Sediment Yield Studies in Upland Catchment Areas in South-East Scotland

Abstract

Although it is known from studies by Thomas (1956), Imeson (1971) and others that degeneration of the vegetation cover by burning, overgrazing or recreational pressure often causes accelerated erosion on hillsides in upland Britain, few data are available to indicate whether this results in high sediment yields from the catchment areas thus affected. In view of the growing concern about the quality of the country's water resources this situation is clearly unsatisfactory and the need to rectify it has been stressed by many authors (Douglas 1969). This can be done in two ways: firstly by sampling the sediment load in upland streams over a period of time, preferably a long one; secondly by measuring the rate of sediment accumulation in reservoirs and other water bodies located in upland areas. This paper presents the results obtained from a recent study of the second type involving two reservoirs in south-east Scotland: North Esk Reservoir in Midlothian, and Hopes Reservoir in East Lothian. Both the reservoirs considered in this paper are located in upland areas within 30 km of Edinburgh (Fig. 1). North Esk Reservoir, the older of the two, lies in the Pentland Hills where it is fed by headwaters of the river North Esk. It was opened in 1850 to supply water to the paper mills located further downstream in the Penicuik area, and has been used continuously for this purpose ever since. Hopes Reservoir is located further east in the Lammermuir Hills where it was commissioned in 1935 to develop Hopes Water, a tributary of the river North Tyne, for public water supply purposes. It now forms part of the South-East of Scotland Water Board supply system. Both reservoirs are of approximately the same small size with areas of 0-10 km2 and 0*13 km2 and capacities of 576 x 103 m3 and 350 x 103 m3 respectively. Their catchment areas of 7 km2 and 5 km2 are also similar in size and physical character (Fig. 1). Both consist of deeply dissected moorland ranging in altitude from c. 300 m to c. 500 m above sea level. Both are underlain by Palaeozoic sedimentary rocks and, in both, glacial deposits are locally important in the valley bottoms. Podsolic soils predominate in both areas, with extensive areas of peat on the hill tops. In each case the vegetation developed on these soils comprises acidic grassland and heather moor types dominated by Deschampsia/Festuca and Calluna vulgaris. Pteridum is common on the lower slopes. In both areas sheep grazing at a stocking rate of c. 2 sheep to the hectare is, and has been for many years, the main form of land use, with burning at intervals of 7-10 years as the primary form of land management.