This floodwater diversion technique is very common in the plains adjacent to great wadis in Central Tunisia. It is a channel system based on the diversion of floodwater from its natural course in wadi beds to the nearby fields. The floodwater is diverted by solid dikes and lateral channels with minimal slope (“mgoud�) and then distributed by an extensive network of drainage channels to the irrigated area with fruit trees, cereals and vegetables (see figure 14). The crops are planted in parallel strips with a surface from 3 to 5 ha; each enclosed by small bunds and separated from the other strips by uncultivated land.

This kind of technique supplies water to the crops only after significant rainfalls. But even though low precipitation does not generate any runoff in the ravines or wadis and leaves the cropping zones completely dry, already one single shower with a high intensity can regen­erate the river and supply the water reserves for a whole vegetation period (Alaya et al., 1993). Water-spreading systems like the mgouds need a careful design and engineering layout to withstand floods and prevent erosion. Since the deviated water is part of the wadi water, the relation between the surface of the impluvium and the cropped area (CCR) cannot be set up.

Figure 14: Mgoud system (Chahbani, 1997 and Oueassar)