ANNUAL FLUCTUATIONS OF GROUNDWATER LEVELS AS A RESULT OF LOADING BY SURFACE MOISTURE

Long-term hydrographs for deep confined aquifers in southern Saskatchewan, Canada, show annual fluctuations characterized by a gradual rise in head from October to May/June and a rapid drop from May/June to October. These fluctuations are distinctly different from the seasonal fluctuations observed in surficial and shallow semi-confined aquifers, which reflect the response of these aquifers to recharge derived from snow melt and early spring rains. On the basis of field data and theoretical considerations, it is demonstrated that seasonal fluctuations of the water table are not transmitted down to the deep aquifers, provided the confining layers are thick and have sufficiently low hydraulic diffusivity. The annual fluctuations in the deep aquifers thus do not reflect seasonal recharge to these aquifers. Plots of moisture accumulation (i.e. precipitation minus calculated areal evapotranspiration) are strikingly similar to the observed hydrographs for the deep aquifers, in timing and magnitude of the annual fluctuations. It is postulated that the annual head fluctuations in the deep confined aquifers reflect changes of the mechanical load on the formations, caused by seasonal changes of the soil moisture, snow, and storage at the water table. The deep confined aquifers thus act as large-scale, inexact, weighing lysimeters.