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Research article2016Peer reviewedOpen access

A sub-field scale critical source area index for legacy phosphorus management using high resolution data

Thomas, I. A.; Mellander, Per-Erik; Murphy, P. N. C.; Fenton, Owen; Shine, O.; Djodjic, Faruk; Dunlop, Paul; Jordan, P.


Diffuse phosphorus (P) mitigation in agricultural catchments should be targeted at critical source areas (CSAs) that consider source and transport factors. However, development of CSA identification needs to consider the mobilisation potential of legacy soil P sources at the field scale, and the control of (micro) topography on runoff generation and hydrological connectivity at the sub-field scale. To address these limitations, a 'next generation' sub-field scale CSA index is presented, which predicts the risk of dissolved P losses in runoff from legacy soil P. The GIS-based CSA Index integrates two factors; mobile soil P concentrations (water extractable P; WEP) and a hydrologically sensitive area (HSA) index. The HSA Index identifies runoff-generating-areas using high resolution LiDAR Digital Elevation Models (DEMs), a soil topographic index (STI) and information on flow sinks and effects on hydrological connectivity. The CSA Index was developed using four intensively monitored agricultural catchments (7.5-11 km(2)) in Ireland with contrasting agri-environmental conditions. Field scale soil WEP concentrations were estimated using catchment and land use specific relationships with Morgan P concentrations. In-stream total reactive P (TRP) concentrations and discharge were measured sub-hourly at catchment outlet bankside analysers and gauging stations during winter closed periods for fertiliser spreading in 2009-14, and hydrograph/loadograph separation methods were used to estimate TRP loads and proportions from quicicflow (surface runoff). A strong relationship between TRP concentrations in quicicflow and soil WEP concentrations (r(2) = 0.73) was used to predict dissolved P concentrations in runoff at the field scale, which were then multiplied by the HSA Index to generate sub-field scale CSA Index maps. Evaluation of the tool showed a very strong relationship between the total CSA Index value within the HSA and the total TRP load in quickflow (r(2) = 0.86). Using a CSA Index threshold value of >= 0.5, the CSA approach identified 1.1-5.6% of catchment areas at highest risk of legacy soil P transfers, compared with 4.0-26.5% of catchment areas based on an existing approach that uses above agronomic optimum soil P status. The tool could be used to aid cost-effective targeting of sub-field scale mitigation measures and best management practices at delivery points of CSA pathways to reduce dissolved P losses from legacy P stores and support sustainable agricultural production. (C) 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (


Critical source area; Diffuse pollution; Phosphorus; LiDAR DEM; Agriculture; Hydrologically sensitive area

Published in

Agriculture, Ecosystems and Environment
2016, Volume: 233, pages: 238-252
Publisher: Elsevier Masson

      Sustainable Development Goals

      Ensure availability and sustainable management of water and sanitation for all
      End hunger, achieve food security and improved nutrition and promote sustainable agriculture

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      Environmental Sciences

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