A Critical Source Area Index for Legacy Soil Phosphorus Management That Uses Water Extractable Phosphorus and Lidar DEMs.

Critical source areas (CSAs) of diffuse phosphorus (P) losses in agricultural catchments must be accurately identified to cost-effectively target mitigation measures and best management practices. CSA indices typically use an agronomic soil P test as a source factor, but environmental soil P tests such as water extractable P (WEP) are better at replicating the chemical interaction between soil P and runoff and differences in the mobilization potential between soil types. CSA indices also use transport factors which don’t consider the control of (micro)topography on runoff generation and hydrological connectivity. To address these limitations, a sub-field scale CSA index was developed which predicts the risk of dissolved P losses in runoff from legacy soil P. It integrates soil WEP data with a Hydrologically Sensitive Area (HSA) index that identifies runoff-generating-areas using 0.25-2 m LiDAR Digital Elevation Models (DEMs), a soil topographic index, and information on microtopographic flow sinks causing hydrologically disconnected pathways. The CSA Index was applied to four intensively monitored agricultural catchments (7.5-11 km²) in Ireland with contrasting agri-environmental conditions. Between 1.1-5.6% of catchment areas were identified as CSAs using a CSA Index threshold value of ≥ 0.5, compared with 4.0-26.5% based on an approach that uses above optimum agronomic soil P status only. Thus CSA maps showed large areas, including those with excessive soil P, at low risk of P loss due to low mobilisation, transport or connectivity potential. Evaluation of the tool using high resolution hydrograph/loadograph data from 2009-2014 at catchment outlets showed a very strong relationship between the total catchment CSA Index value and the total reactive P load in quickflow (r² = 0.86). The tool could be used to target sub-field scale measures such as applying manure to non-CSAs or mitigating delivery points of CSA pathways, to reduce legacy P losses and support sustainable agricultural production.