Impact of urban blue spaces on urban surface temperatures - A seasonal perspective.

Assessments on the cooling potential of natural environments in urban areas has largely focussed on green spaces at very specific spatial and temporal dimensions. Less has been studied so far about the surface thermal heat mitigation effect of urban blue spaces across an entire city and for different seasons and years. This study investigates the impact of urban blue spaces, such as lakes and ponds, on surface temperature for the city of Hannover, Germany. Using remote sensing data, we assess how different lake sizes and surrounding urban land use structures influence water surface temperature, surface temperature cooling intensity and distance across seasons and for three consecutive years. Our results indicate that larger lakes consistently exhibit the coolest water surface temperatures and provide the highest surface cooling intensities. The study identifies a case-specific Threshold Value of Efficiency (TVoE) of 0.70 ha for surface cooling intensity in the hottest months. We found that high proportions of impervious areas and buildings are associated with increased water surface temperatures and reduced surface temperature cooling effects, while high vegetation close to urban blue spaces show significantly lower surface temperature values. Given our limitations in data availability for particular environmental condition indicators (e.g. wind regime, air humidity) and water body characteristics (shape and depth) probably also influencing the thermal effect of blue spaces, we suggest a multi-method approach combining remote sensing and in-situ based environmental measurements to assess the thermal regulation of urban environments.