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Abstract
Dissolved silica was measured in five feeder streams to Loch Leven, sampled at 8-day intervals throughout 1985. Similar mean concentrations (8.7 to 10.7 mg SiO21−1) and narrow ranges of values (5 to 13 mg l−1 overall) were found in all, in spite of a 25-fold variation in subcatchment area (173 to 4301 ha) and a 63-fold variation in mean annual flow (23 to 1452 1 s−1). As a consequence, variation in loading (the product of concentration and flow) within and between sites, mainly reflects variation in flow rather than in SiO2 concentration.
Various methods of calculating loadings were explored, but even for a site where continuous flow records were available, a total of 7 methods gave similar answers, viz. annual mean instantaneous loads of 8208 to 8815 mg SiO2 s−1. It is possible, however, that this close agreement reflects a chance similarity between the frequency distribution of the flow values recorded at the instants of sampling and that of the flows recorded continuously.
Annual loads from the five streams ranged from 7.3 to 448.9 tonnes SiO2, and assuming a prorata loss of SiO2 to the loch from the 14% of the area of its catchment not drained by these waters, a total input of 1157 tonnes SiO2 to the loch can be calculated. Areal losses from the subcatchments ranged from 42.5 to 104.4 kg SiO2 ha−1 y−1 and increased exponentially with catchment area; these values are compared with corresponding data in the literature.
Plots of the year's individual sample results confirm the insensitivity of SiO2 concentration to changes in flow and a multivariate analysis—not reported here—involving rainfall and temperature did not explain the relationships any more effectively. However, a graph of moving averages (each of 3 points) reveals distinct seasonal differences in the concentration-flow relationship, although reference to the timing of major agricultural activities in the catchment provides few clues as to the causes of these patterns.