Unformatted text preview: Soil Temperature Topic # 8
Importance of soil temperature Chemical and biological reactions increase as temperature increases Soil Temperature Affects: Soil development Nutrient availability Organic matter content of soils Basic Concepts Basic
Temperature: a measure of the average kinetic energy of particles in a substance Heat (Energy) flows from hot to cold (thermal gradient) Thermal conductivity: ability of a medium to transmit heat (speed limit) Heat Capacity: ability of a substance to store heat (water 5 times higher than dry soil) Basic Concepts
Heat (energy) moves in three ways: Radiation – as electromagnetic waves (short-wave radiation from sun;
long-wave radiation from Earth) Convection – physical movement of air (hot air rises) Conduction – by collision between atoms in solids and liquids
(from hot sources) H2O & CO2 (from cooler sources) Incoming Solar radiation Greenhouse gases (Heats air)
Outgoing long-wave radiation Long wave - sky Radiated long-wave Short wave radiation enters, but long-wave radiation is largely trapped
Short-wave solar Shortradiation Long wave Earth radiation LW SW Long wave - Earth LW L W L W 1 Energy reaching the soil Some is reflected Some evaporates water (E.T.)
(it take 540 cal/g to evaporate water) Heat flow in Soils – Where do the calories go? From high temperature to low temperature down a thermal gradient. Heat flow: Q dT K At dx Some is radiated back to space as long-wave radiation Some is consumed by heating the soil surface or overlying air Q = quantity of heat - calories
T = temperature, dT is the difference in temperature x = distance between the points where T is measured A = cross sectional area of flow, t = time K = thermal conductivity (cal/s cm C) rock 4-7, water ≈ 1.4; air ≈ 0.06 Thermal Conductivity of Soils Factors Affecting Soil Heating Albedo Slope & aspect Soil cover (veg & mulch) (veg Soil Moisture Heat capacity Evaporation of water Thermal conductivity Figure 7.22 Bulk density and water content affect heat transfer through soils from a warm zone (X1) to a cooler zone (X2). The rate of heat transfer is in proportion to the arrow thickness. Soil compaction increases particle-to-particle contact, which in turn hastens heat transfer because the thermal conductivity of mineral particles is much higher than that of air. If the remaining gaps between particles become filled with water instead of air, thermal conductivity increases still more because water also conducts heat better than air. Therefore wet, compacted soils transfer heat most rapidly. (Diagram courtesy of R. Weil) Typical Albedos Which landscape has a higher albedo?? 2 Slope and Radiation North and south facing slope North contrast in western Montana South slope North Slope North facing slopes receive less direct radiation and are cooler and wetter environments Heat Capacity
Water = 1 cal/cm3 oC Dry soil = 0.2 cal/cm3 oC Which soil will have a greater temperature change from day to night? How much will the temperature change upon addition of 1 cal of energy?
Water = ? o C 1 Dry soil = ? o C 5 Dry soil: low heat storage Wet soil: high heat storage Soil Heat Flow and Water ` Dry soil
Low x-section in dry soil Moist soil ET Heat Cap K ET Heat capacity K Temp Temp Temp Water film –wet soil higher xsection 3 Daily Temperature Changes Regulating Soil Temperature Mulch or vegetative cover Water content Wet soils are cooler & remain cooler because energy is used to evaporate water and the soil has a high heat capacity Mulch insulates soil retains moisture keeps out weeds reduces soil compaction reduces erosion Porosity & compaction
- Mineral soil have greater K than organic Shading Fire-Walking Tips Porous rock/ash Low K Low heat capacity Sweaty feet Evaporation 4 ...
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