Fluvial Eco-Environmental Assessment Based on Exergy Efficiency for Small Rivers in Main Island of Okinawa

Abstract

A new ecosystem assessment method based on exergy effi ciency is employed to evaluate the health of fl uvial ecosystems. The exergy takes into account both energy quantity and energy quality. In this method, the ecological exergy effi ciency, which is calculated as the ratio of organism exergy produced in a system to exergy supplied from outside the system photosynthetically active radiation exergy and organic exergy supplied by fl ood, is defi ned as an indicator of the health of a fl uvial ecosystem. This ecosystem assessment method is applied to three small rivers on the main island of Okinawa, Japan. The results show that the exergy effi ciency of the fl uvial ecosystems takes different values among the three rivers and can clarify the difference in the health between the natural zone and the artifi cial zone with bank protection of the Genga River, which is diffi cult to identify by means of the evaluation using water quality and biological environment.