Selected Water Quality Processes in Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 135

I

Mixed layer

Z

FIGURE 5.6 Light attenuation with depth.

longwave radiation (pyrgeometers), visible light (photometers), or PAR (Strangeways 2003). When
planning a ield study or using the results from measurements by others, attention should be given
as to exactly what radiation measures will be (were) taken.
For aquatic studies, it is not only the light at the surface that is important, but also the light in
the water column or that reaching the substrate. Light typically decreases exponentially from the
surface (Figure 5.6, as described by Beer's law; Chapra 1997) as it is absorbed by the water and the
materials suspended or dissolved in it. The sorption of light also varies with the wavelength. For
example, red is absorbed most quickly, while blue light penetrates the farthest.
Light penetration is most commonly measured using an underwater photometer. A general pro-
cedure is to maintain a surface photometer for measuring surface light and measuring light as a
function of depth in order to estimate the light extinction coeficient. Light attenuation in the water
column is discussed in greater detail for lakes and reservoirs.

5.3 TEMPERATURE
Temperature is a master variable affecting many aspects of habitat and water quality. Temperature
impacts the degradation of materials. For example, for materials degrading following the Q10 rule,
the rate of degradation would double for each 10°C increase in temperature (2 = 1.07 T−20 , where T
is the temperature in degrees Celsius). Temperature impacts isheries growth, reproduction, migra-
tion, susceptibility to diseases, and behavior. Temperature also impacts the timing of invertebrate
reproductive cycles, such as for mayly hatches that hatch at times of the year when temperatures
are within certain ranges. Temperature also affects oxygen solubility and other processes related to
the health of aquatic systems.
Heat variations occur largely as a result of exchanges of heat energy across a water surface.
Models of heat exchange for predicting temperature are commonly used and methods are described
in detail in Chapra (1997) and Martin and McCutcheon (1999). Shortwave and longwave radia-
tion add heat energy, some of which is emitted back into the atmosphere (via blackbody radiation).
Gradients between the water and the atmosphere in terms of the temperature and moisture content
(as measured by relative humidity) result in exchanges due to conduction, convection, and evapora-
tion (Figure 5.7).
The water temperature is a function of surface heat exchange and other factors. For example,
shading by banks and riparian vegetation and processes such as fog formation in the early morning
hours can reduce heat exchange, particularly in low-order streams and rivers. Also, as water move-
ment slows, the impact of heat exchange is greater and the stream is warmer. In very large streams,