Physical Characteristics: Lake Models
33
one side, build up a 10 to 15° slope on the thermocline and redetermine the periods
of the seiches. Record temperatures and the depth of the homogeneous red layer.
7. Allow the system to rest and then with a strong wind (DO NOT SPLASH WATER ONTO
THE HEAT LAMP!) turn the lake over. When mixing is complete, record the
temperatures.
Apparatus and Supplies
1. An aquarium, corresponding to a lake basin, internally about 40 x 20 x 24 cm high,
and insulated on the sides and bottom with 5 cm of polyurethane foam. The
aquarium should be filled with cold water and ice to cool the water to 4°C. Some ice
should be floating at the surface.
2. A heat source (e.g., 250-W lamp) corresponding to the sun. This lamp should be
located directly over the middle of the aquarium, 25 cm above the water surface.
3. Seven mercury thermometers ( - 20 to 110°C) attached by masking tape or other
means to two plastic rulers. Match the thermometers or intercalibrate the
thermometers relative to each other. The rulers should be shortened to 23 cm and
the thermometers positioned horizontally at depths of 1, 3, 5,9, 13, 17 and 21 cm.
The string of thermometers is suspended by masking tape on the inside of the front
wall of the aquarium. In this manner temperatures can be read directly and quickly
at the stated depths. An additional thermometer is required to read temperatures at
intermediate depths. This thermometer should be inserted into the water with as
little turbulence as possible.
4. Catechol violet or methylene blue crystals. Before the dye is added, crystals should
be placed in the fold of a small piece of paper and gently crushed by pressing the two
sides together. In this way the crystals will be small enough so that most will dissolve
completely in the upper quarter of the water column. Use care in handling the
catechol violet. Avoid excessive contact with skin.
5. A fan or blower to generate wind-driven surface currents.
6. A stop watch.
7. A tea strainer to remove ice.
8. (Optional) An electrical resistance thermometer connected to a variable-speed strip
recorder.
Calculations
1. Density versus temperature. Using data on water density as a function of temperature
[e.g., a handbook of physics and chemistry, or pp. 204-205 in Hutchinson (1957)],
make a plot from which you can read density to the sixth decimal place for various
temperatures from 0 to 35°C. Retain this graph for future use in other experiments. On
a single sheet of graph paper, plot thermally inferred density versus depth, and temperature depth after the second wind, at the time of maximum thermal stratification.
Note the difference.
2. Heat budget. There are various ways of expressing heat budgets (see Exercise 4).
The one used here (Birgean Heat Budget) corresponds to the heat gained by a lake
between the times of its lowest and highest heat contents (usually late winter to midsummer). The simplest method of calculating the heat budget is to plot depth (z) on the
vertical axis versus the product A z (t s . z - t w • z ) on the horizontal axis, where A z is the
area of the stratum at depth z and t s • z and tw,z are the maximum (summer) and minimum (winter) temperatures, respectively, at depth z. The integral is measured either
33
one side, build up a 10 to 15° slope on the thermocline and redetermine the periods
of the seiches. Record temperatures and the depth of the homogeneous red layer.
7. Allow the system to rest and then with a strong wind (DO NOT SPLASH WATER ONTO
THE HEAT LAMP!) turn the lake over. When mixing is complete, record the
temperatures.
Apparatus and Supplies
1. An aquarium, corresponding to a lake basin, internally about 40 x 20 x 24 cm high,
and insulated on the sides and bottom with 5 cm of polyurethane foam. The
aquarium should be filled with cold water and ice to cool the water to 4°C. Some ice
should be floating at the surface.
2. A heat source (e.g., 250-W lamp) corresponding to the sun. This lamp should be
located directly over the middle of the aquarium, 25 cm above the water surface.
3. Seven mercury thermometers ( - 20 to 110°C) attached by masking tape or other
means to two plastic rulers. Match the thermometers or intercalibrate the
thermometers relative to each other. The rulers should be shortened to 23 cm and
the thermometers positioned horizontally at depths of 1, 3, 5,9, 13, 17 and 21 cm.
The string of thermometers is suspended by masking tape on the inside of the front
wall of the aquarium. In this manner temperatures can be read directly and quickly
at the stated depths. An additional thermometer is required to read temperatures at
intermediate depths. This thermometer should be inserted into the water with as
little turbulence as possible.
4. Catechol violet or methylene blue crystals. Before the dye is added, crystals should
be placed in the fold of a small piece of paper and gently crushed by pressing the two
sides together. In this way the crystals will be small enough so that most will dissolve
completely in the upper quarter of the water column. Use care in handling the
catechol violet. Avoid excessive contact with skin.
5. A fan or blower to generate wind-driven surface currents.
6. A stop watch.
7. A tea strainer to remove ice.
8. (Optional) An electrical resistance thermometer connected to a variable-speed strip
recorder.
Calculations
1. Density versus temperature. Using data on water density as a function of temperature
[e.g., a handbook of physics and chemistry, or pp. 204-205 in Hutchinson (1957)],
make a plot from which you can read density to the sixth decimal place for various
temperatures from 0 to 35°C. Retain this graph for future use in other experiments. On
a single sheet of graph paper, plot thermally inferred density versus depth, and temperature depth after the second wind, at the time of maximum thermal stratification.
Note the difference.
2. Heat budget. There are various ways of expressing heat budgets (see Exercise 4).
The one used here (Birgean Heat Budget) corresponds to the heat gained by a lake
between the times of its lowest and highest heat contents (usually late winter to midsummer). The simplest method of calculating the heat budget is to plot depth (z) on the
vertical axis versus the product A z (t s . z - t w • z ) on the horizontal axis, where A z is the
area of the stratum at depth z and t s • z and tw,z are the maximum (summer) and minimum (winter) temperatures, respectively, at depth z. The integral is measured either