Evaluation of rainwater runoff storage by urban green spaces in Beijing

As the world population increases, more and more people will live and work in cities. Urbanization involves replacement of vegetated surfaces by impervious surfaces; often causing an increase in the rate and volume of surface runoff of rainwater. Climate change may further increase these fluctuations and the flood risk. Beijing experiences rapid urbanization and the rapid urban extension alter natural hydrological processes. Urban green spaces have positive effects on infiltration and storage of water in the soil, resulting in run-off reduction, nutrient and pollutant removal, and improved groundwater quality. Therefore, urban green spaces in Beijing can play an important role in the water cycle, for the improvement of urban ecology and management. Although some studies found that the green areas in Beijing had positive impact on reduction of storm water runoff, only a few studies could reveal the economic or ecological benefits of rainwater storage by urban green spaces. To evaluate the benefits of rainwater storage by urban green spaces and to provide a reference point for the public and government organizations, we grouped rainwater storage into two ecosystem benefits: rainwater runoff reduction and rainwater purification. Based on inventory data of urban green spaces in Beijing (2009), the amount of rainfall runoff reduction was calculated by the rainwater runoff coefficient of different green types, the benefits of rainwater storage by green space were evaluated by the shadow price method of the unit cost for reservoir and price of reclaimed water. The results showed that, a total volume of 1.54 x 10 super(8)m super(3) rainwater was conserved in these urban green spaces, which almost corresponds to the annual water needs of urban ecological landscape in Beijing, and 2494 m super(3) of potential runoff was stored per hectare of green area. The total economic benefit was 1.34 billion Yuan, which equals three quarters of the maintenance cost of green spaces in Beijing, and the monetary value of runoff storage was 21.77 thousand Yuan per hectare. We also found that the benefits could be ranked by district or county in the same way as urban green areas, but the average benefits per hectare green spaces showed variable trends, which may be related to the varying ISI (Impervious Surface Index) with regions. To reduce the high runoff during rainfall and increase rainwater harvesting as an alternative water supply one constructs drainage facilities. This impli