FATHOM Dynamic Mass Balance Model

Development and Use of the FATHOM Dynamic Mass Balance Model for Florida Bay to Simulate Physical and Water Quality Variability

Principal Investigators: Bernard .J. Cosby, Frank Marshall, and William Nuttle

·     Dynamic mass-balance salinity model driven by flow

·     Assumes basins are completely mixed and manning’s equation governs flow between basins

·     Has capability to perform water quality calculations

FATHOM (Flux Accounting and Tide Hydrology at the Ocean Margin) is a spatially explicit model designed to simulate the movement of water and solutes in Florida Bay in response to runoff, climate, tides and the topography of the bay.  The model maintains a mass-balance of water, salt, nutrients (N and P), dissolved oxygen, and heat (temperature) in each of 54 basins bounded by the shallow shoals that dissect the bay. Fluxes across the shoals driven by differences in water surface elevation on either side. Water velocity on the shoals is calculated as a function of bank width, depth of flow, and bottom roughness using Manning's equation for friction flow. Calculated velocities are used with cross sectional areas of water on the shoals to give water fluxes. Solute fluxes are then calculated from solute concentrations. Model input includes monthly climate data from the mainland and the Keys, estimated average monthly evaporation, monthly coastline runoff, and hourly tide stages along Gulf and Atlantic boundaries of the model.  Monthly changes in mean sea level at the boundaries and long-term trends in annual average sea-level are added to the tide data.  Bathymetry data are derived from a combination of NOAA charts for Florida Bay and bathymetric data collected in Florida Bay by the US Geological Survey. The roughness coefficient for Manning’s equation was derived from flume studies conducted in the northwestern area of the bay. FATHOM simulations of monthly salinity in Florida Bay have been compared with observed monthly salinity data from a long-term monitoring program conducted by the Southeast Environmental Research Center. For the period 1991 - 2002, simulated salinity accounts for greater than 75% of the variation in observed monthly salinity for the 12 years at 20 sites within Florida Bay. 

In addition to simulated salinity values, FATHOM outputs include time-series estimates of physical and hydraulic conditions in Florida Bay. Within each of the 54 basins, the model provides monthly simulated values of residence times, water depths, wetted surface areas, and water volumes. For each shoal the model provides monthly average cross-sections of exchange between basins, monthly gross and residual fluxes of water and solutes across each shoal, and velocity profiles.  The nutrient module in FATHOM is also active to provide simulation of dissolved organic C (TOC), dissolved organic N (TON), and dissolved organic P (TOP) in addition to simulation of NO3, NH4, and PO4. The FATHOM nutrient module also provides simulated values of three combined variables:  dissolved Total N (TN = TON + NO3 + NH4); dissolved inorganic N (DIN = NO3 + NH4); and dissolved Total P (TP = TOP + PO4).

FATHOM as originally developed utilizes best available information for model inputs, i.e. FATHOM was not calibrated as are many numeric models. During the FY12 period, FATHOM will be undergoing a calibration verification process, funded by the US Army Corps of Engineers RECOVER Branch for use with the Southern Coastal Sub-team evaluations of CERP alternatives and assessment of monitoring data for adaptive management purposes.  This task will include:

a)     review and improvement of the regression models producing the western Florida Bay salinity boundary conditions

b)     Review and improvement of the distribution of freshwater inputs (overland, groundwater, creeks) to Florida Bay along the northern part of the FATHOM domain

c)     Review the of the bathymetry in the shallowest depth classes

d)     Review and improvement of the Manning’s equation to govern the flows

Frank Marshall,
Oct 14, 2011, 12:33 PM