Forestry effects on sediment sources and yields in the Balquhidder catchments, central Scotland

Abstract

This study sets out to examine the effects of forestry- operations on sediment sources and yields in a Consortium funded, Institute of Hydrology (IH) instrumented, paired catchment experiment at Balquhidder in the southern Highlands of Scotland. A sediment budget approach was adopted incorporating ongoing IH catchment sediment output monitoring of suspended and bedload from the 7.7 km2 moorland Monachyle catchment and the 6.8 km2, 40% forested Kirkton catchment. Sediment source inputs from tributary streams and mainstream bank erosion were estimated. There is good agreement between source inputs and catchment outputs in the moorland but not in the forest where tributary inputs seem to be over-estimated. Mainstream channel banks contribute around 5% of the total sediment yield in both catchments. Catchment suspended sediment yield accounts for 97% of the total sediment output (less than 3% is bedload) and is about one and a half times higher in the forested catchment where concentration varies more sensitively with streamflow, suggesting greater availability of erodible sediment. These pre-disturbance sediment budgets based on 1982-5 outputs monitoring and 1984-5 source monitoring are used as a background from which to assess the effects of the land-use conversions which began in 1986. In autumn 1985 disturbance began in the forested catchment in the form of road regrading, repairs and construction of timber stacking areas. Clearfelling started in January 1986. Between April and July 1986 about 10-15% of the moorland catchment was ploughed and ditched in preparation for planting. Preliminary analysis of suspended sediment data collected in this post-disturbance phase (1986-7) so far indicates that there have been about three and seven-fold increases in suspended sediment outputs from the moorland and forest catchments respectively. No bedload output data is available yet from IH. In the moorland tributaries bedload yields more than doubled but suspended sediment load in the one moorland tributary monitored did not reflect the increased outputs even though the sub-catchment was ploughed and ditched. The greatest impact of forest clearfelling appears to be accelerated erosion of regraded and more heavily used roads and of recently constructed timber loading areas. These ’bare’ areas are shown to be capable of supplying much of the observed catchment increases in suspended sediment outputs. The elevated sediment yield in the forested catchment before felling and in both catchments after disturbance is attributed to an increase in the sediment supply from new sources (ditches, roads and stacking areas in the forest; plough furrows and ditches in the moorland) and not to an increase in flood magnitude or frequency. Forestry management implications are discussed in relation to the findings.