In most watersheds biophysical and demographic conditions are not static and the impacts of controllable and uncontrollable risks are not uniformly distributed among ecosystem areas. Differences in risk factors assigned to ecosystems in different areas can cause significant differences in their economic values even if they are identical and are in are in landscape contexts that are identical now. Note that the expected value of services from a ecosystem may be affected by the vulnerability of expected service flows from other ecosystems. Difficulties with Environmental Valuation Non-Market Goods Most environmental goods, such as clean air and water, and healthy fish and wildlife populations, are not traded in markets. The only option for assigning dollar values to them is to rely on non-market valuation methods.
Environmental goods are different.
Clean water and air, beautiful views, and to some extent outdoor recreation, can be enjoyed by everyone in the same way as radio and television. The economic value of non-rival or public goods is the sum of all people's willingness to pay. Non-exclusive Goods People cannot be excluded from enjoying most environmental goods and the cost of trying to exclude them is prohibitive. Other than increases in onsite hunting and fishing opportunities, which may be a source of economic benefit to farmers, the environmental benefits of most conservation practices are non-exclusive. Inseparable Goods Conservation practices at a given site contribute in many roundabout ways to environmental goods and result in environmental and economic benefits that accrue over great distances in time and space.