Hydraulique des unités d'interfluves et de bas-fond tourbeux: un exemple en zone de socle altéré (Madagascar)

Abstract

The hydrodynamics of the groundwater system of the upland granito-gneissic plateau of Madagascar is governed by the structure of the underlying aquifer. In the alteritic interfluves, the aquifer has two layers; the peat aquifer is multilayered. Groundwater levels and spring discharges observed over 3 successive years distinguish between two different types of hydraulic behaviour. 1. (1) Under the interfluves, unconfined conditions are found in allochtonous weathered material, and semi-confined conditions in the autochtonous weathered basement. These two systems are separated by a semi-permeable clay layer. The first system responds rapidly to rainfall (approximately 15 days) whereas the second system responds more slowly (approximately 3 months). This leads to a vertical flux between the two aquifers, the direction of which depends on the season; at the beginning of the wet season the flux is downwards, but during the later part of the season the direction reverses. 2. (2) In the low-lying regions, where sequences are superimposed, the direction of the vertical flow changes several times over a year. At the beginning of the wet season the flux is downwards, it reverses at the end of the season. A downwards flux is then observed towards the second part of the dry season. Such behaviour is only observed in the lowest third of the catchment where permeable silts take the place of less permeable peat. This pattern of fluxes suggests a close relationship between surface water and groundwater regimes, and has a major influence on the individual water balances of the two systems.