Naar een klimaatbestendige stad d.m.v. groene en blauwe structuren (KB-14-002-005, KB-01-017-008)

Vegetation and water can help mitigating unfavourable impacts of weather and climate in cities. When deciding upon application of green and blue structures quantification of effects on urban climate in relation to effects in other policy areas may be required. Simple but realistic assessment methods preferably embedded in an interactive tool may be helpful. The goal of the present project is to convert scientific knowledge on effects of green and blue structures on urban climate into pieces of such a tool. We analyse observations from various sources, perform model simulations and attempt tailoring the information to the users’ needs.

Observations in Rotterdam confirm that this city can be much warmer than the surrounding rural area. They also confirm the cooling effect of urban vegetation. Cooling by surface water is less clear, because in particular in late summer water can contribute to nighttime warming of the city. However, especially large water surfaces allow urban ventilation, which has a positive impact on thermal comfort during periods with warm weather. Parts of our main results have been implemented in the Climate Adaptation Atlas (klimaateffectatlas.wur.nl) and in European maps with indicators to assess urban climate (www.eea.europe.eu).

Special observations in the cities of Arnhem and Rotterdam revealed that the contribution from evaporation to city cooling may be considerable. This obviously requires water. The data from Arnhem suggest that in the summer periods of 2012 and 2013 about 60% of the precipitation was evaporated, of which a relatively large portion on wet days.

Simulations with a regional weather model in which grassland around Rotterdam was replaced by deciduous forest suggest that such a landuse change may slightly warm the city, by a few tenths of a degree. This result is in contradiction with results from similar simulations for the Veluwe area and deserves further investigation.