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4.3 Floodwater Harvesting
Flood flows are a feature of all landscapes, including regions of water scarcity. A
large part of the annual flow may occur in one or two floods, but the flow is often so
large that the water passes through the region and cannot be used where the rain fell.
Some advantage can be gained from these large flows by encouraging them to
spread across flat areas (Fig. 11 ). If water can be retained on flat surfaces for a day
or two, the upper soil layers may be saturated or water may percolate downwards to
replenish the local aquifer. In both these circumstances, the water thus ʻharvestedʼ
is available for later use, in the first case for crop growth or to support grazing and,
in the second case, for whatever purpose groundwater is used (Prinz 1996 ; Missaoui
1996 ). Where the terrain is suitable, it may be possible to restrict flow in the river
channel and force water to flow over the floodplain or into an old floodway. Flood
spreading by these means is most easily achieved when the upstream–downstream
gradient of the river valley is quite small. With small longitudinal gradients, water
forced onto the floodplain tends to flow in the downstream direction very slowly,
allowing maximum time for infiltration to occur (Pereira et al. 2002 ).
Often referred to as water spreading or spate irrigation, flood water harvesting
(FWH) is involved in the collection and storage of creek flow for irrigation use
(Prinz and Singh 2000 ). The main characteristics of FWH are a turbulent channel of
Fig. 11 Floodwater harvesting in Balochistan, Pakistan: (a) water diversion structures on ephem-
eral stream (top) and (b) spreading of floodwater on the agricultural land (bottom) (Source: Akhtar
Ali)
A. Yazar and A. Ali