67water flow harvested either by diversion or spreading within a channel bed/valley
floor where runoff is stored in the soil profile (Critchley et al. 1991 ). Two categories
of FWH are macrocatchments and large catchments.
4.3.1 Macrocatchment Water Harvesting
Macrocatchments, sometimes called medium-sized catchments, are characterized
by large flood zones that are situated outside the cropping area. Often farmers must
use structures such as dams or bunds to divert, transfer, collect and store the runoff.
Such systems are often difficult to differentiate from conventional irrigation sys-
tems and are considered FWH as long as the harvested water is available year
around (Mbilinyi et al. 2005 ). Examples of macrocatchments include stone dams,
large semi-circular hoops, trapezoidal bunds, hillside conduit systems and culti-
vated reservoirs, which from 1000 m^2 to 200 ha (Prinz 1996 ).
4.3.2 Large Catchment Water Harvesting
Large catchment water harvesting (LCWH) comprises catchments of many square
kilometers, from which runoff water flows through a large stream bed (also known
as Wadi) necessitating a more complex dam structure and distribution network (Fig.
12 ). There are two main forms of LCWH: Floodwater Harvesting (FWH) within a
stream bed and Floodwater Diversion. FWH involves blocking the water flow in
order to flood the valley of an entire floodplain and force the water to infiltrate the
ground for crop production or pasture improvement. Floodwater diversion is a
method where the river, stream (wadi) or creek bed water is diverted from its natural
course and used to flood nearby areas as an irrigation method (Majdoub et al. 2014 ).
The ʻJessourʼ (or jesr) technique has long been known as a way of exploiting sur-
face runoff water for agriculture in arid regions. It is a typical system in the highlands
Fig. 12 Macrocatchment water harvesting (Source: Akhtar Ali)
Water Harvesting in Dry Environments