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Low Temperature Geothermal Resources: Ground Source Heat Pumps 189
0 250 500
Miles
FIGUre 10.5 Soil thermal regimes for the contiguous United States. The thermal regime is an indication of
soil properties that relates to temperature, moisture content and soil mineralogy. (Modified from USDA Natural
Resources Conservation Service (http://soils.usda.gov/use/thematic/temp_regimes.html.)
0.0 Light, dry
soil
2.0
4.0
Depth (m) 6.0 Wet soil
8.0
10.0
12.0 Winter Summer
14.0
–4.0 0.0 4.0 8.0 12.0 16.0 20.0
Temperature (°C)
FIGUre 10.6 Temperature profile annual extremes in soil, to a depth of 15 meters. The thin lines are ideal-
ized temperature profiles for summer maximum and winter minimum temperatures. There are two curves
for each seasonal extreme, the more extreme curve representing the idealized variation for wet soil, and the
less extreme curve showing the variation of dry soil. The idealized curves assume that the variation is sym-
metrical about a mean temperature. The heavy solid curves are actual measurements from a site in south
Finland representing two different years. (From Lemmelä, R., Sucksdorff, Y., and Gilman, K., Geophysica,
17, 143–54, 1981.)
