Measurement-derived Heat-budget Approaches for Simulating Coastal Wetland Temperature with a Hydrodynamic Model

Abstract

Numerical modeling is needed to predict environmental temperatures, which affect a number of biota in southern Florida, U.S.A., such as the West Indian manatee (Trichechus manatus), which uses thermal basins for refuge from lethal winter cold fronts. To numerically simulate heat-transport through a dynamic coastal wetland region, an algorithm was developed for the FTLOADDS coupled hydrodynamic surface-water/ground-water model that uses formulations and coefficients suited to the coastal wetland thermal environment. In this study, two field sites provided atmospheric data to develop coefficients for the heat flux terms representing this particular study area. Several methods were examined to represent the heat-flux components used to compute temperature. A Dalton equation was compared with a Penman formulation for latent heat computations, producing similar daily-average temperatures. Simulation of heat-transport in the southern Everglades indicates that the model represents the daily fluctuation in coastal temperatures better than at inland locations; possibly due to the lack of information on the spatial variations in heat-transport parameters such as soil heat capacity and surface albedo. These simulation results indicate that the new formulation is suitable for defining the existing thermohydrologic system and evaluating the ecological effect of proposed restoration efforts in the southern Everglades of Florida.

Appendix. Radiant Energy Fluxes

The absorbed solar radiation is calculated as follows:

$$ {Q_{ASR}} = {Q_{SUN}}\left( {1 - \alpha } \right) $$

(A1)

where Q _SUN is the incident solar radiation, and α is the albedo. The long-wave radiation exchange term is calculated using the following equation:

$$ {Q_R} = {R_{water}} - {R_{atmosphere}} $$

(A2)

where R _water is the long-wave radiation emitted by the water surface to its surroundings, and R _atmosphere is the long-wave radiation emitted by the atmosphere that is absorbed by the water cell. These terms are defined as follows:

$$ {R_{water}} = {\varepsilon_w}\sigma T_w^4 $$

(A3)

$$ {R_{atmosphere}} = {\varepsilon_a}{\varepsilon_w}\sigma T_a^4 $$

(A4)

where ε _w and ε _a are the water surface and atmospheric emissivities, respectively, σ is the Stefan-Boltzman constant, and T _w and T _a are water temperature and air temperature, respectively (Jacobs et al. 2002). The water emissivity is treated as a constant and the atmospheric emissivity is calculated using Brunt’s method, which presents it as a function of the atmospheric vapor pressure e _a (Brunt 1932):

$$ {\varepsilon_a} = a + be_a^{0.5} $$

(A5)

where the atmospheric vapor pressure e _a is calculated from Eq. 4 in the text, a and b are empirical coefficients to be determined from field data, and b must be in the units of pressure^−0.5. Equation A5 is a simplistic empirical relation, which has proven to give reasonable long-wave radiation exchange comparable to more complex methods and requires minimal inputs (Viswanadham and Ramanadham 1970; Garrett 1977).