Mitigation of the Urban Heat Island effect by using water and vegetation

Mitigation of the Urban Heat Island effect by using water and vegetation

Slingerland, J.D.

Van de Giesen, N.C. (mentor)
Van der Ven, F.H.M. (mentor)
Brolsma, R. (mentor)
Kleerekooper, L. (mentor)
De Jong, S.A.P. (mentor)

Civil Engineering and Geosciences

Water Management

Water Resources

2012-12-20

Urban areas are confronted with higher temperatures compared to rural areas during summer. Buildings, roads and paved surfaces store the heat during the day and then release it slowly during the evening keeping urban lands hotter than surrounding areas. This phenomenon is called Urban Heat Island effect and the differences can be up to 8°C. A rise in mortality and decrease of work productivity are only some of the consequences. To see if and how vegetation, and water can mitigate this urban heat island effect, measurements are done in the city of Rotterdam, using temperature sensors, sap flow measurements and DTS by fiber optic cables. Measurement results of the temperature sensors show that temperature differences between an urban area and a small park within this urban area can be 3°C, when air temperatures are 25°C. Under these circumstances, temperature in the park is equal to the temperature measured outside the city, meaning that the urban heat island effect is abolished in the park. The results also shows that the urban heat island intensity for the city of Rotterdam is the largest during the night and can be up to 7°C. Trees can help mitigate the UHI by evaporating sap which is transported through the trunk to the leaves. The measurements show an increase of sap flow going further in the growing season, starting from about 10 liter per day towards over 500 liters a day. When this amount of water is divided by the surface area of the tree crown, the considered trees can evaporate 4.5 mm/day. The cooling effect of surface water is hard to measure, mainly because it is not possible to compare the temperature just above the water surface with temperatures above paved surface at the same time. It can be seen that water is a good mitigation measure, because DTS measurements show that a minimum of 14% of daily incoming solar energy is absorbed by surface water. DTS show also that the cooling effect of trees can be up to 5°C, partly by providing shade and partly by evaporation of water through the leaves. The same measurements show that the cooling effect by shade of trees is larger compared to the cooling effect by shade of buildings. When water evaporates form a paved surface this results in a decrease of air temperature. The measurements show that this decrease can be 2°C close to the ground when 1 mm of water evaporates and up to 6°C close to the ground when an infinite amount of water is available. Towards a height of 2 meter, the cooling effect decreases to 1°C and 2°C respectively. The directly measureable cooling effect of vegetation is larger than the cooling effect of water. This is mainly caused by the fact that a large part of the cooling effect of trees is provided by shade, which is of course absent with water. Nevertheless, water is a good mitigating factor of the UHI. Surface water is very use full to absorb incoming solar energy.

Urban Heat Island

http://resolver.tudelft.nl/uuid:cd2976c6-7c06-4277-92d1-a2dc5c054809

2012-12-21

Student theses

master thesis

(c) 2012 Slingerland, J.D.