13ème Congrès Mondial de l'Eau, Montpellier, France

abstract 385

Spatial redistribution of water resources in a Tunisian semi-arid catchment subject to conservation works

Author(s): Guillaume Lacombe, Christian Leduc, Bernard Cappelaere, Mohamed Ayachi, Monique Oi, Mohamed Kefi
Author addresses : Guillaume Lacombe: a, b Christian Leduc: a Bernard Cappelaere: b Mohamed Ayachi : c Monique Oi : b Mohamed Kefi : c a: UMR G-EAU (Cemagref, CIHEAM-IAMM, CIRAD, ENGREF, IRD, SupAgro). MSE, BP 64501, 34394 Montpellier Cedex 5 - France. Tel/Fax : + 33 4 67 14 90 91/72. b: UMR HSM (CNRS, IRD, UMI, UMII). Université Montpellier II, MSE, BP 64501, 34394 Montpellier Cedex 5 - France c: Commissariat régional au développement agricole, Tunisian Ministry of Agriculture, Kairouan, Tunisia

Keyword(s): Hydrological change, Water/soil conservation works, Conceptual model, Semi-arid area, Tunisia.

Article:

Poster:

Session: OS6k Rural water conservation and reuse

AbstractIn

semi-arid areas, hydrological impact of water and soil conservation works (WSCW) has most often been studied at

the local level. Studies on the regional scale are rare. In central Tunisia, the Merguellil catchment (1183 km2) defined

by the big El Haouareb dam has been subject to WSCW for several decades. They consist of contour ridges and

small earth dams collecting hillslope runoff and wadi flow respectively. 97% of the surface areas equipped with

WSCW between 1989 and 2005 are located in the lower area of the Merguellil catchment, downstream the Skhira

upper subcatchment (189 km2). In this lower area, 32% of the surface area is covered by WSCW. A local and a

regional approach were developed. The first one performed water budgets at the reservoir scale. In average, one

third of the flow collected by earth dams is lost through evaporation and 12% are pumped for additional irrigation.

This calculation is in good agreement with field surveys. Simulations with a coupled rainfall-runoff/water balance

model indicated that estimated water withdrawal corresponds to a 50% water shortage risk. In contour ridged

hillslopes, the whole runoff collected in the ditches is probably evapotranspirated without any agricultural yield

increase. The second approach characterized the hydrological changes induced by WSCW at the Merguellil

catchment scale. The space-time variability of the rainfall-runoff relationship was analysed, comparing the runoff

responses of the Skhira upper subcatchment and of the lower area. A non-parametric test, based on a resampling

approach, was applied to the lower area runoff simulated with the GR4J daily rainfall-runoff model. Results indicate

that between periods 1989-1996 and 1997-2005 the runoff produced by rainfall below 40 mm was reduced by

over 70% in the lower area, a significant change at the 95% confidence level. This drop in runoff was estimated at

about 45% when considering all rain depths and at about 30% when including the Skhira upper subcatchment in the

analysis. No runoff change was found for rains above 40 mm in the lower area, nor for any rain depth range in the

Skhira upper subcatchment. Possible sources for the runoff reduction (climate, land use/land cover, or water

exchanges with the aquifer) were considered. The WSCW appear as the most likely cause. Their impact on the

water resources produced and stored in the Merguellil catchment was assessed from the local and regional

approaches. Expressed as percentage of total runoff produced in the catchment, infiltrated and pumped volumes

changed from 54% to 45% and from 18% to 14% respectively. Evaporation changed from 27% to 41%. These

changes are equivalent to a global 19% water resources decrease in the Merguellil catchment. Consequences for the

regional economy could become deleterious, should WSCW development continue in the Merguellil catchment. The

El Haouareb dam located at its outlet is the main recharge point for the overexploited Kairouan aquifer which is the

major water resources for this populated area.

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Spatial redistribution of water resources in a Tunisian semi-arid catchment subject to conservation works