How old is streamwater? Open questions in catchment transit time conceptualization, modelling and analysis

Institute for Water and Watersheds and Department of Forest Engineering, Resources and Management Oregon State University, Corvallis, OR 97331-5706, USA; School of Geosciences, University of Aberdeen, Aberdeen, Scotland, AB24 3UF UK Virginia Water Resources Research Center & Department of Forestry Virginia Tech, Blacksburg, VA 24061, USA Isotope Hydrology Section, International Atomic Energy Agency, Wagramer Strasse 5, A-1400 Vienna, Austria Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ UK Dipartimento di Difesa del Suolo, Università della Calabria, Arcavacata di Rende (CS), Italy Department of Physical Geography & Quaternary Geology, Stockholm University, SE 106 91 Stockholm, Sweden Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, UK Department of Forestry and Environmental Resources, NC State University, Campus Box 8008, Raleigh, NC 27695-8080, USA Swiss Federal Institute of Technology (ETH), Raemistrasse 101, CH-8092, Switzerland Institute of Landscape Ecology and Resources Management Heinrich-Buff-Ring 26, 35392 Giessen, Germany Physical Geography and Quaternary Geology, Stockholm University, Villavägen 16, SE-752 36 Uppsala, Sweden HMGU Munich—Institute of Groundwater Ecology, 85764 Neuherberg, Germany Department of Environment and Agro-Biotechnologies, Centre de Recherche Public-Gabriel Lippmann, 41 rue du Brill, L-4422 Belvaux, Luxembourg Dept of Civil & Environmental Engineering, Universita’ di Padova, Padova I-35131, Italy Division of Hydrology, Uppsala University, Box 256, SE-751 05 Uppsala, Sweden DPRI, Kyoto University, Gokasho Uji, Kyoto, 611-0011, Japan Hydrology and Climate, Physical Geography Division, Department of Geography University of Zurich—Irchel, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland Department of Geology & Geophysics, University of Utah, Salt Lake City, UT 84112, USA University of Edinburgh, Old College South Bridge, Edinburgh EH8 9YL, UK Aquifer Dynamics & GNS Science, PO Box 30368, Lower Hutt, New Zealand School of Geosciences, Department of Geography and Environment, University of Aberdeen, Elphinstone Road Aberdeen, AB24 3UF Scotland, UK Ecole Polytechnique Fédéral de Lausanne, EPFL ENAC ISTE EFLUM GR A0 402, Station 2, Lausanne CH-1015, Switzerland Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721 USA Institute of Hydrology, University of Freiburg, Fahnenbergplatz, 79098 Freiburg, Germany Department of Civil and Environmental Engineering, The University of Melbourne, Victoria 3010, Australia Division of Water Resources Engineering, Royal Institute of Technology, Teknikringen 76, SE-100 44 Stockholm, Sweden The Delft University of Technology, Faculty of Civil Engineering and Geosciences, Water Resources Section, P.O. Box 5048, NL-2600 GA Delft, The Netherlands *Correspondence to: J. J. McDonnell, Institute for Water and Watersheds and Department of Forest Engineering, Resources and Management Oregon State University, Corvallis, OR 97331-5706, USA; School of Geosciences, University of Aberdeen, Aberdeen, Scotland, AB24 3UF UK. E-mail: [email protected]