Soil erosion caused by low-intensity rainfall in Angus, Scotland

Abstract

A slow-moving, occluded weather front brought over 50mm of rainfall in 24 hours to eastern Scotland on 31 March 1992, but rainfall intensities rarely exceeded 4 mm hr −1. Nevertheless, rill erosion caused widespread damage to vulnerable, recently seeded fields, unprotected by vegetation. A reconnaisance survey of 195 fields in the Forfar area of Angus revealed that erosion was particularly severe in bare soil ploughed in a downslope direction; 58 per cent of fields were affected and 30 per cent suffered rill erosion. Of fields with young crops, 10 per cent suffered some erosion and only 2 per cent were rilled. No erosion affected grazing land. At three sites, the volume of redeposited soil in closed depressions gave estimates of soil loss of 1.17–2.22 t ha −1, far in excess of both long-term annual averages and of probable soil loss tolerances for the region. The risk of events of similar magnitude has been estimated for the planting and early growing season using maximum recurrence intervals (RI) based on 1950–1991 rainfall records. For Dundee (15 km south of the study area), RI < 75 yr for March, RI < 32 yr for April, and RI < 24 yr for March and April combined. The risk of an event of such magnitude occurring in March is therefore only half the risk of an April occurrence, indicating that late planting and crop growth increase the erosion risk. Estimates of rainfall erosivity based on rainfall intensities do not adequately predict the erosion hazard in northern Britain, where prolonged frontal rain of low intensity may cause much damage. It is suggested that under conditions of saturated overland flow, sediment yield from rilled fields is related primarily to rainfall duration, and thus large sediment yields are possible where rilling is initiated.