EXERCISE 4
The Heat Budget* of Lakes
One of the most important and interesting
characteristics of a lake is its thermal structure.
The heat content of a body of water is of vital
importance in limnology. The metabolism,
physiology, and behavior of aquatic organisms
are related directly to the temperature of the
aquatic environment. Extreme temperatures restrict the growth and distribution of plants,
animals, and microbes. Because of the high
specific heat of water, large volumes of water
change temperature relatively slowly. Therefore,
large lakes tend to moderate local climates,
provide longer growing seasons for aquatic life,
and serve as integrated recorders of recent climatic phenomena. For these and other reasons, the
thermal structure and heat content of a body of
HEAT CONTENT
water must be known with some degree of
accuracy in limnological studies.
Temperature is related to the heat content of a
body of water but does not measure it. Temperature is a measure of the intensity of heat stored in a
volume of water, not a measure of the amount of
heat stored. Heat (H) is measured in calories and
is a function of the mass (M) of the substance in
grams, temperature (t) in °C, and the specific heat
(s) in cal/g-oC t
H=Mxtxs
A bathtub filled with water at OSC has a much
greater heat content than does a glass filled with
water at a temperature of 25°C.
The general equation which applies to the heat balance for a body of water in a given
time interval is
OR + OE + OL + Oy + Os + OB = 0
where OR = net radiation, OE = latent heat exchange, OL = sensible heat exchange with
the atmosphere, Oy = net advective exchange, Os = change in heat storage, and
OB = conductive heat exchange through the bottom sediments.
Energy added is considered positive, while energy lost is negative. Over the course of
an annual cycle the total heat gain balances the total heat loss. That is, in general and
assuming no change in climate, natural lakes are not becoming warmer or colder from
*The use of the word "budget", as normally defined, is incorrect but the term is firmly
entrenched in the literature of limnology. Reference is to the heat storage capacity of a lake.
t 1 g cal (mean) x 4.1862 = 1 J.
43
The Heat Budget* of Lakes
One of the most important and interesting
characteristics of a lake is its thermal structure.
The heat content of a body of water is of vital
importance in limnology. The metabolism,
physiology, and behavior of aquatic organisms
are related directly to the temperature of the
aquatic environment. Extreme temperatures restrict the growth and distribution of plants,
animals, and microbes. Because of the high
specific heat of water, large volumes of water
change temperature relatively slowly. Therefore,
large lakes tend to moderate local climates,
provide longer growing seasons for aquatic life,
and serve as integrated recorders of recent climatic phenomena. For these and other reasons, the
thermal structure and heat content of a body of
HEAT CONTENT
water must be known with some degree of
accuracy in limnological studies.
Temperature is related to the heat content of a
body of water but does not measure it. Temperature is a measure of the intensity of heat stored in a
volume of water, not a measure of the amount of
heat stored. Heat (H) is measured in calories and
is a function of the mass (M) of the substance in
grams, temperature (t) in °C, and the specific heat
(s) in cal/g-oC t
H=Mxtxs
A bathtub filled with water at OSC has a much
greater heat content than does a glass filled with
water at a temperature of 25°C.
The general equation which applies to the heat balance for a body of water in a given
time interval is
OR + OE + OL + Oy + Os + OB = 0
where OR = net radiation, OE = latent heat exchange, OL = sensible heat exchange with
the atmosphere, Oy = net advective exchange, Os = change in heat storage, and
OB = conductive heat exchange through the bottom sediments.
Energy added is considered positive, while energy lost is negative. Over the course of
an annual cycle the total heat gain balances the total heat loss. That is, in general and
assuming no change in climate, natural lakes are not becoming warmer or colder from
*The use of the word "budget", as normally defined, is incorrect but the term is firmly
entrenched in the literature of limnology. Reference is to the heat storage capacity of a lake.
t 1 g cal (mean) x 4.1862 = 1 J.
43