Purpose
In recent years, cities have been increasingly experiencing the so-called heat island phenomenon due to various reasons such as artificial ground cover and anthropogenic heat. A rise in temperature produces unpleasant outdoor environment and increases energy consumption as a result of the use of air-conditioners.
Pavement made of asphalt or concrete absorbs more heat from the sun, causing the surface temperature to rise occasionally above 70 degrees during the summer, which results in urban heat island and the thermal stress to human bodies. Accordingly, lowering the surface temperature of the pavement will greatly help reduce urban temperature and human heat stress, creating more comfortable urban space.
The authors focused on the watering on paved roads and retention in permeable or porous pavement as measures to lowering surface temperature, and these measures were investigated through field observation for the watering and lysimeter experiments for the water retention in the permeable pavement.
Method
The authors conducted test watering on the surface of roadways in Nagaoka City in August, from 9 a.m. until 3 p.m. Two observation sites were set up, one within the area to be watered and the other about 30 m away from this area. We used about 500 m of total length of the city's snow-melting pipe network. We observed meteorological and thermal variables such as air temperature, relative humidity, globe temperature, wind velocity, solar radiation, atmospheric radiation and surface temperature. The quantity and temperature of water sprinkled from the watering pipe were also measured.
Besides, experimental studies are carried out for the water retention in permeable pavement. We placed a piece of permeable pavement (w=50cm, B=50cm, H=5cm) on the soil packed in a weighable lysimeter. Meteorological and hydrological conditions were measured to quantify the energy balance of the permeable pavement with and without water retention under natural circumstances.
Result and Conclusion
The results of watering showed that the differences in air temperature, globe temperature and relative humidity at two sites with and without watering were a maximum 1 degree, 4 degrees and 4%, respectively. At the site where watering was not conducted, the maximum surface temperature reached 50 degrees, while the surface temperature at the site with watering was lower by 10 to 30 degrees. In addition, the estimated energy balance at the sites showed that considerable latent heat and lateral heat transport contributed to lowering the surface temperature of a paved road.
The lysimeter measurements showed that the surface temperature of the pavement with water retention is reduced by 18 degrees at maximum than without retention, and the latent heat is dominant to the net radiation rather than the heat storage by retention water.
The bio-climatic effect of lowering 20 degrees of the surface temperature is estimated by the numerical analysis of energy balance of a human body. The result indicated that lowering 20 degrees of the surface temperature significantly relieves the human thermal stress and is equivalent to the decrease of air temperature by 3 degrees in terms of human thermal sensation.