6Loomis J. B. , Walsh R. G. Recreation Economic Decisions: Comparing Benefits and Costs (2nd) [ M ]. Venture Publishing Inc, 1997:45-48.
7Bateman I. J, Willis K. G. Valuing Environmental Preferences: Theory and Practice of the Contingent Valuation Method in the US,EU, and Developing Countries [ M]. New York: Oxford U- niversity Press, 1999 : 112 - 116.
8Asafu Adjaye J.. Environmental Economics for Non - economists [ M ]. Singapore: World Scientific Publishing Co Pte Led, 2000 : 55 -62.
9Bjornstad D. J. , Kahn J. R. The Contingent Valuation of Envi- ronment al Resources: Methodological Issues and Research Needs [ M ]. Chehenham: Edward Elgar Publishing Limited, 1999: 188 - 200.
10Davis R. K. Recreation planning as an economic problem [ J]. Natural Resources Journal, 1963 (3) : 239 - 249.