Paving the way for Green

Abstract

Climate change is one of the most pressing matters our society faces. As a result, temperatures are rising across the globe. This effect is especially prevalent in urban areas, where artificial surfaces such as pavements are abundant and vegetation scarce. This effect is called the Urban Heat Island Effect (UHI). Another issue that is caused by climate change are increased rainfall extremes, which lead to a higher risk of flooding. This risk of flooding is higher in urban areas, as vegetation that can hold and absorb water is scarce. Vegetation can thus help mitigate both issues. Residential gardens could be an interesting source of vegetation within urban areas. The city of The Hague, in the Netherlands, is assumed as having one of the highest UHI intensities in the country. Moreover, it is the densest city in the country and thus poses an interesting location to study these gardens.
This study’s objective is to find out what it is that drives people to have vegetation within their gardens, and what effect this could have on both the UHI effect as well as runoff. The main research question entails: “How, and where, can vegetation in residential gardens in The Hague improve climate on a neighbourhood level?”. To answer this question, ArcGIS and SPSS are used. Initially, drivers behind vegetation within gardens are explored. Then, the current effect of vegetation within gardens is analysed. Finally, the potential of vegetation within gardens is calculated. This leads to several neighbourhoods that could be of great interest to policymakers who want to increase the amount of vegetation.
First, four factors emerged that influence vegetation within residential gardens. Green in the public environment, the average value of the residence, and the average size of the garden have a positive impact on vegetation within gardens. The more green in the public environment, or the higher economic value the residence has, the more vegetation there is within gardens according to this study’s findings. In contrast, the average household size instead sees a negative impact. The more people within a household, the more impervious surfaces within gardens.
Next, the effect of vegetation within the gardens on the UHI effect and runoff was calculated. This has shown that across all neighbourhoods, residential gardens reduce on average 2.7% of the local median UHI effect. The largest reduction is observed in Parkbuurt Oosteinde with 6.7%, and the smallest reduction in Westvliet-Oost, with 0.2%. As for runoff during a rainfall event of 100mm, the amount differs between 0.374mm in Vlietbuurt and 21.634 mm in Lage Veld, with an average of 7.3mm. Runoff stands for the amount of rain that cannot be absorbed by the soil.
Finally, the potential for residential gardens was calculated for a scenario in which all impervious surfaces were replaced with vegetation. As for the reduction of the UHI, increases up to 7.8% are noted. In addition, runoff can be reduced by up to 77% of the current amount. With these calculations, three neighbourhoods emerged as having high potential in both cases. These are van Hoytemastraat e.o., Parkbuurt Oosteinde, and Lage Veld. One thing that these neighbourhoods share is that they all consist of a large part of gardens of their total surface area. Policymakers could address these three to test their ideas, as the effects are likely to be easier to identify here than in other neighbourhoods.