The urban permeable surface is an effective practice to conserve rainwater. This study presents a method to calculate the areas of the urban permeable surfaces and assess the benefits of the rainwater retained by them. As a case study in the Xigang District and the Zhongshan District of Dalian City

the areas of the urban permeable surfaces in 2005 were calculated based on the data from aerial photographic survey. Then the volume of the collected water in the year was obtained according to the annual rainfall amount

retaining ratio of the runoff

and infiltrating ratio of ground water. At last

the benefits of the rainwater retained by urban permeable surfaces were estimated using the economic and ecological benefit indexes. The main results from the study can be summarized as follows:(1) The total area of the permeable surfaces was 3 663.30 hm2

which was 51.5% of the total area of the study region. The area ratio of forestry land

grassland

and porous pavement was 6.5:1.3:1.0. (2) In virtue of the urban permeable surfaces

runoff was reduced by 1.69×107 m3

of which 5.0×106 m3 rainwater infiltrated into groundwater layer. Consequently

the total economic benefit of the retained rainwater was 1.4×106 RMB and the ecological benefit was 5.52×107 RMB. Moreover

great social benefit was generated due to controlled soil erosion and improved urban residential environment.