Physical hydronomics (PH) is the specific application of thermodynamics that physically characterizes the governance of water bodies, i.e., the Water Framework Directive (WFD) for European Union citizens. In this paper, calculation procedures for the exergy analysis of river basins are developed within the WFD guidelines and a case study is developed. Therefore, it serves as an example for the feasible application of PH in the environmental cost assessment of water bodies, accordingly to the principle of recovery of the costs related to water services in accordance with the polluter pays principle, one of the milestones of the WFD. The Foix River watershed, a small river located at the Inland Basins of Catalonia (IBC), has been analyzed. Main results, difficulties, and constraints encountered are shown in the paper. Following WFD's quantity and quality objectives previously defined, water costs are calculated and the equivalence between the exergy loss due to water users and the exergy variation along the river are also analyzed. (C) 2009 Elsevier Ltd. All rights reserved.