ASSESSMENT OF WET DEPOSITION MECHANISMS IN AN UPLAND SCOTTISH CATCHMENT

A network of collectors were installed at various altitudes and degrees of exposure in the Allt a Mharcaidh catchment, northeast Scotland, in an attempt to obtain an accurate assessment of wet deposition loading. Results indicate that the quantity and quality of bulk deposition is constant over the whole catchment. "Enhancement deposition" as measured by a filter-gauge interception collector indicated that there was greatest deposition at altitude. The concentrations of all elements, except for hydrogen, were greater than that of catchment bulk deposition at the higher altitudes; at lower altitude enrichment was only appreciable for sodium and chloride. Input/output chloride budgets were used to assess catchment evapotranspiration rates and the relative proportions of enhancement deposition within different altitudinal ranges. The calculation gives a catchment evapotranspiration of 18.5% and a chloride enhancement deposition 2.5 times greater at higher altitudes than at lower altitudes. Rainfall chemistry in this high-level Cairngorm catchment appears independent of the positioning of the rainfall collectors. Different altitudes within the catchment receive an additional loading due to enhancement deposition, dependent upon the frequency of cloud/mist cover. This additional loading must be included in the assessment of total catchment loadings and in the calculation of evapotranspiration. © 1990.