The water cycle is governed by the thermodynamic laws. Therefore Thermodynamics and more specifically Exergoecology, that is the application of the exergy analysis in the evaluation of natural fluxes and resources on Earth, could help to build a solid cost structure based strictly on Physics and far away from price policies or subjectivities. Any type of degradation of water bodies along their rivers, lakes, etc., could then be quantified. Moreover, the physical information of the resource can be unified into universal units (energy units). and the monetary conversion of exergy costs is automatic through conventional energy prices. This paper presents a new discipline named "Physical Hydronomics". as a guide to assess environmental costs included in the European Water Framework Directive (WFD). (C) 2008 Elsevier Ltd. All rights reserved.