Multi-criteria reservoir water management

Abstract

The Plastiras dam was constructed in the late 1950s mainly for electric power production, but it has also partially covered irrigation needs and water supply of the plain of Thessaly. Later, the site has been designated as an environment conservation zone because of ecological and landscape values, while tourist activities have been developed around the reservoir. Irrigation of agricultural land, hydroelectric production, drinking water supply, tourism, ecosystem water quality and scenery conservation have evidently been conflicting targets for many years. Good management would require a multi-criteria decision making. Historical data show that the irregular water release has resulted in a great annual fluctuation of the reservoir water level. This situation could be improved by a rational management of abstractions. Apparently, higher release leads simultaneously to more power production and to irrigation of a larger agricultural land. Moreover, demands for electricity and for irrigation are partially competing to each other, due to different optimal time schedules of releases. On the other hand, higher water release leads to lower water level in the reservoir and, therefore, it decreases the beauty of the scenery and deteriorates the trophic state of the lake. Such degradation affects the tourist potential as well as the quality of drinking water supplied by the reservoir. A multi-criteria approach uses different scenarios for the minimum permissible water level of the reservoir, if a constant annual release is applied. The minimum level concept is a simple and functional tool, because it is understood by people, easily certified and incorporated into regulations. The quantity of water that would be yearly available is a function of the minimum level allowed. The water quality depends upon the trophic state of the lake, mainly the concentration of chlorophyll-a, which determines the state of eutrophication and is estimated by water quality simulation models, taking into account pollutant loads such as nitrogen and phosphorus. The value of the landscape is much depending on the water level of the lake, because for lower levels a dead-zone appears between the surface of the water and the surrounding vegetation. When this dead zone is large, it seems lifeless and the lake appears partially empty. Quantification of this visual effect is not easy, but it is possible to establish a correspondence between the aesthetic assessment of the scenery and the minimum allowed reservoir level. Using results from hydrological analysis, water quality models and landscape evaluation, it seems possible to construct a multi-criteria table with different criteria described against alternatives and with a plot of three relative indices against the minimum level allowed. However, decision making has to take into account the fact that comparison or merging of indices corresponding to different criteria analysis encompasses a degree of arbitrariness. More objective decisions would be possible if different benefits and costs were measured in a common unit. Moreover, management will be sensitive to different social pressures.