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Estimating Changes in Heat Energy Stored Within a Column of Wetland Surface Water and Factors Controlling Their Importance in the Surface Energy Budget
Received 14 February 2005; revised 12 July 2005; accepted 25 July 2005; published 21 October 2005.
 Changes in heat energy stored within a column of wetland surface water can be a considerable component of the surface energy budget, an attribute that is demonstrated by comparing changes in stored heat energy to net radiation at seven sites in the wetland areas of southern Florida, including the Everglades. The magnitude of changes in stored heat energy approached the magnitude of net radiation more often during the winter dry season than during the summer wet season. Furthermore, the magnitude of changes in stored heat energy in wetland surface water generally decreased as surface energy budgets were upscaled temporally. A new method was developed to estimate changes in stored heat energy that overcomes an important data limitation, namely, the limited spatial and temporal availability of water temperature measurements. The new method is instead based on readily available air temperature measurements and relies on the convolution of air temperature changes with a regression-defined transfer function to estimate changes in water temperature. The convolution-computed water temperature changes are used with water depths and heat capacity to estimate changes in stored heat energy within the Everglades wetland areas. These results likely can be adapted to other humid subtropical wetlands characterized by open water, saw grass, and rush vegetation type communities.
 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).
 A simplified surface energy budget for wetlands takes the form (Figure 2)
where Rn is net radiation, W is changes in heat energy stored in wetland surface water, Gveg is biomass storage (heat energy stored in the vegetation), λE 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 (λE). Typically, Rn 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.
 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.  developed relations between mean weekly equilibrium stream temperature and mean weekly stream temperature, and Mohseni et al.  examined the relation between mean weekly air and stream temperatures. Edinger et al.  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.
1 A. Castillo and W. B. Shoemaker, Center for Water and Restoration Studies, Florida Integrated Science Center, U.S. Geological Survey, 9100 NW 36th Street, Suite 107, Miami, FL 33178, USA. (email@example.com)
2 D. M. Sumner, Center for Aquatic Resource Studies, Florida Integrated Science Center, U.S. Geological Survey, 224 West Central Parkway, Suite 1006, Altamonte Springs, FL 32714, USA. (firstname.lastname@example.org)
U.S. Department of the Interior, U.S. Geological Survey
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