Aquifer Recharge of the River Adour floodplain in southwest France
In recognition of the important function of wetlands as regulators of water regimes and supporters of a rich biodiversity, the French government instituted a national programme for research into wetlands, (Programme National de Recherche sur les Zones Humides, PNRZH) in 1995. This initiative is directed to address the problem of environmental degradation of wet zones, in terms of their hydrology, vegetation and fauna, with the intention of developing effective management and restoration strategies. The floodplains of the River Adour were one of the twenty sites selected under this initiative and our involvement included a study on the mechanism of aquifer recharge in a section of the floodplain of the River Adour, between Saubusse and Le Vimport, one of the major wetland areas in southwest France.
The study focused primarily on a major flood event observed during 6 19 March 1999. The March flood allowed 2.1 x 106 m3 of water to be stored in the sub-surface as the water-table rose to the surface of the floodplain to give complete saturation. After the passage of the flood wave, the water-table then declined to a level that restored the original volume of water stored in the sub-surface. The 7 km2 floodplain, between Le Vimport and Subusse, has an estimated total sub-surface storage capacity of ca. 6.3 x 106 m3 down to a layer of impermeable clay. During summer low water periods, the sub-surface storage is estimated to be ca. 4.2 x 106 m3 (i.e., 67% of the total available sub-surface storage in the zone studied). A hydrologic balance was calculated for the flood event that considered inputs and outputs to the floodplain. The input volume from stream overflow, surface runoff, rainfall, and input from a small stream in the floodplain was estimated to be 6.9 x 106 m3 (of which 5.6 x 106 m3 is retained on the surface). Water outputs including drainage channels, direct drainage by the river, and evapotranspiration were estimated to be 7.5 x 106 m3. These volumes represented ca.1.4% of the total flood event flow of 5.40 x 108 m3. Relatively low nitrate concentrations and associated low conductivities were observed in the water sampled at a number of sites placed within the study zone.
This study, on a part of a largely agricultural catchment, illustrates the importance of aquifer recharge mechanisms in attenuating peak flows during flood events. The substantial recharge evident during flood events suggests that further extraction of water for agricultural purposes would present few problems since the aquifer could still be replenished but with a lower surface flood depth during flood events. This would, however, present problems of enhanced nitrate concentrations in the sub-surface during the summer months as fertilizer infiltrated the more extensively depleted aquifer. In practice, over-extraction of water for irrigation purposes has already led to problems during low water periods that have required releases from dams located much further upstream to provide additional support for summer baseflows and additional extractions would therefore be unwise.
For more details contact Dr Brian Astin
Selected Publications
BRUNET, R-C, ASTIN, K B 2003. The role of the floodplain in regulating aquifer recharge during a flood event of the River Adour in Southwest France. Wetlands, 23(1), 190-199.
