A simple, dynamic, hydrological model for mesotidal salt marshes

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 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). Model outputs were used to predict the amount of soil pore space available to process tidal inflows in Winant Slough, finding the volume available to range from 11.3 % to 23.7 % of the incoming marsh tidal prism, depending on the maximum tide height. Inundation frequencies of the top 10 cm of soil over a 14.8-day tidal cycle were estimated to be 18.3 % for the area closest to the tidal creek and 59.3 % for the area furthest from the creek. A simulated scenario for 30 cm of sea level rise was predicted to decrease the soil pore space available to process incoming tidal water and increase inundation frequency of the top 10 cm of soil; this substantial change in hydrology would impact the marsh’s ability to purify incoming water and the zonation of its vegetation. The model is relatively easy to apply to salt marshes and can provide informative hydrology predictions to land managers, ecologists, and biogeochemists.

Impact/Purpose

Salt marshes have been recognized in the scientific literature for the valuable ecosystem services they provide to their surrounding communities and environments. Services include water purification, flood mitigation, habitat provision, and carbon sequestration. The movement of rain and tide water across the surface and through the soil (i.e. the hydrology) of these marshes affects the delivery of these services. However, measuring existing marsh hydrology and predicting any impacts to it that may be caused by environmental changes is a difficult endeavor. This research project was conducted to improve the understanding and predictability of salt marsh hydrology. This was achieved by intensely measuring water levels at several Pacific Northwest salt marsh sites and developing a dynamic model using the resulting data. The model we developed is simple in structure, has few parameters, and can predict salt marsh water levels under various scenarios. Model outputs for one of the research sites were used to predict the amount of soil pore space available to process tidal inflows, finding the volume available to range from 11.8 % to 24.7 % of the incoming marsh tidal prism, depending on the maximum tide height. This available volume was predicted to decrease in a simulated scenario of 30 cm of sea level rise, which may have implications for the delivery of water purification services. The simplicity of the model enhances its utility by expediting its transfer and application to new settings. Researchers that need water level data to assess marsh processes and ecosystem services would be interested in the model we developed. Land managers would also be interested in the model for its ability to predict changes in marsh hydrology under future scenarios. The tool we developed is valuable to the agency in its ability to aid in predicting potential impacts to ecosystem services and other salt marsh functions that depend on hydrology.

Citation

Marois, D. AND J. Stecher. A simple, dynamic, hydrological model for mesotidal salt marshes. Elsevier Science Ltd, New York, NY, 233:106486, (2020). [DOI: 10.1016/j.ecss.2019.106486]

DOI: A simple, dynamic, hydrological model for mesotidal salt marshes