1. Introduction

Figure 1. Location of study sites in the wetland areas of southern Florida. Modified from Lietz [2000]. [larger image]

[2] Suitable spatial and temporal definition of changes in heat energy stored in a column of wetland surface water are frequently needed to make local and regional energy budget estimates of latent heat fluxes; that is, the energy equivalent of evapotranspiration. Uncertainties in the characterization of surface energy fluxes limit the reliability of hydrologic analyses, and handicap efforts to manage water resources. The purposes of this paper are to (1) identify when and where changes in stored heat energy in wetland surface water are a considerable component of the surface energy budget and (2) introduce new equations for computing changes in wetland stored heat energy that rely on measured changes in air temperature rather than measured changes in water temperature. Reliance on air temperature instead of water temperature was considered desirable because air temperature data are more readily available. Additionally, air temperature monitoring is less expensive and less labor intensive than water temperature monitoring. The new equations for computing changes in heat energy stored in wetland surface water are applied in a case study of the Everglades areas of southern Florida (Figure 1).

[3] A simplified surface energy budget for wetlands takes the form (Figure 2)

where R_n is net radiation, W is changes in heat energy stored in wetland surface water, G_veg is biomass storage (heat energy stored in the vegetation), E is the latent heat flux, and H is the sensible heat flux. The units for these surface energy fluxes are watts per square meter (W m^-2). The terms on the left side of the energy budget equation are commonly called the available energy (Ae) for evapotranspiration because this energy is partitioned between sensible heat (H) and latent heat (E). Typically, R_n is the dominant component of a surface energy budget; however, this does not hold true universally because W can be considerable at locations with surface water. In fact, W sometimes can be the dominant component of a surface energy budget.

Figure 2. Simplified surface energy budget for the Everglades. [larger image]

[4] Previous studies have investigated surface energy fluxes using land-based hydrometeorological methods [Brutsaert, 1982; Monteith and Unsworth, 1990; Abtew and Obeysekera, 1995; Bidlake et al., 1996; Campbell and Norman, 1998; German, 2000; Lott and Hunt, 2001; Sumner, 2001; Jacobs and Satti, 2001; Wilson et al., 2002; Small and Kurc, 2003] and satellite-based methods [Anderson et al., 1997; Norman et al., 2003; Liu et al., 2003; Bastiaanssen et al., 2002; Islam et al., 2002].

Because a change in surface water temperature reflects a change in stored heat energy, previous studies of surface water temperature are considered relevant, including those of equilibrium surface water temperature [Edinger et al., 1968; Bogan et al., 2003] and heat exchange at the air-water interface [Mohseni et al., 1998]. The latter studies, however, differ from the analysis described herein because they focus on absolute water temperatures rather than changes in water temperature. For example, Bogan et al. [2003] developed relations between mean weekly equilibrium stream temperature and mean weekly stream temperature, and Mohseni et al. [1998] examined the relation between mean weekly air and stream temperatures. Edinger et al. [1968] introduced the concept of equilibrium water temperature, defining it as the water temperature at which the sum of the heat fluxes through the surface water is zero.