paper
LIU Changming, ZHENG Hongxing, ZUO Jianbing
2009, 31(8): 1295-1302.
Efficient Urban Rainwater Harvesting (URH) can alleviate water shortage and urban soil erosion, reduce urban non-point pollution and water logging, and improve urban ecological environment. Currently, most URH projects lack an effective and full-scale Cost-Benefit Analysis (CBA) based on economy principles, while paying more attention to the calculation of the economic benefits from tap-water saving and the basic construction. Therefore, systematic and scientific CBA indices are calculated on the basis of the combination of hydrology and economy in this paper. The potential calculation formulas of typical rainwater harvesting are discussed in details, including the potential of tank, sunken grassland, man-made pools, and recharged wells. Meanwhile, the costs of rainwater project are divided into fixed cost, operating cost, and marginal cost. The benefits of rainwater project fall into 7 categories, being followed by the benefit from tap-water saving, groundwater recharging, national fiscal increase, less social loss due to less pollution, less operational costs of the municipal water discharge system, less flood control costs, and the alleviation of land subsidence.
Beijing has been suffering from water shortage since 1980s. With the fast increase in both economy and population, the continuous increase in water demand has worsened the water shortage. According to the latest statistical data, the annual average volume per capita is only 245 m3 in Beijing. This amount equals to 1/8 of the national average and 1/30 of the international average. Consequently, Beijing is among the world’s top ten cities suffering from water shortage. On the other hand, Beijing is also seriously affected by local floods. In order to take full advantage of rainwater to alleviate water shortage, more and more Urban Rainwater Harvesting Projects (URHP) have been put into practice in Beijing. According to the statistical data, 350 URHP have been finished in 2007, 267 of which are located in the urban areas. In this paper, the model of CBA on URH is used to evaluate the 267 projects. The results are: 1) In general, Beijing’s rainwater harvesting projects is remarkably beneficial, with the index of benefit α reaching 2.0; 2) based on the values of α, the districts can be divided into four categories. The first class is non-benefit districts with the α values lower than 1.0, which includes Chaoyang, Xicheng, Dongcheng and Fengtai districts, accounting for 22.2% of the total area. The second class is low-benefit districts with the α values between 1.0 and 1.5, which includes Tongzhou, Pinggu and Shijingshan districts, accounting for 16.7%. The third class is general-benefit districts with the α values between 1.5 and 3.0, including 9 other districts, accounting for 50.0% of the total. The forth class is high-benefit districts with α values above 3.0 (included), including 2 districts, accounting for 11.1%; 3) The spatial distribution of α shows that the projects in suburban areas are more beneficial than those in urban and central areas. Meanwhile, regression analysis reveals a positive coordination between the value of α and the area of the district (r=0.65, R2=0.423). The results indicate that a lower α value in urban or central areas can be attributed to the limited space and rainwater harvesting style, smaller rainwater cisterns and more porous pavements for rainwater infiltration.
