Abstract
Subsurface temperature data is usually only accessible as point information with a very limited number of observations. To spatialize these isolated insights underground, we usually rely on interpolation methods. Unfortunately, these conventional tools are in many cases not suitable to be applied to areas with high local variability, like densely populated areas, and in addition are very vulnerable to uneven distributions of wells. Since thermal conditions of the surface and shallow subsurface are coupled, we can utilize this relationship to estimate shallow groundwater temperatures from satellite-derived land surface temperatures. Here, we propose an estimation approach that provides spatial groundwater temperature data and can be applied to natural, urban, and mixed environments. To achieve this, we combine land surface temperatures with anthropogenic and natural processes, such as downward heat transfer from buildings, insulation through snow coverage, and latent heat flux in the form of evapotranspiration. This is demonstrated for the city of Paris, where measurements from as early as 1977 reveal the existence of a substantial subsurface urban heat island (SUHI) with a maximum groundwater temperature anomaly of around 7 K. It is demonstrated that groundwater temperatures in Paris can be well predicted with a root mean squared error of below 1 K by means of satellite-derived land surface images. This combined approach is shown to improve existing estimation procedures that are focused either on rural or on urban conditions. While they do not detect local hotspots caused by small-scaled heat sources located underground (e.g., sewage systems and tunnels), the findings for the city of Paris for the estimation of large-scale thermal anomalies in the subsurface are promising. Thus, the new estimation procedure may also be suitable for other cities to obtain a more reliable insight into the spatial distribution of urban ground and groundwater temperatures.
Highlights
Natural in situ temperatures usually do not substantially vary at a depth of more than 10-20 m
Due to the low well density within the studied urban area, especially within the city center, we investigate the auxiliary use of satellite-derived land surface temperatures to estimate spatial groundwater temperature distribution
This study demonstrated that the spatial heterogeneity of the investigated heat islands mainly depends on vegetation cover and building density, whereas its total extent and intensity are related to the population size
Summary
Natural in situ temperatures usually do not substantially vary at a depth of more than 10-20 m. Ground and groundwater temperatures are close to the annual mean values in the atmosphere, and commonly only a marginal attenuated influence of the coupled seasonal temperature variation in the atmosphere can be detected [1]. The knowledge of the spatial and temporal variability of temperature in urban ground is crucial for proper planning of geothermal energy use. Elevated temperatures offer enhanced opportunities for geothermal heating applications, whereas warmer groundwater is less useful for cooling applications [10,11,12,13,14,15,16,17]. Aside from its role for the geothermal potential, thermal anomalies can influence chemical transport in shallow urban groundwater that often serves as a freshwater resource [18, 19]. Anthropogenic accumulation of heat threatens the stability of groundwater ecosystems [20, 21]
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