Turbidity
is a measure of how water scatters light. Water with a lot of small particles
scatters lots of light and has a cloudy, or murky, appearance. The murkier the water appears the higher the
turbidity. Turbidity is determined by shining a controlled light source into a
sample of water. The mount of light that is scattered in a direction that is 90
degrees away from the incoming light is proportional to the turbidity. Turbidity is different than clarity. Clarity
is determined by measuring the greatest depth that a Secchi disk (which is sort
of a white disk a foot in diameter) can be observed from the surface.
Clarity
measurements have several advantages over turbidity measurements. Clarity is
directly related to what a person sees, it is easy to explain and understand,
it is sensitive to conditions over a range of depths, and there is a long
measurement record for Lake Tahoe. The Tahoe Research Group at the University of
Davis has been measuring clarity since the 1960s. Some of these records have
been collected from sites in the middle of the lake and provide an excellent
indication of overall health of the lake.
Turbidity
measurements have several advantages over clarity measurements. Unlike clarity
measurements, turbidity measurements can be made from a moving boat or by
unattended instruments. This makes it practical to produce maps of turbidity or
to remotely collect turbidity data at a fixed location more often than clarity
measurements can be made. Unlike clarity measurements, turbidity measurements
can be made in water a few inches deep, so it works well near shore. This makes
it possible to consider the influence of localized on shore activates on the
lake water quality.
A
turbidity measurement is made on water from a single depth, so it does not say
much about conditions at other depths unless multiple measurements are made at
different depths at the same location. In our studies we operate on a moving
boat. We continuously pump water from a depth of 1-meter depth in the lake
through an instrument and measure the turbidity. A GPS and computer system
records and displays the value of the turbidity, the water temperature, the
boat location, and occasionally other parameters.
Clarity
and turbidity measurements complement each other. Clarity is a great indication of the overall health of the lake
and is well understood by the public. Turbidity is great for spatially detailed
investigations that can be used to determine which portions of the lakeshore
are contributing undesirable material to the lake.
Turbidity
is sensitive to both biological and geological particles in the water.
Turbidity is expressed in units of NTU.
To
calibrate a turbidity instrument, standards with known turbidity (a know NTU
value) are placed into the instrument. Then an empirical relationship is
developed between the voltage outputs of the instrument and the NTU values of
the standards.
We
are also occasionally measuring the chlorophyll fluorescence which is
proportional to the concentration of chlorophyll in the water. This measurement
is made by illuminating a water sample with one wavelength of light and
measuring how much fluorescence there is at a second wavelength. The light
wavelengths are selected so that the measurement is sensitive to the
concentration of chlorophyll in the water sample. The instrument is calibrated
by determining an empirical relationship between the response of the instrument
and measurements of chlorophyll obtained by filtering water samples and
measuring chlorophyll on the filter. The chlorophyll measurements can also be
made from a moving boat and provide an excellent indication of the algae
concentrations.