Validation of the UK critical loads for freshwaters: Site selection and sensitivity

Critical loads maps for UK freshwaters have been produced on a 10×10 km grid square basis, and used to map critical load exceedances under various deposition scenarios. A single lake or stream site was selected to represent the most sensitive water body in each grid square using predefined criteria. In the UK a major programme of data screening and validation has been undertaken in order to address issues of accuracy and validity. A major part of this validation exercise, the within-square variability study, is designed to test the extent to which the site chosen for mapping represents the most sensitive water body within each grid square or mapping unit. Sampling of all lake sites in thirty-two randomly chosen 10 × 10 km grid squares has shown that in two thirds of cases, the selection exercise has identified a site in the lowest critical load class within a square. However, up to a third of all sites selected to represent grid squares could be replaced by more sensitive sites with a critical load smaller by at least one Skokloster class. The mean overestimate of "diatom model" critical loads for sulphur in the within-square variability study is 0.188 keq ha^-1 yr^-1. This means that current critical load maps show overestimates for some grid squares. In order to determine where the most sensitive site has not been identified, further work on catchment scale classification of freshwater sensitivity is being carried out.