Soil Temperature

Energy balance Incoming shortwave from sun Incoming shortwave from sun Outgoing longwave from earth-atmosphere Outgoing longwave from earth-atmosphere

Overall balance in soil: Daily (diurnal): Daily (diurnal): –Net gain in day –Net loss at night Annual: 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. amount of thermal energy that moves through an area of soil in a unit of time.

Heat flux diagram handout Annual: Annual: –Winter heat flux –Summer heat flux

Re: heat flux diagram: Notice: 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 (39.1 ⁰ 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 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 Diurnal pattern Annual pattern

Soil temperatures cycles handout Notice: 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 Specific heat/heat capacity Thermal conductivity Thermal conductivity

Specific heat/heat capacity Ability to store heat 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 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 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. 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. 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. 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). 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. Sandy soils contain less water (lower porosity) and therefore heat more quickly than clay.