6Costanza R,D Arge R, Groot R,et al. The value of the world's ecosystem services and natural capital. Nature, 1997,387(15) : 253-260.
7Mitsch W J,Gosselink J G. The value of wetlands: importance of scale and landscape setting. Ecological Econom- ics,2000,35(1) :25-33.
8Brown T C,Taylor J G,Shelby B. Assessing the direct effects of stream flow on recreation: A literature review. Water Resources Bulletin, 1992,27 (6) : 979-989.
9Daubert J,Young R. Recreational demands for maintaining in stream flows:A contingent valuation approach. A- merican Journal of Agricultural Economics, 1981,63(4): 666-675.