Atmospheric loading effects on free-draining lysimeters

Abstract

Temporal atmospheric pressure variations introduce an artifact into percolation measurements in free-draining lysimeters. This anomaly is associated with transient pressure gradients that occur as fluid and gas pressures within the lysimeter equilibrate with atmospheric pressure changes at the drain. On the basis of harmonic (Fourier) analysis of a large (20 m × 10 m × 1.8 m) free-draining lysimeter, hourly percolation was maximally affected by pressure variations of ∼2 cycles per day, with the effect decreasing with decreasing frequency. Compared to the calculated pressure differences at the drain, the measured phase delay between percolation and the atmospheric pressure was less by ∼45, which we attribute to an additional phase lag existing between the percolation and pressure difference at the drain. These pressure-induced changes in percolation rate can influence the outcome of water balances conducted via free-draining lysimeters. © 2011 by the American Geophysical Union.