| Popis: |
This project investigates applications of GIS and remote sensing to advance the hydrological understanding and improve the management of the water resources in large semi-arid regions. In the Lake Chad Basin, Africa, it is demonstrated how remotely sensed data can contribute significantly to a groundwater problem, something which historically has not often been achieved, particularly in semi-arid areas. In semi-arid areas, water is scarce and groundwater is often the only perennial resource available for the population. In the central part of Lake Chad Basin, this study focuses on the Quaternary aquifer which covers a vast surface area of 500,000 km 2 and provide most of the water used by human activities (Eberschweiler, 1993; FAO-Schroeter and Gear, 1973; UNESCO-PNUD-CBLT, 1972). So far, there are significant differences in the estimations of recharge and discharge phenomena of the Quaternary aquifer. Another scientific issue is the presence across the Quaternary aquifer of large piezometric depressions (Eberschweiler, 1992; FAO-Schroeter and Gear, 1973; Greigert, 1979; Schneider, 1969; Schneider and Wolff, 1992; UNESCO-PNUD-CBLT, 1972). Although, various theories about their formation and their mechanism have be raised (Aranyossy and Ndiaye, 1993; Dieng and Ledoux, 1987; Dieng et al., 1990; Durand, 1995), up to now little or no evidence has been gathered to confirm a particular explanation. An analysis of the basin's data shows that the use of GIS and appropriate remotely sensed data can greatly enhance the information currently available to hydrologists and hydrogeologists. The use of GIS and remote sensing to map groundwater recharge and discharge areas constitutes, in this kind of environment, a novel application. In the centre of the Lake Chad Basin, this approach has highlighted our knowledge of recharge and discharge processes, and it has enabled mapping major recharge and discharge areas. Among the outcomes, this approach has compiled, for the first time, evidence that the piezometric depressions correspond to very low infiltration areas. The fact that the rainfall recharge is considerably limited leads us to believe that evapotranspiration processes dominate the vertical exchanges, and that the piezometric depressions correspond to discharge areas. In the dunefields, a multitude of small discharge areas are revealed by vegetation indices, which show that the vegetation remains very active during the dry season. Such areas correspond to active oases, and were mapped in the Manga and at the border of the Harr and Kanem regions. Surprisingly, there are neither active oases, nor any tree layers, over the piezometric domes. One can deduce that the transpiration processes are lower than in the rest of the dunefield, and thus that the net recharge of the aquifer might be higher. Overall, throughout the Quaternary aquifer, surface characteristics (topography, soil permeability and vegetation) appear to have a strong influence on recharge and discharge processes. A groundwater model of the whole of the Quaternary aquifer was developed to explore novel applications of GIS and remote sensing in groundwater modelling. The model has allowed new knowledge of the aquifer system to be gained and has offered a first quantification of the groundwater reserves. The model's calibration in steady-state was first conducted independently of applications of CIS and remote sensing to map groundwater recharge and discharge areas. Outcomes clearly back up the information revealed with GIS and remote sensing. It was then possible to use remote sensing and GIS to improve the calibration of the model with a finer definition of recharge and discharge areas. The steady-state model has given information on the value and the distribution of long-term regional recharge and discharge. Rainfall recharge takes place in the dunefields, but appears to be small (less than 1 mm/yr in the Manga and less than 5 mm/yr in the centre of the Harr and Kanem). The model shows the necessity of representing the piezometric depressions as discharge areas. A good representation is obtained for a discharge rate below 3 mm/yr. The contribution from Lake Chad is very different from previous estimations and is thought to be less than 100E+06 mVyr (Carmouze, 1983; Isiorho, 1996; Roche, 1980). The model was then extended to a transient simulation from 1960 to 2000. Satellite archived data and GIS have allowed a comprehensive reconstruction of the fluctuations of the extent of Lake Chad. These data were implemented into the groundwater model in order to assess the impact of this major environmental change on the aquifer. This novel application, which makes an intensive use of remote sensing and GIS in the model, demonstrates the value of archive satellite data for long-term groundwater modelling. The model shows that the impact of the shrinkage of Lake Chad on the aquifer is limited in space to the Lake's region. It also reveals that the aquifer's reserves are threatened by the increase of the population in densely populated areas (Maiduguri, N'Djamena and "zone de concentration"). The drop of the water table in the dunefields could be related to a decrease of the rainfall recharge by more than 50%. Overall, the water budget of the aquifer is characterised by the endorheism of the system, with most of the outflows assured internally. Also, with regard to water resources management, the Quaternary aquifer offers a paradox: it is characterised by vast reserves, but a small renewable resource (recharge). It is, to the best of our knowledge, the first time that a large superficial aquifer of the Sahel has been modelled so thoroughly and understood as a whole. This study clearly illustrates the value of GIS and remotely sensed data in the hydrogeology of semi-arid areas. Successful applications of the Lake Chad Basin leads us to believe that in other semi-arid regions of the world, remote sensing and GIS could bring valuable assistance to hydrologists and hydrogeologists. |
