A classification of acidified catchments in Britain

To compare catchments of possible acidified waters in Britain an extensive database was compiled for nearly 600 catchments of lakes and streams for which chemical data were available. Information on the database included map-derived data, pollutant deposition estimates and hydrological characteristics. To stratify the catchment database a randomly selected subset of 328 catchments was used to derive eight classes of catchments using the classification program TWINSPAN. Several classifications were generated by this method using an increasing number of catchment parameters. TWINSPAN classes were defined on the basis of some catchment parameters (indicators) but were shown to be associated with other variables. A general linear model (GLM) analysis of the available chemical data was used to test the different classifications. A TWINSPAN classification which excluded soil and geology characteristics gave the best GLM model because soil and geology types were poorly correlated with other catchment parameters. However, because of the recognized importance of soils and geology in acidification processes, these parameters were introduced into the GLM model as separate GLM classes. The resulting three-class model (TWINSPAN class, soil type, geology type) proved superior to both the one-class model and to two-class models which included either soil or geology. It was demonstrated that the TWINSPAN classes were associated with particular geographical areas and particular monitoring programmes. The TWINSPAN classification was used to evaluate a set of catchments selected as long-term monitoring sites for Britain. In general, the set covered a wide range of catchment types as defined by the classification. It was possible to identify minor shortfalls in classes of catchment selected and suggest possible additions to the sampling programme. Whilst the classification procedure has been applied to possible acidified waters, the method is of general relevance to all catchments and waters of more diverse nature.