Study On The Relationship Between Runoff Reduction Capacity Trading Effect Of Functional Area And Landscape Pattern

In order to improve the cities’ regulation and storage capacity for stormwater runoff,China put forward the concept of "sponge city" in 2013,its core principle is to mitigate runoff through source control.As an urban landscape with practical functional use,the functional area is the main place to reduce runoff at source.However,due to the different natural base,scale and use,its runoff reduction capacity is quite different,which hinders the comprehensive promotion of regional sponge city.In order to fully utilize the differences in the runoff reduction capacity in different functional areas,a "capacity trading" approach based on functional areas have emerged,and its implementation effect is affected by the distribution pattern of functional areas to a certain extent.In order to discuss the relationship between capacity trading effect and landscape pattern,part of Tianjin city,Nanjing city and Yangzhou city were selected as the study objects,analyzed the landuse of study areas and conducted landscape classification.Landscape index method and capacity trading approach were used to analyze the scale dependence of landscape pattern and capacity trading effect,and determine the scale relationship between the two.Random forest regression model was used to identify key landscape factors that affect capacity trading effect.The main research results obtained are as follows:(1)Four landscape types included residential area,commercial area,industrial area and educational area.In the three study areas,the landscape pattern of Nanjing study area was characterized by the largest number of landscapes,high patch density,high degree of landscape fragmentation,close distance between patches,and relatively concentrated patch distribution.The landscape pattern of Tianjin and Yangzhou study areas was characterized by a small number of landscapes,low patch density,low degree of landscape fragmentation,long distance between patches,and relatively scattered patch distribution.(2)The landscape pattern and capacity trading effect presented relatively consistent scale dependence.The landscape composition and distribution tended to be stable within the critical scale,reflecting the characteristics of the study areas’ overall landscape pattern,and the capacity trading effect was close to the overall optimal effect.The critical scale in Tianjin study area was 2000～2500 m,including 60～100 functional areas,which could achieve 80%～87%capacity trading effect.The critical scale in Nanjing study area was 1000～1400 m,including 30～65 functional areas,which could achieve 78%～82%capacity trading effect.The critical scale in Yangzhou study area was 1500～2000 m,including 25～50 functional areas,which could achieve 87%～94%capacity trading effect.The critical scale could reflect the appropriate planning scope of sponge city,collaborating with multiple functional areas to jointly undertake the transformation and construction responsibilities of sponge city,significantly reducing the overall construction cost.(3)The four landscape types were significantly correlated with surrogate cost and traded capacity in different study areas.Residential area and commercial area were the main landscape types that affect the capacity trading effect in Tianjin and Nanjing study areas,the cumulative relative importance to surrogate cost was 32.1%and 26.9%respectively,and the cumulative relative importance to traded capacity was 30.6%and 27.0%respectively.Educational area and residential area were the main landscape types that affect the capacity trading effect in Yangzhou study area,the cumulative relative importance to surrogate cost was 27.0%and to traded capacity was 28.0%.(4)Number of patches(NP),Patch density(PD)and the Shannon’s diversity index(SHDI)were proved the main landscape indices that affect the capacity trading effect in different study areas.There was a significant negative correlation between NP,SHDI and surrogate cost,while they were significantly positive correlated with traded capacity.PD presented a significant positive correlation with surrogate cost,and a significant negative correlation with the traded capacity.In different study areas,the cumulative relative importance of the three main indices factors to surrogate cost was 38.3%,35.4%and 35.9%respectively,and the cumulative relative importance to traded capacity was 35.8%,38.5%and 36.2%respectively.(5)The landscape pattern with rich landscape types,uniform distribution,and full mixing provided necessary conditions for capacity trading,and the staggered distribution of functional areas with complementary capacity relationships was conducive to achieve most of the capacity trading effect in a smaller scale.Taking the size of the appropriate planning scope of sponge city as the standard to divide the difficulty of sponge city construction,Nanjing study area was easier,Yangzhou study area was the second,Tianjin study area was the most difficult.Taking the achieving speed of capacity trading effect as the standard to divide the difficulty of sponge city construction,Yangzhou study area was easier,Nanjing study area was the second,Tianjin study area was the most difficult.