8. STATE-OF-THE-ART FOR SURFACE WATER QUALITY MODELLING

8.1 Definition of surface water quality domain Surface water is generally defined as any water that is present at the surface of the Earth. This covers rivers, lakes, oceans and estuaries, as well as artificial bodies (reservoirs and canals). The recent publication of the Water Framework Directive (WFD) establishes a “legal definition ” (Article 2, number 1) that, although in different terms, includes the previously listed bodies of water, EC (2000). The WFD also defines the water status, referred to a water body or a limited portion of it, based on the concepts of ecological and chemical quality, which may be excellent, good or poor. In general terms, water quality is assessed on the basis of a set of physical, chemical and biological variables, most of them concentrations of chemical entities or organisms, that are compared with criteria often a function of the type of water body and of its uses. Water quality models are tools that aim at representing the processes that govern the time evolution or the spatial variability of these variables. They cover physical characteristics, such as temperature, pH and conductivity (or salinity), but als