Observations and Modeling of Profile Soil Water Storage above a Shallow Water Table

Abstract

The storage capacity of a soil profile (soil water storage capacity [SWSC]) is the depth of water required to raise a shallow water table to the land surface. The concept of SWSC is fundamental to many hydrological processes, including surface runoff by saturation excess, expansion, and contraction of wetlands, and estimation of the length of an overland flow plane. A model is introduced and tested to estimate SWSC using simultaneous observations of shallow water table fluctuations and soil moisture in a shallow, sandy soil (hyperthermic Aeric Alaquods). The water table at the selected site fluctuated between a shallow depth and the land surface during the summer, allowing frequent observation of surface inundation and profile storage. An equation of the form SWSC = AdB + Cd + D adequately described the variability of SWSC with d, depth to the water table. It is shown that parameters A, B, C, and D are easily derived from basic physical properties of the soil horizons, including porosity and water retention. The SWSC can be significantly limited by the capillary fringe above the water table, encapsulated air (the volume of air trapped under positive pressure beneath the water table), or the presence of a clay pan at shallow soil depths. The capillary fringe had some influence on SWSC in this sandy soil, but encapsulated air as high as 11.0% of the soil volume was observed at the site. Encapsulated air reduced the available soil storage and resulted in a rapid rise in water table. Ignoring encapsulated air significantly overestimated profile storage. Storage results including and excluding air encapsulation were compared as a function of water table depth.