Analysis of long duration piezometric records from Burkina-Faso used to determine aquifer recharge.
Résumé
An 8-year water-level record for an observation well in a granite environment in Ouagadougou (Burkina Faso) is analysed using a lumped-parameter hydrological model. The model computes aquifer levels from rainfall and potential evapotranspiration data, and is calibrated with observed levels. Very satisfactory calibration is achieved, although aquifer levels have been dramatically declining since 1978. It appears that, even with small computational time-steps, a unique solution for calibration is only possible if the precise storage coefficient is known or if surface runoff data are available. In the absence of such data, multiple calibrations displaying the same agreement with observed data give different values for aquifer recharge, although relative variation is the same from year to year. When used in conjunction with a long set of in-situ rainfall records, however, the various sets of parameters applied result in almost-identical extension of water-level data, an important advance. The model shows that the 1978-1985 period is typified by the lowest water levels encountered in at least 60 years, and that a return to a rainfall sequence near the long-term average would cause the level to rise, although only after a period of 7 to 10 years.