Physical Processes - Light and Thermal Energy

In the United States, reservoirs are distributed differently from natural lakes. They are often
located in areas of water demand or frequent flooding and large reservoirs are rarely constructed in
areas where natural lakes are plentiful.
The geographic location and the basin shape of a lake or reservoir are important to the things
that happen to and in lakes. The shape of the lake basin is often studied as a topic called lake
morphometry, the measurement of lake shape. Measurements such as depth and area of a lake are
morphometric measurements.
Many reservoirs have shapes that have been termed dendritic. Dendritic means that the
numerous channels and arms of the lake branch like a tree into smaller channels and arms. This means
that for the same surface area, lakes often have much more shoreline than a round lake of similar area.
Lake Keowee, a Duke Power Co. reservoir in South Carolina, has an area of approximately 74.4 sq.
kilometers. Lake Keowee has approximately 480 kilometers of shoreline although a round lake of
similar area would have only a little more than 30.5 kilometers. As you can see, Lake Keowee is not
only more than 10 times as complicated as a round lake, ten times as much shoreline is available for
housing, docks, beaches, septic tanks, and boats. The impact that this development has on ordinary
lakes is increased where more people share the same quantity of water. Table 1.2.1. summarizes the
differences between natural lakes and reservoirs.
1.2.5.3 Physical Processes - Light and Thermal Energy
Source and Fate of Light Energy. All life, all ecosystems are dependent on solar energy.
The amount and quality of this energy is dependent on the latitude on earth, local climate, altitude, and
the season. The fate of this energy in the aquatic ecosystem is dependent on the optical characteristics
of the lake or reservoir.
Almost all of the energy from the sun arrives on the earth as electromagnetic The complete
spectrum of this ranges from wavelenghts associated with X-rays (10 nm or less) to wavelengths
extending through microwaves and into the radio wavelengths (10's of meters in length). According to
Herman and Goldberg (1978), more than 99% of the energy exists in a range of approximately 275 to
5000 nm. For the entire solar spectrum, the integrated average rate of this energy input is termed the
"solar constant" or approximately 1.353 (103) Watts per sq. meter. The atmosphere, the earth-sun
distance and other factors result in significant variations of this `constant' that can and do affect climate
(Herman and Goldberg 1978).
Solar inputs of energy are considered as factors independent of lake processes, reservoir
operations, or most other terrestrial interventions on the ecosystem. As shown in Figure 1.2.2, solar
energy has its peak intensities in the visible wavelengths. We see these as colors ranging from blues in
the shorter wavelengths to reds in the longer wavelengths. The actual wavelength of this incoming
moreover, is very important to its fate in aquatic systems.
1.2-5