Use values include:

●Direct use values, concerning actual use of a resource for either commercial purposes or   recreation。

●Indirect use values, such as society’s benefits from ecosystem functions。

●Option values, that is, inpiduals’ willingness to pay for the option of using a resource in the future。

On the other hand, non-use (or passive use) values involve:

●Existence values, which reflect the fact that people value resources for moral reasons, unrelated to current  or  future use。

●Bequest values, which reflect people’s willingness to pay to  ensure  that  their  heirs  will  be  able  to use a natural resource in the   future。

Environmental valuation is based on people’s will- ingness to pay (WTP) an amount of money in order to avoid environmental degradation, or their willingness to accept (WTA) compensation in exchange for environ- mental impacts incurred [34,35]。 Environmental valua- tion has been used in the US since the 1960s, and was further established in Europe and other countries during the 1980s and 1990s [20,36]。 The increasing desire to incorporate monetary values for environmental impacts originates from ‘‘market failure’’, which means that the market place does not reflect the true social worth of the environment [35]。 Economic valuation seeks to in- ternalize the environmental impacts that have been traditionally viewed as externalities, in order to come up with more informed and fair choices。 The challenge is therefore twofold:  firstly, it is necessary to identify   the

ways in which environmental change affects well-being and secondly, to estimate the value of these changes through a variety of direct and indirect valuation techniques [37]。 Consequently, the accuracy of economic valuation depends on the identification and quanti- fication of the environmental change as well as the estimation of people’s preferences for, or against, this change。

3。2。Environmental  valuation methods

3。2。1。‘‘Demand curve’’ vs。 ‘‘non-demand curve’’ approaches

Nowadays, several techniques are available, which differ in data requirements, assumptions regarding economic agents, and values that they are able to capture。 Broadly speaking, valuation techniques are pided into the following two categories: the ‘‘non- demand (market price)’’ methods and ‘‘demand curve’’ approaches。

In the first category, the most commonly used techniques are the replacement cost, the damage avoided cost, the substitute (or alternative) cost, and the productivity change cost。 These methods are based on the cost of replacing environmental services, or the cost of avoiding damages, the cost of providing substitute services, and the cost incurred due to changes in productivity。 The principal assumption of these methods is that if people incur costs to avoid damages caused by lost environmental services or to replace the services of ecosystems, then those services are worth at least what people have paid to replace them。 Some examples where these methods might be applied include the  following:

●Valuing land destruction by estimating the restora- tion cost。

●Valuing improved water quality by measuring the cost of filtering and chemically treating  water。

●Valuing noise pollution by determining the cost of control measures。

Although ‘‘non-demand’’ approaches can easily be applied, economic theory shows that the full value of a good can only be estimated by examining the demand curve, which shows how much of the good people would use at varying prices。 Consider the following case。 Market price reflects the interaction of demand and supply。 The equilibrium market price is equal to the marginal value of an item, that is, the value of the last unit consumed (Fig。  1)。