Scale effects on headwater catchment runoff timing, flow sources, and groundwater-streamflow relations Brian L. McGlynn, 1 Jeffrey J. McDonnell, 2 Jan Seibert, 3 and Carol Kendall 4 Received 16 July 2003; revised 2 March 2004; accepted 22 April 2004; published 28 July 2004. [1] The effects of catchment size and landscape organization on runoff generation are poorly understood. Little research has integrated hillslope and riparian runoff investigation across catchments of different sizes to decipher first-order controls on runoff generation. We investigated the role of catchment sizes on riparian and hillslope dynamics based on hydrometric and tracer data observed at five scales ranging from trenched hillslope sections (55–285 m 2 ) to a 280-ha catchment at Maimai on the west coast of the South Island, New Zealand. The highly organized landscape is comprised of similar headwater catchments, regular geology, steep highly dissected topography, relatively consistent soil depths, and topographically controlled shallow through flow. We found a strong correlation between riparian zone groundwater levels and runoff for the headwaters, whereas the water tables in the valley bottom of the larger catchments were uncorrelated to runoff for 14 months of record. While there was no clear relationship between catchment size and new water contribution to runoff in the two storms analyzed in detail, lag times of tracer responses increased systematically with catchment size. The combination of hydrometric and tracer data allowed assessment of the runoff contributions from different parts of the landscape. Runoff was generated consistently in headwater riparian zones. This agreed also with the observed variations of tracer ( 18 O and silica) responses for the different catchments. During wetter antecedent conditions or during larger events (>30 mm under dry antecedent conditions) hillslope and valley bottom floodplains did contribute to event runoff directly. We propose that analysis of landscape-scale organization and the distribution of dominant landscape features provide a structure for investigation of runoff production and solute transport, especially as catchment-scale increases from headwaters to the mesoscale. INDEX TERMS: 1719 History of Geophysics: Hydrology; 1860 Hydrology: Runoff and streamflow; 1871 Hydrology: Surface water quality; 1866 Hydrology: Soil moisture; KEYWORDS: scale, water age, runoff generation, landscape organization Citation: McGlynn, B. L., J. J. McDonnell, J. Seibert, and C. Kendall (2004), Scale effects on headwater catchment runoff timing, flow sources, and groundwater-streamflow relations, Water Resour. Res., 40, W07504, doi:10.1029/2003WR002494. 1. Introduction [2] The effect of different scales on hydrologic variables is one of the major unresolved issues in the hydrological sciences. Sivapalan and Kalma [1995] called for continued and sustained research on scale problems in hydrology. Much effort has been directed to the more theoretical aspects of scaling [Blo ¨schl, 2001], but less progress has been made in experimental studies of watershed hydrology [Uhlenbrook et al., 2002]. While scaling of runoff genera- tion processes has been cited as a major need for model formulation at the mesoscale [Uhlenbrook and Leibundgut, 2002], process hydrological research has been examined almost exclusively at the headwater catchment scale [McDonnell and Tanaka, 2001]. More problematic perhaps is the fact that intercomparisons between different ‘‘same scale’’ catchments have been few and far between [Jones and Swanson, 2001], resulting in rather poor progress in determining commonalities across different catchments and catchment positions about how water and solutes are delivered to streams. Thus most current field-based research focuses on the idiosyncrasies of individual hillslopes [e.g., Freer et al., 2002], riparian zones [Seibert et al., 2003], or small catchments [Williams et al., 2002]. As a result, defining the dominant controls on runoff generation and transferring knowledge from one place to another has been difficult [Bonell, 1998]. [3] Hydrologists continue to grapple with the key question posed by Sivapalan and Kalma [1995]: ‘‘Are there certain preferred time and spatial scales at which conceptualizations of hydrological response may be feasible?’’ Recently, Brown et al. [1999] and Shanley et al. [2002] examined how runoff composition varies across scale. These results have so far been highly equivocal. In fact, they focus exclusively on how ‘‘tracers’’ behave across scale. Without detailed hydrometric information, they may only provide part of the explanation on the first-order controls on the age, origin, and flow paths of 1 Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana, USA. 2 Department of Forest Engineering, Oregon State University, Corvallis, Oregon, USA. 3 Department of Environmental Assessment, Swedish University of Agricultural Sciences (SLU), Uppsala, Sweden. 4 U.S. Geological Survey, Menlo Park, California, USA. Copyright 2004 by the American Geophysical Union. 0043-1397/04/2003WR002494$09.00 W07504 WATER RESOURCES RESEARCH, VOL. 40, W07504, doi:10.1029/2003WR002494, 2004 1 of 14