UNCORRECTED PROOF 2 Towards an improved understanding of the 3 nitrate dynamics in lowland, permeable 4 river-systems: Applications of INCA-N 5 Andrew J. Wade * , D. Butterfield, P.G. Whitehead 6 Aquatic Environments Research Centre, School of Human and Environmental Sciences, The University of Reading, 7 Whiteknights, Reading RG6 6AB, UK Accepted 12 April 2006 Summary The Integrated Catchment Model of Nitrogen (INCA-N) was applied to the Lambourn and Pang river-systems to integrate current process-knowledge and available-data to test two hypotheses and thereby determine the key factors and processes controlling the movement of nitrate at the catchment-scale in lowland, permeable river-systems: (i) that the in-stream nitrate concentrations were controlled by two end-members only: groundwater and soil-water, and (ii) that the groundwater was the key store of nitrate in these river-systems. Neither hypothesis was proved true or false. Due to equifinality in the model structure and parameters at least two alternative models provided viable explanations for the observed in-stream nitrate concentrations. One model demonstrated that the seasonal-pattern in the stream-water nitrate concentrations was controlled mainly by the mixing of ground- and soil-water inputs. An alter- native model demonstrated that in-stream processes were important. It is hoped further mea- surements of nitrate concentrations made in the catchment soil- and ground-water and in-stream may constrain the model and help determine the correct structure, though other recent studies suggest that these data may serve only to highlight the heterogeneity of the sys- tem. Thus when making model-based assessments and forecasts it is recommend that all pos- sible models are used, and the range of forecasts compared. In this study both models suggest that cereal production contributed approximately 50% the simulated in-stream nitrate load in the two catchments, and the point-source contribution to the in-stream load was minimal.  c 2006 Elsevier B.V. All rights reserved. 11 KEYWORDS 12 Water quality; 13 Pollution; 14 Nitrogen; 15 Nitrate; 16 Model; 17 Uncertainty 18 Introduction 19 The Water Framework Directive (2000/60/EC) requires that 20 all European Union water-bodies, that are not heavily mod- 0022-1694/$ - see front matter  c 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2006.04.023 * Corresponding author. Fax: +44 118 975 5865. E-mail address: a.j.wade@reading.ac.uk (A.J. Wade). Journal of Hydrology (2006) xxx, xxx– xxx available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jhydrol HYDROL 15339 No. of Pages 19, Model 6+ 13 May 2006 Disk Used ARTICLE IN PRESS