Research On The Cooling Effect Of Urban Parks And Its Impact Factors

Land use and cover are changing with rapid urbanization,which leads to significantly higher urban temperatures than rural temperatures,creating an urban heat island effect with many ecological effects that are worsening in the context of global climate change.Alleviating the urban heat island effect and improving the urban thermal environment are the main elements of urban habitat construction.Dominated by vegetation,an urban park can provide significant cooling effects and is an effective way to improve the urban thermal environment.Accurately and easily quantifying the cooling effect of urban parks and identifying its influence factors,so as to provide scientific references for better planning and management of urban parks and then improving urban thermal environment.First of all,we address the shortcomings of the methods for quantifying the cooling effect(i.e.,cooling intensity)of urban parks based on remotely sensed land surface temperature data with high uncertainty(fixed radius method)or large workload(turning point method),to propose optimized methods(equal radius method and equal area method).We select 246 parks in seven major cities(i.e.,Shenyang,Beijing,Xi’an,Shanghai,Chengdu,Wuhan,and Guangzhou)in China to verify the applicability of the new method through one-way ANOVA,correlation analysis,and regression analysis.Then,based on Landsat-8and Gaofen-2 imagery,we quantify the cooling intensity of urban parks in Changsha by the equal radius method to reveal its spatial heterogeneity,and clarifies the influencing factors of cooling intensity at the park scale by correlation analysis,multiple regression analysis and variance partitioning.Finally,we use a typical urban park(i.e.,Shawan Park)to reveal the internal heterogeneity of cooling intensity in urban parks and analyze its response to changes in land cover and vegetation community structure based on air temperature data and sample plot data by field measurement.The results of this study are as follows:First,the equal radius method(i.e.,the buffer radius of the park is equal to the radius of a circle with the same size as the park)is a fast and effective method to determine the buffer zone and quantify the cooling intensity of urban parks.Compared with the fastest method(i.e.,fixed radius method,which means all parks have equal buffer radii),the equal radius method calculates and creates buffers of their unique radii for each park with higher accuracy.Compared with the turning point method(i.e.,extracting the temperature at the first turning point of the park buffer to calculate the temperature difference between the inside and outside of the park),which has the highest current accuracy,the equal radius method has no significant difference with its results in estimating the cooling intensity of the park,the spatial heterogeneity of the cooling intensity,the logarithmic relationship between the cooling intensity and the park area size,and calculating the TVoE(Threshold value of efficiency)of urban park,but the calculation speed is faster.Second,the cooling intensity of 153 parks in Changsha ranged from-1.00 to 6.63℃,with an average cooling intensity of 1.06℃,of which 80% of the parks exerted a cooling effect.The geometric morphology of the park,the landscape pattern in the park,and the thermal environment outside the park significantly affect the spatial pattern of cooling intensity of urban parks.Based on the logarithmic relationship between park size and cooling intensity,the TVoE of urban park in Changsha is about 0.7 ha.When the park area is doubled,the vegetation cover inside the park is increased by 10%,the average patch area of vegetation is increased by 1 ha,the edge density of vegetation patches is decreased by 1m/ha or the temperature outside the park is increased by 1℃,the park cooling intensity can be increased by 1.60℃,0.1℃,0.27℃,and 0.17℃ respectively.The geometric morphology of the park is the most important part of drivers and independently explains 30% of the variations of the cooling intensity of urban park.The landscape pattern in the park and the thermal environment outside the park each independently explain 5% and 4% of the variations of the cooling intensity of urban park.There are strong interactions between the geometric morphology of the park and the landscape pattern in the park,and they jointly explained 30% of the variations.At the same time,these two groups of factors suppress the independent effect of the thermal environment outside the park.We suggest increasing park size and optimizing the spatial pattern of tree cover to enhance the cooling intensity,especially in the hot areas of the city,to improve the thermal environment and alleviate the urban heat island.Third,there is obvious spatial heterogeneity in the cooling intensity within the parks.The cooling intensity of Shawan Park can reach 1.62℃ in summer,and the cooling intensity of 15 monitoring sites ranges from 0.69 to 2.94℃.Land cover and vegetation community structure significantly influence the cooling intensity within the park.The cooling intensity of the park in summer has a significant positive correlation with canopy coverage,shrub coverage,average tree height,average canopy height,tree density and trunk sectional area,and a significant negative correlation with sky view factor(SVF).The canopy cover and tree density are the main factors affecting the cooling intensity of each site,in which a 10%increase in canopy cover and 10 per/ha increase in tree density could increase the cooling intensity of park sites by 0.94℃ and 0.02℃,respectively.Optimizing the land cover and vegetation community structure within the park,especially increasing the vegetation cover and tree planting density,can help to enhance the cooling intensity and improve the park microclimate.In summary,this study improves the quantification of cooling intensity in urban parks,reveals the spatial heterogeneity of urban parks in Changsha and the internal heterogeneity of cooling intensity in a typical urban park(i.e.,Shawan Park),comprehensively analyzes the relationship between park cooling intensity and its influencing factors,systematically elucidates the driving mechanism of park cooling intensity.The findings can provide important reference for urban park planning,design and management.