Modelling long-term diffuse nitrate pollution at the catchment-scale: Data, parameter and epistemic uncertainty

A model of catchment-scale nitrate transport is presented for a small, rural headwater basin (Alton Pancras: <10 km2) in Dorset, UK, for the period 1930–2007. Estimates of annual nitrogen (N) loading were based on parish land-use data, held in the UK’s National Archives, and previously reported figures of typical UK N loadings from livestock, fertiliser, ploughing of permanent pasture, atmospheric deposition, biological fixation and crop uptake. Loading calculations were performed within an uncertainty framework to allow for the reliance on literature data sources. Loading calculations show that all significant sources must be included not just fertiliser application which, at most, contributes 50% of N input in any given year. A simple algorithm was used to transform estimated catchment N loading (1930–2007) into a river nitrate response (observed data: 1980–2004). This assumed N-loads were delayed by some catchment mean travel time (MTT), ta, attenuated according to a Peclet number, Pe, converted into solute concentrations by a factor, α, to increase some initial baseline river concentration, Cb. Simple graphical translation of estimated catchment N loading to the river concentration data suggested a MTT of around 37 years. As Pe → ∞, the transport model simplified to a linear relationship between catchment N load and river nitrate concentration response lagged by the MTT. Hence, the model results suggest that, in this catchment, advection is the dominant mechanism for transport of diffuse pollution from land to river: there is little or no dispersion present. The MTT (ta) was then reconsidered using an estimated distribution of unsaturated zone depths in the Alton Pancras catchment. Conclusions suggest that, in modelling of long-term nutrient transport, a detailed source term is of much greater importance than a complex hydrogeological model. Implications of epistemic uncertainty, long-term prediction and management of diffuse agricultural pollution are discussed.