A simple, dynamic, hydrological model for mesotidal salt marshes

Abstract

Salt marsh hydrology presents many difficulties from a measurement and modeling standpoint: bi-directional flows of tidal waters, variable water densities due to mixing of fresh and salt water, significant influences from vegetation, and complex stream morphologies. Because of these difficulties, there is still much room for development of a truly mechanistic model of salt marsh groundwater and surface-water hydrology. This in turn creates an obstacle for simulating other marsh processes, such as nutrient cycling, that rely heavily on hydrology as a biogeochemical control and as a mode of nutrient transport. As a solution, we have used water level data collected from a well transect in Winant Slough, a mesotidal salt marsh on the Oregon coast, to create and calibrate a simple, empirical dynamic marsh hydrology model with few parameters. The model predicts the response of a marsh's water table level to tides and precipitation as a function of surface elevation and distance from tidal channel. Validation was conducted using additional well data from a separate transect in Winant Slough (achieving a standard error of 2.5 cm) and from two other mesotidal marshes in Tillamook Bay, Oregon (achieving standard errors of 3.1 cm and 3.6 cm). Inundation frequencies of the top 10 cm of soil were estimated from model outputs to be 18.3% of a 14.8-day tidal cycle for the area closest to the tidal creek and 59.3% for the area furthest from the creek. Model outputs were also used to predict the amount of soil pore space available to receive incoming tide water in Winant Slough, finding the volume available to range from 12.5% to 24.7% of the incoming marsh tidal prism volume, depending on the maximum tide height. Incrementally increasing sea level rise scenarios ranging from 15 cm to 75 cm predicted an exponential decrease in soil pore space available to receive incoming tidal water and an approximately linear increase in inundation frequency of the top 10 cm of soil; this substantial change in hydrology would impact the marsh's ability to process incoming water and could alter the zonation of vegetation. The model is relatively easy to apply to salt marshes and can provide informative hydrology predictions to land managers, ecologists, and biogeochemists who may not have the time or expertise required to apply more complex models.