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The substantial heat source for the earth is Solar radiation, which is responsible for the surface temperature. The radiated solar energy warms the near surface ground and this in turn warms the atmosphere and the underground. The regular energy from the sun on the surface of the earth amounts to an average of ca. 0,75 kW/m². Different surface types and elevations, as well as both seasonal and daily changes to the amount of sun exposure, lead to local and time deviations from the median value.
The surface temperature penetrates with decreasing intensity into the ground. The penetrated depth and the speed in which the heat is transported depends on the heat transfer ability of the ground.
The heat transfer in the ground can be distinguished between conductive and convective heat transfer.
During the convective heat transfer the heat movement occurs through the material (for example ground and seepage water), on the other hand the conductive transfer results from the transportation of energy through push "processes" between molecules.
There is also another form of surface heating, that comes out of the earth and passes to the surface. It is called the earth heat flow. It originates in the production of heat from the decay of radioactive isotopes. However this form of heating is much weaker than the sun.
In the continental crust of the earth the heat flow density is defined as the heat flow per unit area perpendicular to the standard area, so there are regional differences (Hurtig & Oelsner, 1979). The heat flow density in the Berlin area amounts to ca. 80 mW/m².
The near surface groundwater temperature is therefore essentially caused by the exchange of heat between the sun, the earth's surface and the atmosphere, with a much smaller amount from the earth heat flow directed at the surface.
The regional average annual temperature at the surface in Berlin, under anthropogenically unbiased conditions, is ca 8,5 to 9 °C.
Daily fluctuation affect the ground only up to a depth of about 1,5 m, while seasonal fluctuations affect a depth of ca. 20-30 m. Below this depth the seasonal influence is no longer registered, This is the beginning of the so called neutral zone. At this point temperatures rise depending on the heat transfer ability of the rocks and the regional heat flow density (Fig. 2).
In the Berlin area the average rise in temperature in this region, until a depth of ca. 300 m, is 3 °C per 100 m.
The thermal conductivity of soils is mostly controlled by water content, although, obviously, the type of soil is also important. For an average clay, the thermal conductivity in W/(m K) is 0.25 for no soil moisture, about 1.0 for 10% soil moisture (% by volume), 1.25 at 14% (a bend in the thermal conductivity curve occurs here), 1.67 at 30%, and about 2.0 at 50%. That will give you a rough idea.