Soil TemperatureSoil Temperature

Energy balanceEnergy balance

Incoming shortwave from sunIncoming shortwave from sun Outgoing longwave from earth-Outgoing longwave from earth-

atmosphereatmosphere

Overall balance in soil:Overall balance in soil:

Daily Daily (diurnal):(diurnal):– Net gain in day Net gain in day – Net loss at night Net loss at night

AnnualAnnual::– Net gain in summerNet gain in summer– Net loss in winterNet loss in winter

Soil heat fluxSoil heat flux

amount of thermal energy that amount of thermal energy that moves through an area of soil in a moves through an area of soil in a unit of time.unit of time.

Heat flux diagram handoutHeat flux diagram handout

AnnualAnnual::– Winter heat fluxWinter heat flux– Summer heat fluxSummer heat flux

Re: heat flux diagram:Re: heat flux diagram: Notice: Notice:

– Crossover at ~ 4 m.Crossover at ~ 4 m. penetration lag of cold and warm transmitted to penetration lag of cold and warm transmitted to

depthdepth

– Spring, fall: transitionalSpring, fall: transitional Turnovers are important triggers for soil animalsTurnovers are important triggers for soil animals

– Constant temp. at depthConstant temp. at depth ““depth of zero annual range”depth of zero annual range” MAST (mean annual soil temp.)MAST (mean annual soil temp.)

– About 1About 1°C warmer than mean annual air temp (39.1°C warmer than mean annual air temp (39.1⁰ Duluth)⁰ Duluth) high lat :20 m high lat :20 m midlat: 15 m midlat: 15 m Tropics: 10 mTropics: 10 m

MAST corresponds roughly to the water MAST corresponds roughly to the water temperature measured in groundwater wells 30 – temperature measured in groundwater wells 30 – 50’ deep50’ deep

MAST observations at individual stations, superimposed on well-water temperature contours.

Geothermal heatGeothermal heat(not volcanic type)(not volcanic type)

Transfers heat to building from soilTransfers heat to building from soil– In summer, soil can remove heat from In summer, soil can remove heat from

buildingbuilding– In winter, can transfer heat from soil to In winter, can transfer heat from soil to

buildingbuilding– Horizontal tubing in trenches within Horizontal tubing in trenches within

zone of MASTzone of MAST

Soil temp cyclesSoil temp cycles

Diurnal pattern

Annual pattern

Soil temperatures cycles Soil temperatures cycles handouthandout

Notice:Notice:1. Decreased amplitude and increased lag 1. Decreased amplitude and increased lag

time with depthtime with depth Changes in conductivity with depthChanges in conductivity with depth

2. Diurnal temp wave discernible to about 2. Diurnal temp wave discernible to about 0.8 m0.8 m

3. Annual temp wave to 14 m 3. Annual temp wave to 14 m

Thermal propertiesThermal properties

Specific heat/heat capacitySpecific heat/heat capacity

Thermal conductivityThermal conductivity

Specific heat/heat capacitySpecific heat/heat capacity

Ability to store heatAbility to store heat– Amount of heat required to raise Amount of heat required to raise

temperature of 1 g of substance by 1 degree temperature of 1 g of substance by 1 degree CC

greater the heat capacity of a greater the heat capacity of a substance, the more heat it can gain (or substance, the more heat it can gain (or lose) per unit rise (or fall) in temperaturelose) per unit rise (or fall) in temperature

Soil: 0.2 cal/g Soil: 0.2 cal/g Water: 1.0 cal/gWater: 1.0 cal/g

..

light dry soils experience greater light dry soils experience greater seasonal temperature swings at a seasonal temperature swings at a

given depth than wet soils.given depth than wet soils.

Thermal conductivityThermal conductivity

Ability to conduct heatAbility to conduct heat– Affected by:Affected by:

moisturemoisture texturetexture

1. moisture1. moisture

thermal conductivity of water is thermal conductivity of water is about two to three times about two to three times greatergreater than that of soil.than that of soil.

– In saturated soils, pore spaces filled with In saturated soils, pore spaces filled with water rather than airwater rather than air

Wet soils have higher conductivity than dryWet soils have higher conductivity than dry

2. texture:2. texture:

thermal conductivity of air is about thermal conductivity of air is about one hundred times one hundred times lessless than that of than that of soil .  soil .  

– Finer soils have more particle-to-particle Finer soils have more particle-to-particle contact and smaller pore spaces, contact and smaller pore spaces, therefore increased conductivitytherefore increased conductivity. .

Conductivity increases as texture becomes Conductivity increases as texture becomes increasingly fine.increasingly fine.

Notice that adding water makes Notice that adding water makes texture have opposite effect (wet sand texture have opposite effect (wet sand

higher cond. than wet clay)higher cond. than wet clay)

Texture determines how quickly soil Texture determines how quickly soil will heat (in spring) or cool (in fall).will heat (in spring) or cool (in fall).

Sandy soils contain less water (lower Sandy soils contain less water (lower porosity) and therefore heat more porosity) and therefore heat more quickly than clay.quickly than clay.