Preprint published in Applied Thermal EngineeringPavement-watering as a technique of cooling dense urban areas and reducing the urban heat island effect has been studied since the 1990's. The method is currently considered as a potential tool for and climate change adaptation against increasing heat wave intensity and frequency. However, although water consumption necessary to implement this technique is an important aspect for decision makers, optimization of possible watering methods has only rarely been conducted. We propose an analysis of pavement heat flux at a depth of 5 cm and solar irradiance measurements to attempt to optimize the watering period, cycle frequency and water consumption rate of a pavement-watering method applied in Paris over the summer of 2013. While fine-tuning of the frequency can be conducted on the basis of pavement heat flux observations, the watering rate requires a heat transfer analysis based on a relation established between pavement heat flux and solar irradiance during pavement insolation. From this, it was found that watering conducted during pavement insolation could be optimized to a frequency of every 30 minutes and water consumption could be reduced by more than 76% while reducing the cooling effect by less than 10%

Urban heat island (UHI) countermeasures are of growing interest for cities. Field studies of their micro-climatic effects are scarce, yet are essential to properly evaluate their effectiveness and that of anti-UHI policies. The standard approach to determining their micro-climatic effects is to study the difference in measurements made at case and control stations. However, measurements conducted during a pavement-watering experiment in Paris, France reveal that this method mistakes preexisting differences for pavement-watering effects. An alternative approach based on a twosample t-test was therefore developed and tested with the pavement-watering field trial as a case study. The proposed method proved able to determine the effects of pavement-watering, without misinterpreting preexisting differences. In the process of the case study, watering was found to reduce maximum daily heat stress, while having smaller statistically significant UHI-reducing effects. The greatest effects were reached during the day for all parameters with maximum reductions of 0.79°C, 1.76°C and 1.03°C for air, mean radiant and UTCI-equivalent temperatures and a 4.1% increase in relative humidity, while UHI-mitigation reached up to-0.22°C. The methodology developed is not specific to pavement-watering and recommendations for its improvement and its application to the field-evaluation of other UHI countermeasures are made.

Paving materials can negatively impact the urban climate, but knowledge of the thermal and climatic behavior of multilayer urban structures is lacking, particularly under heatwave conditions. To this aim, a laboratory-scale experiment was developed to characterize pavement samples under heatwave conditions. Surface albedo and evapotranspiration are confirmed as the dominant parameters for surface heating. The thermal properties of the underlying layers of pavement structures also impact their behavior and contribution to the urban climate. In particular, the combination of high effusivity and diffusivity of the granite sidewalk structure allow it to exhibit "cool" behavior during the day and "hot" behavior at night despite its relatively high albedo. A solar transmission index is proposed, including both the radiative and the thermal properties of a structure's constitutive layers, to rank structures by their ability to transmit absorbed radiation in depth. Future work with the developed experimental platform will aim to evaluate the performance and optimize the watering method of pavement-watering for different kinds of pavement structures. Keywords: urban heat island; urban paving materials; thermal properties; heat waves Vz downwards conductive heat flux at depth z [W/m²] Subscript/superscript α albedo [-] emissivity [-] n layer n ref/up reflected or upwards z depth [m]

Published in Urban ClimatePavement-watering has been studied since the 1990's and is currently considered a promising tool for urban heat island reduction and climate change adaptation. However, possible future water resource availability problems require that water consumption be optimized. Although pavement heat flux can be studied to improve pavement-watering methods (frequency and water consumption), these measurements are costly and require invasive construction work to install appropriate sensors in a dense urban environment. Therefore, we analyzed infrared camera measurements of pavement surface temperatures in search of alternative information relevant to this goal. Firstly, surface temperature reductions of up to 4°C during shading and 13°C during insolation were found. Secondly, the infrared camera successfully detected temperature spikes indicative of surface drying and can therefore be used to optimize the watering frequency. Measurements made every 5 min or less are recommended to minimize relevant data loss. Finally, if the water retaining capacity of the studied pavement is known, optimization of total water consumption is possible on the sole basis of surface temperature measurements

Urban water networks can contribute to the energy transition of cities by serving as alternatives sources for heating and cooling. Indeed, the thermal energy potential of the urban water cycle is considerable. Paris is taken as an example to present an assessment of the field performance of a district-scale waste water heat recovery system and to explore potential techniques for emergency cold recovery from drinking or non-potable water networks in response to heat-waves. The case heat recovery system was found to provide significant greenhouse gas emission reductions (up to 75%) and limited primary energy savings (around 30%). These limited savings are found to be mainly due to the performance of the heat pump system. Three emergency cold recovery techniques are presented as a response to heat-waves: subway station cooling, ice production for individual cooling, and "heat-wave shelter" cooling in association with pavement-watering. The cold generation potential of each approach is assessed with a special consideration for mains water temperature sanitary limitations. Finally, technical obstacles and perspectives are discussed.

Heat-related mortality is of growing concern for cities faced with the combined effects of increasing heatwave frequency and intensity and stronger urban heat islands (UHI). In cities around the world, high air temperatures have been found to have strong repercussions in terms of heat-related mortality for populations aged 65 years and older, especially nighttime temperatures. In response, many measures have been proposed to counteract the effects of UHI such as cool roofs and materials or urban greening. While these approaches are promising and are rightfully explored, behavioral adaptation measures have not received as much attention. Given the importance of nighttime temperatures on heat-wave mortality and the importance of sleep quality for individuals to recover from intense daytime heat exposure, adapting sleeping habits to reduce sleep time exposure to intense heat may help reduce the health impacts of heat-waves. In this paper, outdoor and indoor temperature measurements conducted over the summer of 2015 in the bedrooms of two apartments in Paris, France are analyzed. The potential for this kind of behavioral adaptation to reduce occupant exposure to high sleep time temperatures is quantified and discussed. The policy implications of our findings and their practicality are also mentioned.

Due to their tendency to absorb heat, urban materials participate in the formation of urban heat islands thus contributing to increased health risks during heat waves. Since 2013, the city of Paris has experimented in situ pavement-watering campaigns as an emergency cooling tool during heatwaves. These studies have highlighted the influence of the materials being watered on the optimal watering strategy to adopt. In this regard, a laboratory experiment was developed to study the thermal behaviour of various urban materials under heatwave conditions with or without watering. Here, results from watering an asphalt road structure with twelve different rates are presented to fine-tune the process for optimal cooling. The sample undergoes a heatwave like 24-hour cycle inside a climate chamber. Two distinct cooling regimes are highlighted versus the watering rate, corresponding to the increase of evaporation with the watering rate until maximum evaporation rate is reached. This aspect was used to maximise the cooling efficiency of the method while minimising the water consumption. Using the surface heat budget, the evaporative cooling flux was also determined. Pavement-watering was found to have a great impact on heat stored in the pavement and released to the atmosphere. Results otherwise compare favourably with field observations.

Many cities are expected to face a strong increase in the frequency and intensity of heatwaves by the end of the 21 st Century due to climate change. In Paris, the frequency of heatwaves could rise from an average of one day per year to 14-26 days per year, with temperatures reaching up to 50°C. Since 2012, pavementwatering is viewed as a potential tool for emergency cooling by the city while scientific work on the technique has found its application to be best suited to densely built urban areas, compared for example to urban greening whose impact may be hindered by lack of available planting space. This paper proposes an interdisciplinary approach combining urban physics with social sciences to develop such a GIS model for pavement-watering as an emergency response to heatwaves in Paris. It is built on performance criteria derived from previous work are input into a Geographic Information System to identify urban areas where pavement-watering would be most effective. In addition, a heat-related health risk assessment is conducted, using microclimatic, urban and socioeconomic layers, to single out areas where heatwave risk is highest in public spaces, combining high temperatures, pedestrian traffic and local population vulnerability. The microclimatic hazard dataset includes a physical model of park and water body cool islands assuming they are driven by thermal diffusion. The resulting tool has significant flexibility in defining the thresholds of the different indicators. The mapping scheme identified a total of 50 to 200 km of high priority areas for pavement-watering, requiring between 1,400 and 5,800 m 3 /day of non-potable water, equivalent to 0.6 to 2.6 L/day per capita. Limitations due to data quality or resolution are discussed as well as paths for further improvement.