The effect of rainfall intensity on soil erosion and particulate phosphorus transfer from arable soils

Soil erosion, in the form of transported suspended sediment in overland flo w, is often associated with high rates of particulate phosphorus (PP) (tota l P > 0.45 mu m) transfer from land to watercourses. Particulate P may prov ide a long-term source of P for aquatic biota. Twenty-two Sites for winter overland flow monitoring were selected in south-west England within fields ranging from 0.2-3.8 ha on conventionally-managed arable land. Fields were situated on highly porous, light textured soils, lacking impermeable horizo ns and often overlying major aquifers. Long arable use and modem cultivatio n methods result in these soils capping under rain impact. Overland flow wa s observed when rainfall intensity approached the modest rate of 0.8 mm hr( -1) on land at or near to field capacity. Low intensity rainfall (<2 mm hr( -1)) produced mean suspended sediment losses of 14 kg ha(-1) hr(-1), with a ssociated PP transfer rates of 16 g ha(-1) hr(-1). In high intensity rainfa ll (>9 mm hr(-1)) mean PP losses of 319 g ha(-1) hr(-1) leaving the field w ere observed. As might be expected, there was a good relationship between P P and suspended sediment transfer in overland flow leaving the sites. The c apacity of light soils to cap when in arable use, combined with heavy or pr olonged rainfall, resulted in substantial discharges, soil erosion and asso ciated PP transfer. Storms with heavy rain, typically of only a few hours d uration, were characterised by considerable losses of PP. Such events, with return periods of once or twice a winter, may account for a significant pr oportion of total annual P transfer from agricultural soils under arable cr ops. However, contributions from less intense rain with much longer duratio n (around 100 hours per winter in many arable districts of the UK) are also demonstrated here. (C) 1999 IAWQ Published by Elsevier Science Ltd. All ri ghts reserved.