Depth to Groundwater 1995
To find out the groundwater depths at first a model of the height of groundwater level above sea-level was calculated from data collected at the groundwater measuring sites (using the methods described in the text accompanying Map 02.12 – Groundwater Levels). The depth to groundwater for areas with a confined groundwater is defined as the distance between the lower edge of the covering mantle (or the upper edge of the groundwater aquifer) and the surface. Measurements taken from the geological drillings are used to establish values for the depth of confined groundwater areas. From the Barnim flat upland approximately 100 drillings were considered. From the Teltow flat upland nearly 40 drillings were considered. Through this process of measuring and sampling, one has a model of the upper surface of the groundwater (hydro-contour lines – lines of the same groundwater level). Interpolation of data was made with the program SICAD-SCOP (GIS software).
Then a Difference Model was calculated with SICAD-SCOP using the Model of the Upper Surface of Groundwater and the Terrain Elevations Model (model of the altitude of ground about sea-level) of the Environmental Information System (UIS). The width of the grid was 100 m. Depths to groundwater are divided into seven depth classifications portrayed as a map of different heigth levels to groundwater. The first results revealed regional inadequacies of the Terrain Elevations Model, so it became necessary to carry out the calculations several times to insure accuracy. The obvious mistakes were corrected in the Terrain Elevations Model and the Groundwater Depth Model was repeated. In order to differentiate depths to groundwater in the range of up to 4 m, which are important for vegetation, an irregular division of classification was chosen.
For smaller areas it would be possible to obtain more exact results using smaller widths of grid to interpolate the data, provided that the density of data of the Terrain Elevations Model is included. Value limits used for classification of depths to groundwater also can be chosen arbitrarily.
The exactness of the data collected for the Groundwater Depth Model is directly dependent on the quality of the Terrain Elevations Model. Therefore, any miscalculations in the Terrain Elevations Model also apply to the Groundwater Depth Model.
The following points should be considered, to avoid false interpretations:
- Narrow strips at the edge of surface waters, which have some connection to groundwater, are not portrayable in the chosen scale
- Because of the state of the data, the Terrain Elevations Model will show some inaccuracies. This relates on the one hand to areas in the outlying districts (forests and agricultural areas) with not enough points of elevation and on the other hand to areas that were not yet developed at the time when the measurements were taken. Because of anthropogenic landfills, some of the map showing depressions with a low depth to groundwater no longer exist.
- Areas of confined groundwater existing under thick relatively impermeable, obstructive boulder marl layer can usually be assumed to have depths to groundwater of more than 10 m. The lower edge of the groundwater obstructing layer is assumed to be the upper surface of the groundwater. Sandy interstratifications in these boulder marl layer, within which near-surface perched groundwater can also appear, are narrowly limited spatially, and their sites can hardly be localized, and cannot be portrayed.
- Upper surface of the groundwater is subject to strong variations in areas near wells, depending upon withdrawal amounts. Depths to groundwater of more than 10 m can occur here. These areas cannot be portrayed by these scales either.
- It is to be noted that not all wet areas potentially valuable for the protection of biotopes and species can be read in the Map of Depth to Groundwater (depth less than 1.0 m). This includes areas that have no connection to groundwater and are moistened by flooding programms or periodic natural flooding (such as the Tiefwerder Meadows).