplain below takes
place. The goals of both agroecosystems are to work
within the constraints and limits of the natural rainfall
regime.
FIGURE 6.6 Fruit and olive trees in the Negev Desert near Avdat in Israel. Rainwater is harvested from the surrounding hillsides
to provide soil moisture for the orchard.
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