PREDICTING FRESH-WATER CRITICAL LOADS FROM NATIONAL DATA ON GEOLOGY, SOILS AND LAND-USE

Using information on geology, soils and land use, a map has been gener ated for Great Britain which indicates five classes of sensitivity of surface waters to acidification. This map has been used for designing sampling strategies for mapping critical loads of acidity for freshwat ers. This paper evaluates the freshwater sensitivity map using a data set of water chemistry collected as part of the UK critical loads prog ramme. Discriminant analysis was used to predict five critical load cl asses from information on geology and soil sensitivity for freshwater sites. This showed geology and soil information on correctly predict a pproximately 50% of all critical loads classes. In addition, 77% of si tes fall within one critical loads class of that predicted Predictions may be improved by including other variables eg altitude and geograph ical location. Differences between lake, stream and reservoir sites ar e also examined Ranges of critical loads values were determined for ea ch of the five classes of surface water sensitivity. While a trend in critical load values was evident between classes, there was significan t overlap. A simplified sensitivity map with only three classes relate d more closely to critical loads values. The paper demonstrates the us efulness of the surface water sensitivity map for assessing acidificat ion at a national scale, but highlights the difficulties of predicting critical loads for individual sensitive catchments using national dat a.