『Abstract
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.
keywords: Nitrate; Model; Catchment; Long-term; Land use; Groundwater』
1. Introduction
2. Study catchment
3. Methods
3.1. Catchment N-source estimation
3.1.1. Nitrogen loading model
3.1.2. Land-use categories
3.1.3. Inorganic feltilisers
3.1.4. Livestock
3.1.5. Ploughing of permanent pasture
3.1.6. Biological fixation
3.1.7. Denitrification
3.1.8. Crop uptake
3.1.9. Load estimates: uncertainty
3.1.10. Load estimates: sensitivity
3.2. Catchment-scale transport model
3.3. Transport parameter estimation
4. Results
4.1. Catchment N-load estimation
4.1.1. Inorganic fertiliser application
4.1.2. Animal excreta
4.1.3. Enhanced mineralisation from ploughing permanent grassland
4.1.4. Crop uptake
4.1.5. Total load estimation
4.1.6. Sensitivity of total and component load estimates
4.2. Transport model
4.2.1. Parameters
4.2.2. Catchment response to N-load inputs
5. Discussion
5.1. nitrogen loading estimates
5.2. Uncertainty in load estimates
5.2.1. Estimates of total N-load
5.2.2. Sensitivity of total load to component load uncertainty
5.3. The use of external data
5.4. Solute transport model
5.4.1. Epistemic uncertainty in long-term diffuse solute transport
6. Conclusions
Acknowledgements
References