Diurnal fluctuations of water tables induced by atmospheric pressure changes

Abstract

The shallow water table at the Bonneville Salt Flats, Utah, fluctuates 1.5–6.0 cm per day during the summer, and 0.5–1.0 cm per day during the winter. Highest water levels occur in late afternoon, whereas lowest levels occur in mid-morning. Similar fluctuations occur in many other localities having shallow, fine-grained aquifers. The fluctuations are attributed to temperature-related atmospheric pressure changes acting on the capillary zone. This proposed mechanism involves the transfer of water from below the water table to the capillary fringe in response to lower pressure, and transfer of water from the capillary fringe to the water table in response to higher pressure. Rapid volume changes of air entrapped in the capillary pores account for the expulsion or infusion of capillary water. Laboratory experiments with an artificial aquifer confirm that such fluctuations can occur rapidly when the only variable is pressure. In this case no intervening confining layer exists, so the mechanism is distinctly different from the mechanism which causes blowing wells. The correct interpretation of fluctuations of several centimeters can be of considerable importance in monitoring small changes of water levels in manometers emplaced to monitor seepage through dams or levees, or in measuring small drawdowns in observation wells during pumping tests of fine-grained aquifers.