Landscape, regional and global estimates of nitrogen flux from land to sea: Errors and uncertainties

Regional to global scale modelling of N fluxfrom land to ocean has progressed to datethrough the development of simple empiricalmodels representing bulk N flux rates fromlarge watersheds, regions, or continents on thebasis of a limited selection of modelparameters. Watershed scale N flux modellinghas developed a range of physically-basedapproaches ranging from models where N fluxrates are predicted through a physicalrepresentation of the processes involved,through to catchment scale models which providea simplified representation of true systemsbehaviour. Generally, these watershed scalemodels describe within their structure thedominant process controls on N flux at thecatchment or watershed scale, and take intoaccount variations in the extent to which theseprocesses control N flux rates as a function oflandscape sensitivity to N cycling and export. This paper addresses the nature of the errorsand uncertainties inherent in existing regionalto global scale models, and the nature of errorpropagation associated with upscaling fromsmall catchment to regional scale through asuite of spatial aggregation and conceptuallumping experiments conducted on a validatedwatershed scale model, the export coefficientmodel. Results from the analysis support thefindings of other researchers developingmacroscale models in allied research fields. Conclusions from the study confirm thatreliable and accurate regional scale N fluxmodelling needs to take account of theheterogeneity of landscapes and the impact thatthis has on N cycling processes withinhomogenous landscape units.

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