Merguellil catchment, Kairouan
Size: ca. 4 200 km²
Wadi Merguellil is the second main river of central Tunisia. The Merguellil catchment has limited and fragile water resources and faces an increasing water demand (mainly for agriculture and drinking water), which leads to groundwater overexploitation. Typical of the high pressure over Tunisian water resources, this region is considered by national authorities as a pilot for developing new integrative approaches of water management. Several previous national and international (IWMI, FAO, EU FP6...) studies were performed there.
The hilly Merguellil upstream (1200 km2, 200-1200 m a.s.l.) contrasts with the very flat downstream Kairouan plain (more than 3000 km2). Rainfall is highly variable in time and space (annual means of 300 mm in the plain and 500 mm in the heights; extremes of 108 and 703 mm in Kairouan). River flow is not perennial. Aquifers represent the only permanent and reliable water storage. Soil and water conservation works, affecting more than 25% of the upstream area, significantly reduce the surface runoff and the stream flow. The big El Haouareb dam (95 Mm3) loses most of its water by 1) infiltration in karstic fissures and 2) evaporation; it has often been completely dry for the last decade.
Before the 20th century, the little population, mainly nomadic, exploited the region as an extensive grazing area. Deep changes occurred in the last century, with a spectacular demographic growth and a rapid development of agriculture, in response to local and national incitements. Irrigation is now widely spread all over the Kairouan plain and depends on groundwater; main crops are cereals, olive trees, tomato, melon, water melon, chilli. In the upstream, crops are more various and irrigation less present. Social and economic constraints (increasing the general welfare of the people, maintaining the rural population) are strong and sometimes antagonistic balancing (equity vs water productivity). Because of the semi-arid climate, the regional water resources are naturally very variable with time. Conservation works in the upstream (higher evaporation and infiltration on the spot) decrease the available resource and increase its vulnerability. Added to the continuous increase in pumpings, this makes the situation worse every day and moves away the idea of a sustainable development. Climatic change in the next decades should even damage the present state of imbalance between resource and demand.