The Effects of Truncating the Capillary Fringe on Water‐Table Dynamics During Periodic Forcing

Abstract

AbstractThe hydrodynamic response of the capillary zone is an important factor for understanding and predicting the propagation of groundwater head changes in unconfined aquifers. While the response can be predicted for a capillary fringe that is not bounded above by the ground surface, the influence of an upper boundary is poorly understood. In this study, we present laboratory sand column experiments for a range of water‐table depths below the ground surface, corresponding to truncation factors that represent the range of capillary‐zone truncation. Periodic forcing from below is related to observed amplitude attenuation and phase lag of the water‐table position. An abrupt decline of the water‐table occurs if the capillary fringe is truncated during the entire oscillation period, whereas a sudden increase is observed when the capillary fringe is truncated for only part of the period. Fluctuations during truncation are always larger than without, which is ascribed to meniscus behavior at the ground surface. Air entrapment will reduce the capillary fringe, which is confirmed through a watering test. Simulation using both Cartwright's “wetting and drying” model and Richards' model with dynamic effective porosity are used to evaluate experimental results, with the latter model providing a better match for large capillary‐fringe truncation.