Barometric tides in partly-saturated confined aquifers in Botswana

Barometric fluctuations in Botswana are dominated by atmospheric tides with frequency components of 6, 8, 12, and 24 h, as well as quasi-periodic components with a periodicity of 4–7 days. These tides cause periodic water level fluctuations in the majority of the boreholes in fracture aquifers. Many of these aquifers are partly saturated and show free water table characteristics, but are separated from direct atmospheric influence by less-pervious beds near the surface. The barometric effect is caused by an imbalance between the water pressure change in the well and in the aquifer. A theoretical model is presented that includes the compression of the aquifer and the associated vertical flow through the aquifer to the water table. The model qualitatively explains the observed frequency-dependent phase and amplitude response. The generally observed strong barometric effects are explained by a combination of high porosity and low conductivity.