Transcript Soil Temperature

Soil Temperature
Energy balance
Incoming shortwave from sun
 Outgoing longwave from earth-atmosphere

Overall balance in soil:

Daily (diurnal):
– Net gain in day
– Net loss at night

Annual:
– Net gain in summer
– Net loss in winter
Soil heat flux

amount of thermal energy that moves
through an area of soil in a unit of time.
Heat flux diagram handout

Annual:
– Winter heat flux
– Summer heat flux
Re: heat flux diagram:

Notice:
– Crossover at ~ 4 m.
 penetration lag of cold and warm transmitted to
depth
– Spring, fall: transitional
 Turnovers are important triggers for soil animals
– Constant temp. at depth
 “depth of zero annual range”
 MAST (mean annual soil temp.)
– About 1°C warmer than mean annual air temp (39.1⁰
Duluth)
 high lat :20 m
 midlat: 15 m
 Tropics: 10 m
MAST corresponds roughly to the water temperature
measured in groundwater wells 30 – 50’ deep
MAST observations at individual stations,
superimposed on well-water temperature contours.
Geothermal heat
(not volcanic type)

Transfers heat to building from soil
– In summer, soil can remove heat from
building
– In winter, can transfer heat from soil to
building
– Horizontal tubing in trenches within zone of
MAST
Soil temp cycles
Annual pattern
Diurnal pattern
Soil temperatures cycles handout

Notice:
1. Decreased amplitude and increased lag time
with depth
 Changes in conductivity with depth
2. Diurnal temp wave discernible to about 0.8 m
3. Annual temp wave to 14 m
Thermal properties

Specific heat/heat capacity

Thermal conductivity
Specific heat/heat capacity

Ability to store heat
– Amount of heat required to raise temperature
of 1 g of substance by 1 degree C

greater the heat capacity of a substance,
the more heat it can gain (or lose) per
unit rise (or fall) in temperature
Soil: 0.2 cal/g
.
Water: 1.0 cal/g
light dry soils experience greater seasonal
temperature swings at a given depth than
wet soils.
Thermal conductivity

Ability to conduct heat
– Affected by:
 moisture
 texture
1. moisture

thermal conductivity of water is about two
to three times greater than that of soil.
– In saturated soils, pore spaces filled with
water rather than air
 Wet soils have higher conductivity than dry
2. texture:

thermal conductivity of air is about one
hundred times less than that of soil .
– Finer soils have more particle-to-particle
contact and smaller pore spaces, therefore
increased conductivity.

Conductivity increases as texture becomes increasingly
fine.
Notice that adding water makes texture
have opposite effect (wet sand higher cond.
than wet clay)

Texture determines how quickly soil will
heat (in spring) or cool (in fall).

Sandy soils contain less water (lower
porosity) and therefore heat more quickly
than clay.