UNCORRECTED PROOFS HYP5022 HYDROLOGICAL PROCESSES SCIENTIFIC BRIEFING Hydrol. Process. 16, 0 – 0 (2002) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.5022 The zone of vegetation influence on baseflow revealed by diel patterns of streamflow and vegetation water use in a headwater basin Barbara J. Bond, 1 * Julia A. Jones, 1 Georgianne Moore, 2 Nathan Phillips, 3 David Post 4 and Jeffrey J. McDonnell 5 1 Department of Forest Science Oregon State University 321 Richardson Hall Corvallis, OR 97331-5752 USA 2 Department of Geosciences, Oregon State University, Corvallis, OR 97331, USA 3 Boston University, Geography Department, Boston, MA 02215, USA 4 CSIRO Land and Water, PMB PO Aitkenvale, Queensland 4814, Australia 5 Department of Forest Engineering, Oregon State University, Corvallis, OR 97331, USAž TS1 *Correspondence to: Barbara J. Bond, Department of Forest Science, Oregon State University, 321 Richardson Hall, Corvallis, OR 97331-5752, USA. E-mail: barbara.bond@orst.edu Abstract Water use by vegetation can be closely linked to streamflow patterns on a variety of time scales. However, many of the details of these linkages are poorly understood. We compared diel (24 h) patterns of transpirational water use with streamflow patterns in a small headwater basin that displays a marked diel variation during summer months. The study site was in western Oregon. Our objectives were to: (1) determine the phase shift, i.e. the time lag between maximum transpiration and minimum streamflow, and the strength of the correlation at that time lag; (2) determine the amount of streamflow that is ‘missing’ during each diel cycle (i.e. the difference between base flow, defined by the daily maxima, and actual flow) and use it to estimate the zone, or area, of vegetation that influences daily streamflow patterns; (3) test and refine a conceptual model of how the coupling between vegetation water use and streamflow changes over the period of summer drought in this basin. We found that vegetation water use in the summer is coupled to streamflow over time scales of 4 to 8 h, and water-use-related fluctuations accounted for 1 to 6% of summer base flow. Direct evaporation from the channel was an order of magnitude less than the diel streamflow decrease. Transpiration within only 0·1 to 0·3% of the basin area accounted for the diel variation in streamflow. As the basin drained further through the summer, the coupling between vegetation and streamflow was diminished and occurred at longer time scales, and the zone of vegetation influence became smaller. This pattern is in accordance with our conceptual model, which attributes the summer decline in the strength of the vegetation–streamflow coupling to the increasing depth of plant-available water in the soil profile. Although this study is preliminary, we believe it is an important first step in describing better the coupling of vegetation water use to streamflow. Copyright  2002 John Wiley & Sons, Ltd. Introduction A central issue in hydrology today is to establish relationships between hydrological and biological processes in ecosystems (Rodriguez-Iturbe, 2000). To advance catchment-scale hydrological modelling, we need to couple key catchment areas to streamflow patterns and to identify and quantify components of the water balance (Beven and Freer, 2001; Seibert and McDonnell, 2002). One approach has involved defining hydrologically similar sub-units of basins (Becker et al., 2000) or ‘hydro- logic response units’ (Leavesley, 1983ž). Though these units are spatially Q1 explicit, they generally do not define how soil, sub-soil, and vegetation 1 2 Received 17 October 2001 Copyright  2002 John Wiley & Sons, Ltd. 1 Accepted 27 February 2002