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Berlin Environmental Atlas

02.14 Groundwater Temperature (Edition 2014)

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Groundwater Temperatures and the Annual Temperature Curve

The major heat source for the near-surface subsoil to a depth of approx. 20 m is the solar irradiance which reaches the earth’s surface. This is substantially responsible for the surface temperature.

The near-surface soil is heated by irradiated solar energy, and passes the heat on to the atmosphere and to the subsoil. The annual total of that part of the solar irradiance which impacts upon a horizontal surface, the so-called global irradiance, averages approx. 1000 kWh per sq. m and year in the State of Berlin. A number of parameters at the air-surface interface affect the local thermic climate. Colour, composition, surface roughness, degree of ground cover, the water balance and the orientation of solar irradiation upon urban surfaces determine how much energy is absorbed and "stored" in the building structures, and how much of that is passed on to the atmosphere or the subsoil.

Basically, the temperatures of the earth's surface, and hence, too, the heat entry or exit, are subject to periodic fluctuation in a cycle of one year, which corresponds to the progression of the seasons.

The surface temperature penetrates the soil with decreasing intensity. The penetration depth and the speed with which the heat is transported depend on the heat transfer capacity of the soil.

Subsoil heat transfer can be distinguished as either conductive and convective heat transfer.

While convective heat transfer involves the movement of the heat via moving medium, such as groundwater or seepage water, conductive transfer results from the transmission of energy through the successive impact of molecules.

Compared with solar irradiance, the main heat source for the near-surface area, the geothermal flow caused by the decay of radioactive isotopes in the subsoil is of much less importance.

In the continental crust of the earth, the geothermal flow density, which is defined as the thermal flow per unit of area perpendicular to the standard area, varies regionally. According to Hurtig & Oelsner (1979) and Honarmand & Völker (1999), the mean thermal flow density in the State of Berlin is between approx. 80 and 90 mW/sq. m. Based on this, an energy total of between 0.7 and 0.8 kWh per sq. m and year can be calculated, which is only approx. 1/1000 of the global irradiance.

The near-surface groundwater temperature is essentially determined by the exchange of heat between the sun, the earth's surface and the atmosphere, and to a much lesser degree by the geothermal flow towards the surface.

The regional average annual temperature at the surface in Berlin, given no anthropogenic effects, is approx. 8.0 to 8.5°C.

While the daily fluctuations affect the soil only to a depth of approx. 1.0 m, the seasonal fluctuations reach depths of between 15 and a maximum of 25 m. Below this depth, in the so-called neutral zone, where seasonal effects can no longer be ascertained, the temperatures rises depending on the heat transfer capability of the rock and the regional geothermal flow density (Fig. 2).

In the Berlin area, the average temperature increase to a depth of approx. 300 m is 3°C per 100 m.

Seasonal Progression of Groundwater Temperature
Fig. 2: Seasonal Progression of Groundwater Temperature

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