Light and Heat in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 336

Radiation terms Nonradiation terms

Air-water
interface

Solar
shortwave
radiation

Atmospheric
longwave
radiation

Water
longwave
radiation

Conduction
and
convection

Evaporation
and
condensation

Net absorbed radiation Water-dependent terms

FIGURE 12.10 Components of the heat balance. (From Chapra, S.C., Pelletier, G.J., and Tao, H., QUAL2K:
A modeling framework for simulating river and stream water quality, Civil and Environmental Engineering
Department, Tufts University, Medford, MA, 2007. With permission.)

Similarly, evaporation is based on the gradient of the water vapor pressure between water and
air. So, if there is no gradient (the air is saturated, such as at 100% relative humidity), there is no
heat transfer. Evaporation is also enhanced by wind (Martin and McCutcheon 1999). The typical
magnitudes of each of these processes at midlatitudes are illustrated in Figure 12.11.

12.4 WATER DENSITY
One of the basic properties of water is its density. Because of its unique properties, water is most
dense at 4°C and then decreases as it heats or cools (Figure 12.12). The density of water is also
inluenced by the materials dissolved or suspended in it. For example, seawater is about 2.5% denser
than freshwater.

12.5 LAKE STRATIFICATION
Water quality is profoundly affected by the tendency of many lakes not to mix completely in the
vertical dimension, at least during some portions of the year. The degree of mixing is often based on

Solar
radiation
35-350

Atmospheric
radiation

Back
radiation

Conduction Evaporation

260-420 250-400 -40-60 30-1000

5-25 5-15

Diffusion

-5-10 Conduction

FIGURE 12.11 Heat exchange luxes, typical midlatitude daily average (W m -2 ). (From Wool, T.A.,
Ambrose, R.B., and Martin, J.L., WASP7 temperature and fecal coliform: Model theory and user's guide;
supplement to water quality analysis simulation program (WASP) user documentation, U.S. EPA, Region 4
Water Management Division, Atlanta, GA, 2008; USEPA WASP Workshop.)