Soil saturation index of salt marshes subjected to spring-neap tides: a new variable for describing marsh soil aeration condition

Organized spatial distribution of plants (plant zonation) in salt marshes has been linked to the soil aeration condition in the rhizosphere through simplistic tidal inundation parameters. Here, a soil saturation index (ratio of saturation period to tidal period at a soil depth) is introduced to describe the soil aeration condition. This new index captures the effects of not only the tidal inundation period and frequency but also the flow dynamics of groundwater in the marsh soil. One-dimensional numerical models based on saturated flow with the Boussinesq approximations and a two-dimensional variably saturated flow model were developed to explore the behaviour of this new soil aeration variable under the influence of spring-neap tides. Simulations revealed two characteristic zones of soil aeration across the salt marsh: a relatively well aerated near-creek zone and a poorly aerated interior zone. In the near-creek zone, soils undergo periodic wetting and drying as the groundwater table fluctuates throughout the spring-neap cycle. In the interior, the soil remains largely water saturated except for neap tide periods when limited drainage occurs. Although such a change of soil aeration condition has been observed in previous numerical simulations, the soil saturation index provides a clear delineation of the zones that are separated by an 'inflexion point' on the averaged index curve. The results show how the saturation index represents the effects of soil properties, tidal parameters and marsh platform elevation on marsh soil aeration. Simulations of these combined effects have not been possible with traditional tidal inundation parameters. The saturation index can be easily derived using relatively simple models based on five non-dimensional variables. Copyright (C) 2010 John Wiley & Sons, Ltd.