Light and Heat in Lakes and Reservoirs - Hydro-Environmental Analysis: Freshwater Environments - page 329

12 Light and Heat in Lakes

and Reservoirs

12.1 DISTRIBUTION OF LIGHT AND HEAT EXCHANGE
Our primary source of light is radiation, which comes from the sun. At speciic locations on
the earth, the magnitude of the incoming solar radiation varies with the altitude of the sun,
which varies daily and seasonally (Figures 12.1 and 12.2). The summer solstice or midsum-
mer occurs on the longest day and shortest night of the year, when the earth's axial tilt is most
inclined toward the sun. The summer solstice occurs in June in the Northern Hemisphere and
in December in the Southern Hemisphere. Conversely, the winter solstice occurs when the
earth's axial tilt is most inclined away from the sun, and corresponds to the shortest day and
longest night of the year. The range of seasonal variations in solar radiation varies with the
earth's latitude.
Solar radiation also varies daily (Figure 12.3). The magnitude of peak solar radiation as well
as day length varies seasonally (Figure 12.4). The magnitude of the radiation reaching the water's
surface at a particular location on the earth and the time of year is also reduced by the atmospheric
absorption and relection from clouds, and, of the remaining radiation, some is relected from the
water's surface (Figures 12.5 and 12.6).
The intensity of solar radiation striking the earth's surface varies in wavelengths from ultra-
violet (UV) through the visible to the infrared. Some of the lower UV wavelengths are completely
absorbed by the atmospheric ozone layer, so organisms on Earth have developed no tolerance to it.
Similarly, the atmosphere absorbs some of the infrared wavelengths but it is more transparent to
visible light (wavelengths from 380 and 760 nm; Figure 12.7).
Of the visible light striking the water's surface, some of it penetrates into the water and some
of it is relected. The wave energy of the relected light typically decreases, becoming longer-
wavelength thermal energy. This longwave radiation can be relected back to the water's surface
by clouds.
Both longwave and shortwave radiation are important contributors to the heating of lakes and
reservoirs (see Martin and McCutcheon 1999). Also important is the portion of the shortwave,
visible spectrum that photosynthetic organisms are able to use in the process of photosynthesis,
referred to as photosynthetically active radiation or photosynthetically available radiation (PAR).
Plant pigments such as chlorophyll or accessory pigments such as carotenoids absorb the radia-
tion. Chlorophyll, for example, is most eficient in capturing red and blue light (at 665 and 465 nm;
Figure 12.8).

12.2 LIGHT AND WATER INTERACTIONS
Depending on the color and turbidity of the water, the visible light striking the water's surface can
penetrate to considerable depths. The depth to which light penetrates the water affects the thermal
characteristics of lakes and reservoirs as well as the water quality, for example, by inluencing the
distribution of aquatic plants. An understanding of the light environment and the factors affecting it
is a critical component of lake management.