CRITICAL LOADS FOR SOILS AND WATERS IN A SELECTED SCOTTISH CATCHMENT

In the UK, critical loads have been mapped for both soils and freshwat ers and the maps indicate that discrepancies may occur between these t wo receptors over sensitive areas of the UK. Freshwater critical load maps were prepared by calculating the Henriksen critical load for the most sensitive water body in each 10 km grid square. Critical loads fo r soils were calculated according to the mineralogy and associated soi l properties of the dominant soil at a 1 km resolution. To examine the differences between the soil and freshwater data sets it is necessary to calculate critical loads at a smaller scale using the catchment as the focus for study. This was done by selecting a catchment on granit ic parent material in the North of Scotland. Data on water chemistry, collected on a weekly basis, was used to calculate temporal variations in critical loads for freshwaters using the Henriksen method. Soil sa mpling across the catchment was conducted on a grid based system to pr ovide estimates of spatial variability in sensitivity. Profile charact eristics and soil chemical data obtained from detailed soil sampling p rogrammes were used in the PROFILE model to determine the spatial vari ation in critical loads for soils. In general, the results show that t he critical loads for soils tend to be lower than those for freshwater . The spatial variation in the soil critical load tends to be small wh ilst the temporal variation in critical load for freshwaters is large. in order to account for these differences it is important to identify the key processes within the catchment which play a major role in con trolling streamwater chemistry. This procedure improves the relationsh ip between critical loads for soils and waters.