Author Contributions
Conceptualization, J.A.V.S. and M.W.; methodology, J.A.V.S. and M.W.; software, J.A.V.S.; validation, M.W., S.B., and J.R.; formal analysis, M.W., S.B., and J.R.; investigation, J.A.V.S., M.W., and S.B.; resources, J.A.V.S.; data curation, J.A.V.S. and M.W.; writing—original draft preparation, J.A.V.S.; writing—review and editing, M.W., S.B., and J.R.; visualization, J.A.V.S.; supervision, J.R.; project administration, J.A.V.S., M.W., and J.R. All authors have read and agreed to the published version of the manuscript.
Figure 1.
Topographic map of Nuremberg’s location within Germany (left) and the location of the 38 Nuremberg groundwater monitoring wells sampled for groundwater temperature shifts (right). Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.
Figure 1.
Topographic map of Nuremberg’s location within Germany (left) and the location of the 38 Nuremberg groundwater monitoring wells sampled for groundwater temperature shifts (right). Source: Esri, Maxar, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community.
Figure 2.
Recalculation of the original IMD grid (A,A*) to IMDr = 100 (B) and IMDr = 500 (C,C*). For IMDorig (A,A*), sealing values of 0% or 100% were extracted for a large number of observation wells. A dependency between sealing density and the annual temperature shift could, therefore, not be determined. In contrast, in (C*), with a circular buffer of 500 m, it was possible to derive different urban settings and to work out a statistical correlation between the individual parameters. Base map: Geobasisdaten © Bayerisches Vermessungsverwaltung 2020.
Figure 2.
Recalculation of the original IMD grid (A,A*) to IMDr = 100 (B) and IMDr = 500 (C,C*). For IMDorig (A,A*), sealing values of 0% or 100% were extracted for a large number of observation wells. A dependency between sealing density and the annual temperature shift could, therefore, not be determined. In contrast, in (C*), with a circular buffer of 500 m, it was possible to derive different urban settings and to work out a statistical correlation between the individual parameters. Base map: Geobasisdaten © Bayerisches Vermessungsverwaltung 2020.
Figure 3.
The subsurface urban heat island SUHI of Nuremberg is shown as an isotherm map with a contour interval of 2 K, calculated from more than 400 temperature data. The temperature peaks reached up to 17 °C and were mainly measured near the center. In addition, the 38 observation wells are shown as a function of the calculated annual temperature shift from the years 2015–2020. The temperature shifts ranged from −0.02 K/a to +0.21 K/a. Base maps: CORINE Land Cover, CLC2012; Geobasisdaten © Bayerisches Vermessungsverwaltung 2020.
Figure 3.
The subsurface urban heat island SUHI of Nuremberg is shown as an isotherm map with a contour interval of 2 K, calculated from more than 400 temperature data. The temperature peaks reached up to 17 °C and were mainly measured near the center. In addition, the 38 observation wells are shown as a function of the calculated annual temperature shift from the years 2015–2020. The temperature shifts ranged from −0.02 K/a to +0.21 K/a. Base maps: CORINE Land Cover, CLC2012; Geobasisdaten © Bayerisches Vermessungsverwaltung 2020.
Figure 4.
Comparison of different temperature–depth logs from five observation wells (A–F), recorded with the TLC-meter. A positive trend can be seen for the temperature profiles A–C and F. Temperature–depth log D shows constant temperatures over the entire observation period. In the case of the temperature–depth logs, only measurement series from the same months can be compared with each other, as otherwise temperature-related convections have too great an influence. Plot F shows the absolute temperature shift from 2015–2020 as a temperature–depth plot for each individual observation well.
Figure 4.
Comparison of different temperature–depth logs from five observation wells (A–F), recorded with the TLC-meter. A positive trend can be seen for the temperature profiles A–C and F. Temperature–depth log D shows constant temperatures over the entire observation period. In the case of the temperature–depth logs, only measurement series from the same months can be compared with each other, as otherwise temperature-related convections have too great an influence. Plot F shows the absolute temperature shift from 2015–2020 as a temperature–depth plot for each individual observation well.
Figure 5.
Recorded temperature curves of four data loggers. All evaluated data loggers show a constant, positive temperature trend within the last years. Although the data loggers are located within the neutral zone, seasonal oscillations are indistinctly (A) to clearly (B–D) visible, which can be attributed to temperature-related convection currents within the groundwater monitoring well.
Figure 5.
Recorded temperature curves of four data loggers. All evaluated data loggers show a constant, positive temperature trend within the last years. Although the data loggers are located within the neutral zone, seasonal oscillations are indistinctly (A) to clearly (B–D) visible, which can be attributed to temperature-related convection currents within the groundwater monitoring well.
Figure 6.
The map shows IMDr = 500 categorized into three classes: class 1 in green ranges from 0–30%, class 2 in orange contains IMD values 30–60%, and class 3 is the highly sealed, inner-city area, shown in red, with values from 60–100%. In addition, the monitoring wells are shown with their mean annual changes in groundwater temperature from 2015 to 2020. Base map: Geobasisdaten © Bayerisches Vermessungsverwaltung 2020.
Figure 6.
The map shows IMDr = 500 categorized into three classes: class 1 in green ranges from 0–30%, class 2 in orange contains IMD values 30–60%, and class 3 is the highly sealed, inner-city area, shown in red, with values from 60–100%. In addition, the monitoring wells are shown with their mean annual changes in groundwater temperature from 2015 to 2020. Base map: Geobasisdaten © Bayerisches Vermessungsverwaltung 2020.
Figure 7.
(A,B): Plots of IMDr = 500, against groundwater temperature and temperature change. The linear trends are clearly visible. Plot (C): temperature changes against groundwater temperature. There is only a very weak positive correlation between the individual values. Plot (D): statistical evaluation of GWT and groundwater shift for different urban settings, derived from the degree of sealing IMDr = 500.
Figure 7.
(A,B): Plots of IMDr = 500, against groundwater temperature and temperature change. The linear trends are clearly visible. Plot (C): temperature changes against groundwater temperature. There is only a very weak positive correlation between the individual values. Plot (D): statistical evaluation of GWT and groundwater shift for different urban settings, derived from the degree of sealing IMDr = 500.
Table 1.
Geological units of the Nuremberg subsurface with its hydraulic and thermal properties, modified after [
45] and [
25].
Table 1.
Geological units of the Nuremberg subsurface with its hydraulic and thermal properties, modified after [
45] and [
25].
System | Chronostratigraphy | Lithostratigraphy | Lithology | Hydrogeology | λsat [W/(m × K)] | Thickness [m] |
---|
Quaternary | - | Sediments | q | clay-gravel | Aquifer 3 | 2.4 5 | 0–30 4 |
Triassic | Norian | Löwenstein-F. 1 | kmBO 1 | Sst 1 | Aquifer 3 | 3.0 2 | 90 4 |
kmBM 1 | SSt | 3.0 2 |
kmBU 1 | Sst | 2.9 2 |
Carnian | Mainhardt-F. | 2.9 2 |
Hassberge-F. | kmBl + C 1 | Sst | 3.0 2 | 40 4 |
Steigerwald-F. | kmL 1 | Clst 1 | Aquifer 3 | 2.5 2 | 30 4 |
Stuttgart-F. | kmS 1 | SSt | Aquifer 3 | 2.6 2 | 4–30 4 |
Ladinian/Carnian | Benk-F. | kmE 1 | Clst | Aquiclude 3 | 2.1 2 | 20–30 4 |
kmBe 1 | Sst | Aquifer 3 | - | 90 4 |
Table 2.
Types of data collection and specifications of the devices in use.
Table 2.
Types of data collection and specifications of the devices in use.
Type | Device | Accuracy | Interval of Measurement | Depth of Data Acquisition | No. of Sampling Well |
---|
TLC-meter | Solinst/HT Hydrotechnik | ±0.1 K | ≤5 a | Vertical log, 0–30 m | 27 |
Data logger | Aquitronic Beaver ATP10 | ±0.2 K | 12–24 h | Discrete depth, 10–27.5 m | 11 |
Table 3.
Determined temperature shifts of the groundwater.
Table 3.
Determined temperature shifts of the groundwater.
Parameter | Unit | Total Database | Temperature Logs | Data Logger |
---|
Observation well, count | [–] | 38 | 27 | 11 |
Depth of Data Acquisition | [m b.g.l.] | 10–27.5 | 10–26 | 10–27.05 |
Min Temperature shift | [K/a] | −0.02 | −0.02 | +0.05 |
Max Temperature shift | [K/a] | +0.21 | +0.21 | +0.15 |
Mean Temperature shift | [K/a] | +0.07 | +0.05 | +0.10 |
Median Temperature shift | [K/a] | +0.06 | +0.04 | +0.08 |
Table 4.
Calculated Pearson correlation coefficients (r) of IMD against mean temperature shift and IMD against groundwater temperature, for different grid sets of IMD calculated via focal statistics.
Table 4.
Calculated Pearson correlation coefficients (r) of IMD against mean temperature shift and IMD against groundwater temperature, for different grid sets of IMD calculated via focal statistics.
Raster Re-Calculation | Focal Statistics Radius in [m] | | |
---|
Original data set | Original data set | −0.12 | +0.18 |
Mean/Circular | 50 | −0.13 | +0.32 |
Mean/Circular | 100 | −0.06 | +0.36 |
Mean/Circular | 250 | +0.11 | +0.60 |
Mean/Circular | 500 | +0.35 | +0.67 |
Mean/Circular | 750 | +0.33 | +0.68 |
Mean/Circular | 1000 | +0.31 | +0.63 |
Table 5.
Groundwater temperatures and temperature shifts for different IMD sealing classes.
Table 5.
Groundwater temperatures and temperature shifts for different IMD sealing classes.
Parameter | | Unit | IMDr = 500 in [%] |
---|
| 0–30 | 30–60 | 60–100 | 0–100 |
---|
Observation well | count | [–] | 5 | 17 | 16 | 38 |
Temperature shift | Mean | [K/a] | +0.03 | +0.07 | +0.08 | +0.07 |
Median | [K/a] | +0.02 | +0.07 | +0.07 | +0.06 |
Min | [K/a] | −0.01 | −0.02 | −0.00 | −0.02 |
Max | [K/a] | +0.10 | +0.15 | +0.21 | +0.21 |
Std Dev | [K/a] | +0.04 | +0.04 | +0.05 | +0.05 |
Groundwater temperature | Mean | [°C] | 11.0 | 13.3 | 13.9 | 13.2 |
Median | [°C] | 10.8 | 13.1 | 13.6 | 13.2 |
Min | [°C] | 10.2 | 11.6 | 13.0 | 10.2 |
Max | [°C] | 12.0 | 15.5 | 16.0 | 16.0 |
Std Dev | [°C] | 0.7 | 1.2 | 0.9 | 1.3 |
Table 6.
Comparison of groundwater temperature shifts in different studies for Central Europe.
Table 6.
Comparison of groundwater temperature shifts in different studies for Central Europe.
Location | Period | Land Use Classification | Temperature Shift | No. of Sampling Locations | Data Acquisition | Depth | Sampling Location |
---|
Germany/Baden-Württemberg [16] | 2000–2015 | - | 0.012 K/a | 1468 | - | <40 m | well |
Germany/Bavaria [17] | 1992/’94–2019 | - | 0.28 K/(10 a) (median) | ≤32 | TCL-meter | 20 m | well |
Austria [15] | 1994–2013 | CLC | 0.7 K/(19 a) (~0.04 K/a) | 227 | - | <30 m | well |
Germany/Nuremberg | 2015–2020 | IMD | 0.07 K/a | 38 | TCL-meter | <30 m | well |