Soil erosion and conservation on land cultivated and drained for afforestation

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Abstract
Erosion of soil from pre‐afforestation plough furrows has been measured on four soil types in Scotland for 12 to 18 month periods between 1987 and 1990. Rainfall‐run‐off was also measured at one site. Run‐off is directly proportional to furrow length and rainfall intensity, and for a wide range of intensities (typically > 6 mm hr−1) 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 significant soil loss is of the order of 25 cm3 s−1 m−1, which is in accord with 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 with the total rainfall in the rainstorm — typically 40–80% by the hydrograph peak and approaching 100% by the end of the hydrograph; i.e. long term storage is negligible. A positive relationship was recorded between furrow length, slope angle and sediment yield, with deposition predominating in furrows less than 30 m in length on slopes less than a few degrees. Soil loss is proportional to the excess streampower expended by the run‐off with an exponent in the range 1–1.5. For the soils examined, significant differences in soil loss when comparing sites for low power expenditure become undifferentiated at high power expenditures. 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 erosion 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.