Springs drive downstream nitrate export from artificially-drained agricultural headwater catchments Brandon C. Goeller ⁎ ,1 , Catherine M. Febria 2 , Helen J. Warburton, Kristy L. Hogsden 3 , Kathryn E. Collins 4 , Hayley S. Devlin, Jon S. Harding, Angus R. McIntosh School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand HIGHLIGHTS • Regional groundwater drove seasonal N-loads in nine catchments over four years. • Spring and artificial drainage inputs ex- plained 60% of downstream N-load var- iability. • Catchment nutrient management should address groundwater and farm- scale N inputs. GRAPHICAL ABSTRACT abstract article info Article history: Received 27 November 2018 Received in revised form 27 February 2019 Accepted 20 March 2019 Available online 21 March 2019 Editor: José Virgílio Cruz Excessive nutrient loading from small agricultural headwaters can substantially degrade downstream water quality and ecological conditions. But, our understanding of the scales and locations to implement nutrient atten- uation tools within these catchments is poor. To help inform farm- and catchment-scale management, we quan- tified nitrate export in nine one-kilometre-long lowland agricultural headwaters fed by tile and open tributary drains in a region with high groundwater nitrate (b1 to N15 mg L -1 NO 3 -N) over four years. Across-catchment differences in upstream spring water nitrate concentrations predicted differences in annual nitrate loads at catch- ment outlets (range b1–72 megagrams NO 3 -N 365 d -1 ), and nitrate loads were higher in wet seasons and wet years, reflecting strong groundwater influences. Partitioning the sources of variability in catchment nitrate fluxes revealed that ~60% of variation was accounted for by a combination of fluxes from up-stream springs and contri- butions from tile and open tributary drains (46% and 15%, respectively), with ~40% of unexplained residual var- iation likely due to groundwater upwellings. Although tile and open tributary drains contributed comparatively less to catchment loads (tile drains: b0.01 and up to 50 kg NO 3 -N d -1 ; open drains: b5 kg and up to 100 kg NO 3 -N d -1 ), mitigation targeted at these localised, farm-scale sources will contribute to decreasing downstream nitrate Keywords: Agricultural land-use Groundwater Headwater catchments Nitrate‑nitrogen flux Subsurface drainage Science of the Total Environment 671 (2019) 119–128 Abbreviations: BMP, best management practice; CSA, critical source area; RTP, rapid transfer pathway; VCA, variance components analysis. ⁎ Corresponding author. E-mail address: Brandon.Goeller@niwa.co.nz (B.C. Goeller). 1 Current address: National Institute of Water and Atmospheric Research (NIWA), PO Box 11115, Hamilton 3216, New Zealand. 2 Current address: University of Windsor, Great Lakes Institute for Environmental Research, 2990 Riverside Drive West, Windsor, Ontario, Canada. 3 Current address: National Institute of Water and Atmospheric Research (NIWA), 10 Kyle Street, Christchurch 8011, New Zealand. 4 Current address: Department of Conservation, 73 Rostrevor St, Hamilton 3240, New Zealand. https://doi.org/10.1016/j.scitotenv.2019.03.308 0048-9697/© 2019 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv