Adapting classical water quality diagrams for ecohydrological and policy applications

Ecological values of water have gained increasing attention over the past decades in both (eco)hydrological research and water resources management. Water quality is an important ecological steering variable, and graphical water quality diagrams may aid in rapid interpretation of the hydrochemical status of a site. Traditionally used water quality diagrams for showing multiple variables (e.g. Stiff, Maucha) were developed primarily for hydrogeological purposes, with limited information on ecologically relevant nutrient parameters. This paper presents adapted classical water quality diagrams that retain the traditional information on ions for hydrogeological characterization, and additionally provide information on nutrients for ecological water quality characterization. A scaling factor is used for the minor ions to visually get them across more equally compared to the macro-ion ions in the water quality diagram. Scaling of minor ions is presented based on average concentrations, as well as on water quality policy norms. Four different water quality diagrams are presented, all with the same ions included, but with different appearances to suit different preferences of individual users. Regional, national and continental scale data are used to illustrate how the different diagrams show spatial and temporal water quality characteristics. The adapted diagrams are innovative with respect to adding comprehensive visual information on the four ecohydrologically relevant nutrient species levels (NO3, NH4, PO4, K), advanced insight in redox status from the combination of four redox sensitive parameters (Fe, NO3, SO4, NH4) and the option to scale minor ions relative to average measured concentrations or to water quality policy norms. Using policy norms for scaling has the advantage of providing an ‘alarm function’ of exceedance of norms when concentrations surpass the ring used in the diagram. We discuss possible standardisation of scaling factors to enable comparability between sites