SOIL-EROSION AND CONSERVATION ON LAND CULTIVATED AND DRAINED FOR AFFORESTATION

Erosion of soil from pre-afforestation plough furrows has been measure d on four soil types in Scotland for 12 to 18 month periods between 19 87 and 1990. Rainfall-run-off was also measured at one site. Run-off i s directly proportional to furrow length and rainfall intensity, and f or a wide range of intensities (typically > 6 mm hr-1) a small amount of soil is flushed out of the furrows. However, for furrow,spacings of 3.8 m, a critical downslope run-off increment associated with signifi cant soil loss is of the order of 25 cm3 s-1 m-1, which is in accord w ith a storm of five years return period and a maximum intensity of 25 mm hr-1. The total run-off volume for any hydrograph is commensurate w ith the total rainfall in the rainstorm - typically 40-80% by the hydr ograph peak and approaching 100% by the end of the hydrograph; i.e. lo ng term storage is negligible. A positive relationship was recorded be tween furrow length, slope angle and sediment yield, with deposition p redominating in furrows less than 30 m in length on slopes less than a few degrees. Soil loss is proportional to the excess streampower expe nded by the run-off with an exponent in the range 1 - 1.5. For the soi ls examined, significant differences in soil loss when comparing sites for low power expenditure become undifferentiated at high power expen ditures. For the rainfall regimes monitored, maximum soil losses were in the region of 40 kg per meter run-length of furrow, when soil peds were ripped from the bed. Laboratory data concerning the critical eros ion threshold power and shear stress to erode soil peds are in general accord with the threshold furrow run-lengths defined using the field data for a five year storm and the soil losses observed.