ERT tomography to investigate the formation of new groundwater in wooded solid rock locations
Report of the project:
Groundwater from hard rock aquifers with forested catchments plays an important role in the drinking water supply in Germany. This water often contains less anthropogenic contaminants and is basis for the local water supply in the German low mountain ranges via spring captures.
Now, the "future of the forest" is threatened by climate change, and with it this groundwater resource. Tree species adapted to the new climatic conditions are needed that maintain the ecosystem function of the forests and thus their function for groundwater recharge. The spatially and temporally heterogeneous and dynamic water availability is a central parameter for the selection of suitable tree species.
ERT measurement in Solling
Source: BGR
To explore the spatially and temporally variable water content of the soil in the spruce forest (Picea albis) and the soil-groundwater path, we perform high-resolution electrical resistivity tomography (ERT) measurements. Primarily we observe the natural infiltration processes, but in addition we carry out one irrigation test with an isotope tracer.
The geoelectric array measurements comprise 3421 individual dipole measurements. An array measurement takes approx. 40 minutes and is repeated every 8 hours. The measuring array is 17 m long and 7 m wide and the electrodes are spaced respectively at distances of 0.5 and 1.0 m. The measurements are inverted and the resistivities are interpreted as proximal parameters for the changing water content in the subsurface.
The ERT measurements are combined with local TDR measurements near one spruce tree, observations of the xylem flow in the spruces of the array area, measurements of the water balance parameters (precipitation, evaporation, temperature, canopy throughfall) and modeling of the water balance. The geoelectric measurements carried out in the spruce stand in Solling in cooperation with the Northwest German Forest Research Institute since March 2019 show a spatially and temporally heterogeneous picture of the resistivity (Fig. 1).
We detected a zone of reduced resistivities surrounding the central spruce while the other spruces in the array area do not show such a striking anomaly. The resistivities below all spruce trees down to 0.5 m are mostly lower than at some distance to these trees, where blueberries (Vaccinium myrtillus) grow.
Fig. 1: Distribution of the specific resistance in the ERT array area (supervision on October 8, 2019). The spruce locations are marked with black rectangles, blueberries grow in the upper left quarter
Source: BGR
The resistivities in the different depths down to approx. 1 m and in smaller and larger distances to the spruce trees differs significantly (Figures 2 and 3) with time. The permanent observation includes the very dry spring 2020, where we observed extraordinarily high resistivities in the rooting areas of the spruce trees. In this spring the highest resistivities, indicating low water contents, we found at a depth of 0.6 m. The resistivities in the upper 0.5 m increased after rain in June 2020 indicating higher water content in the spruces rooting areas.
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A resistivity decrease below the rooting area of the spruces (deeper than 0.5 m) is interpreted as indication of percolation processes towards the groundwater. According to the preliminary interpretation, such processes are primarily observed in winter, but also locally at other times after higher rainfall.