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Several models and risk assessments that estimate nutrient transfer from agricultural land have been developed. The majority of these associate increased particulate or total nutrient transfer with increased slope and do not make any inferences on the impact of slope on the transfer of nutrients in solution. These models and risk assessments are increasingly used for water quality assessments as part of the implementation of the European Union Water Framework Directive. To illustrate that these conventional assumptions may be ill-founded when considering total phosphorous, total nitrogen, total carbon, soluble reactive phosphorous, total dissolved phosphorous and nitrate transfer on slope ranges representative of agricultural land in North West Europe we present results from laboratory rainfall experiments using a large soil flume, a silt loam soil, slopes of 3%, 6% and 9% and a rainfall intensity of 47 mm hr− 1. The results show that: (1) with the exception of nitrate, nutrient transfer was as variable between repeats as between slopes; (2) discharge, time since onset of runoff, sediment concentration, slope and sediment size were all dominant controls over nutrient transfer and (3) there was a relationship in the ratio of nutrient concentration to sediment concentration with slope which was attributed to the impact of sediment source and proportion of organics due to different surface connectivity, flow threads and ponding. Consequently, this questions the used of models and risk assessments which rely on slope as a driver when assessing nutrient transfer from low slopes. |
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