Alleviating Temperature And Increasing Humidity Of Urban Green Space And Its Value Estimation

The environmental problems have become the hot topics among the people in recent years. Temperature and humidity are closely related to the comfort of human life, and more importantly, it will be harmful to human health when the temperature is too high or the humidity is too low. Green spaces in the city have played an inestimably important role in regulating temperature and humidity and improving the ecological environment. The construction of urban green spaces also have been changed to focus on the ecological function rather than the artistic function, and the related researches have more practicability. The study abroad on ecological benefits started early and there have been many achievements. The trend of their study has been transformed from the qualitative description to quantitative research. While, the domestic research, which started late in this field, is still in a stage of focusing on qualitative description. Even lack of systematic description, the quantitative research on benefits of ecological system has made a lot of meaningful attempt and has obtained some achievement of great significance. However, the study on cooling and humidifying effect and comfort index of greenbelts in Xi’an City is relatively smaller in number than any other places, therefore this study is of great importance. In this study, in August 2014, four or five different plant cummunities has been selected respectively from the residential areas, round-city-parks and roadside green space in Xi’an as the research objects. The selected plant communities can be summarized as the following three types: arbor forest, bush, and lawn. The air humidity on the height of 0.15 m, 0.5m, 1.0m and 1.5m above ground was measured at daytime in summer in order to find out the effects of different green spaces on regulating the temparature and humidity. The research results show that:First, for each plant community, the cooling humidification effect weakens vertically from the top to the bottom, with the best effect on the hight of 15 cm above ground. For arbor forest and bush, from 15 cm to 100 cm above ground, the cooling effect weakens as the height increases; while from 100 cm to 150 cm above around, the cooling effect enhances as the height increases. For lawn, the cooling effect weakens vertically as the height increases. For all these three types, among the four vertical heights, the best cooling effect is on the height of 15 cm above ground.Second, in summer, the temperature is high and the humidity is low. The humidity of all kinds of green space is higher than that of the cement ground. In the green space, a variety of plants are growing, whose transpiration makes more water in the air, thereby increasing the relative humidity of the environment. Therefore, the green space has a positive effect on the environment, and various types of plants play a very important role in the humidifying effect.Third, for the park green space,the environment temperature is relatively low in the morning, and the cooling effects of arbor forest, bush, and lawn are all small. From 10: 00 to 16: 00, the environment temperature is higher,and the cooling effects of three types of plant communities also increase,with the most obvious cooling effect from 12: 00 to 14: 00. Then, from 16: 00, the atmospheric temperature begins to drop slowly, with the cooling effects beginning to wane. During a day, the greatest drop in temperature usually occurs during 12: 00 to 14:00, and the tiniest drop in temperature occurs in the morning and the evening.Forth, for the plant communities in the residential areas, the day temperature of arbor forest change relatively slowly, with the highest temperature at 14: 00 to 16: 00; while the day temperatures of bush and lawn change greatly, with the highest temparature at 12: 00 to 14: 00 in accordance with the time of the highest atmospheric temperature. The humidity changes from high to low and then from low to high again. Compared with the cement land, the average highest temperatures of arbor forest, bush and lawn are 1.6℃, 1℃ and 0.6℃ lower respectively; the average lowest humidity of arbor forest, bush and lawn are 3.5%, 0.9%, 0.5% higher respectively. The results show that all kinds of green space can perform the function of lowering the temperature and increasing the humidity, with the arbor forest showing the best effect.Fifth, if Xi’an builts up an area of 14000 hm2 green space, it can absorb heat of 4.0 x 1014 J, with per hectare greenfield absorbing heat of 310 million J every day on average, equalling the cooling effect of four 1000 W air conditioners.Last, taking the residential electricity prices for reference, the economic value of building green space in summer is 55.6 million yuan, with per unit 0.4 yuan/m2.