Variation in freshwater critical loads across two upland catchments in theUK: Implications for catchment scale management

In the UK the "critical loads" approach has been used to derive maps based on the 10km x 10km national grid. However, this grid based approach is inap propriate for catchment scale management and these maps cannot be used for "stock at risk" assessments of the number of water bodies or lengths of str eams in a given area that may be vulnerable to acidification. Critical load s are determined across two large river catchments in England (The Duddon) and Wales (The Glaslyn). High resolution, digital datasets are used to char acterise the attributes of each subcatchment in terms of land cover, soil, geology, topography and topology. Empirical models used to examine the rela tionship between these attributes and critical loads indicate that the form er can be used to account for significant variation in the latter. However, these relationships can vary from catchment to catchment. Thus, although t his approach provides the potential for identifying sensitive surface water s on a catchment wide basis, it is likely that models will need to be param eterised on a catchment specific basis.