Methodology
In order to obtain a statement regarding the groundwater temperature distribution over a large area, a Kriging interpolation was first of all carried out. The prerequisite for the use of this method is that the groundwater temperature has been regionalized, i.e., it is a locality dependant variable. Such a variable is e.g. characterized by the fact that neighbouring points are more similar to each other than are mutually remote points.
The temperature distribution in the near-surface subsoil is very greatly influenced by the local climatic conditions at the surface, and the intervals between measurement points are in some cases very great in the state of Berlin. Potential small-scale temperature changes in the near-surface subsoil are hence often not registered, so that the progression of the temperature isolines may be subject to great imprecision.
Basically, a check of local representation should be carried out for each measurement point. However, the cost of testing would be very high, and the results would include areas in which the spatial representation of a measurement point would be insufficient to permit precise statements on temperature distribution in the subsoil of an area.
The basis for the qualitative processing was provided by the maps 04.02, Long-Term Mean Air Temperature, 1961-1990 (2001 Edition), and 06.01, Actual Use of Built-Up Areas (2011 Edition).
In addition, for larger areas in which no temperature data were available for the deeper subsoil between 50 and 100 m depth below terrain surface, synthetic temperature profiles and virtual boreholes at an interval of 1000 m were generated based on similar site characteristics, and using the known structure of the geological subsoil. These synthetic temperature profiles were used for the prognosis of the temperature distribution. Here too, it was evident that based on the intervals between synthetic measurement points, not all spatial structures at the surface and in the subsoil could be ascertained, and thus taken into account in terms of their effect on temperature conditions.