Inverse modelling of diffuse pollution risks in agricultural catchments
Milledge, D, Lane, S N, Heathwaite, L and Reaney, S
Watershed scale processes can influence instream aquatic ecosystems, through delivery of fine sediment, solutes and organic matter. Certain areas in a watershed are critical sources, where the ability both to entrain material and to connect it to the drainage network controls the level of delivery. The models that have been developed to identify these areas tend to be reductionist and, given the multitude of processes that could be included in a model to guarantee that it can work in many situations, results in the development of ever more complex models. Here we outline a new approach based on ‘inverse modelling’. We invert SCIMAP, a simple risk based model with an explicit treatment of hydrological connectivity, and use a Bayesian approach to determine the risk that must be assigned to different land uses in order to explain spatial patterns of measured instream solute concentrations. We apply the model to identify the key sources of Nitrate and Phosphate risk in 11 UK catchments across a range of hydrological and agricultural conditions but focussing on Hampshire Avon, Eden and Wensum catchments. The model results show that: 1) certain landuses are consistently high or low risk; but 2) the risks associated with different land uses vary both between catchments and between nutrients; and 3) that the dominant sources in the catchment are often a function of the spatial configuration.