Flow paths and phosphorus transfer pathways in two agricultural streams with contrasting flow controls Per-Erik Mellander, 1 * Phil Jordan, 2 Mairead Shore, 1 Alice R. Melland 3 and Ger Shortle 1 1 Agricultural Catchments Programme, Teagasc, Johnstown Castle Environment Research Centre, Wexford, Co. Wexford, Ireland 2 School of Environmental Sciences, University of Ulster, Coleraine, UK 3 National Centre for Engineering in Agriculture, University of Southern Queensland, Toowoomba, Australia Abstract: In this paper, we analyse 4 years of data from simultaneous high-frequency monitoring of streamflow and phosphorus (P) concentration. This was carried out to investigate hydrological flow paths and P transfer pathways from diffuse sources in two intensively farmed river catchments (~10 km 2 ) with contrasting flow controls and dominating flow paths. Catchment scale P loss was viewed on an annual and event flow basis and related to hydrological flow paths. A grassland catchment with mostly poorly drained soils, and a higher Q 10 :Q 90 ratio (60 compared with 24), had three times higher annual P loss than an arable catchment with mostly well-drained soils (1.04 compared with 0.34 kg TP ha À1 ) despite the arable catchment having larger areas with high soil P status and more discharge. Neither of the catchments indicated supply limitations. The magnitude of the P losses from the two catchments was not defined by land use, source pressure or discharge volume but rather by more basic rainfall-to-runoff partitioning influences that determine proportions of quickflow and slowflow. There were larger differences between the years than between the catchments, and the P loss of the arable catchment appeared more sensitive to climate. The results confirmed the need to manage the quickflow components of runoff to moderate P transfers. Therefore, in order to further reduce diffuse pollution it may be necessary to account for the contrast in hydrological function before or in addition to any of the other factors known to influence P losses from catchments (such as soil P and land use). Schemes designed to attenuate diffuse P after mobilization from soil surfaces can then be targeted (and resourced) more effectively. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS diffuse pollution; agriculture; eutrophication; high-frequency water quality monitoring; nutrient management; climate Received 28 February 2014; Accepted 2 December 2014 INTRODUCTION Hydrological flow paths have been widely investigated at hillslope and catchment scales (e.g. Quinn et al., 1991; McGlynn et al., 2002; Soulsby et al., 2003). Progress has specifically been made in understanding water transit and storage times of catchments (Kirchner, 2009; McDonnell et al., 2010), and the connection of landscape to streams (McGuire and McDonnell, 2010) via aboveground quickflow pathways (e.g. Dunne et al., 1991) and belowground slowflow pathways (e.g. Freer et al., 2002). Such insights are valuable to applied studies, for example, when investigating nitrogen (N) and phosphorus (P) mobilization and transfers. These are key components of diffuse pollution from agriculture with consequential ecological impacts (Schindler et al., 2008; Conley et al., 2009; Paerl, 2009). Studies of diffuse nutrient transfer pathways at the hillslope scale have been important in providing concep- tualization of hydro-chemical processes and have also been influential in developing mitigation policies, which have been scaled up to large catchments (Dils and Heathwaite, 1999). Specific policies to moderate P transfers from land to water have used the understanding of P loss in fast surface and near-surface runoff pathways from soils of high soil P concentration. Similarly, fast surface runoff following fertilizer applications exacer- bates incidental nutrient losses (Preedy et al., 2001), and both of these observations have resulted in subsequent policy formulation to reduce these pressures, by either avoiding soil P build-up or reducing the probability of incidental losses through closed fertilizer application periods, respectively (Humphreys et al., 2008), and especially from hydrologically sensitive areas (e.g. Easton et al., 2008). Hydrological control on P loss from agricultural land during the closed slurry spreading winter period was best described by the stream discharge flashiness by Jordan et al. (2012), which is most likely *Correspondence to: Per-Erik Mellander, Agricultural Catchments Programme, Teagasc, Johnstown Castle Environment Research Centre, Wexford, Co. Wexford, Ireland. E-mail: Per-Erik.Mellander@teagasc.ie HYDROLOGICAL PROCESSES Hydrol. Process. (2015) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.10415 Copyright © 2014 John Wiley & Sons, Ltd.