Eawag – Swiss Federal Institute of Aquatic Science and Technology, Überlandstr. 133, 8600 Dübendorf, Switzerland

now at: Department of Geography, University of Zürich, Winterthurerstr. 190, 8057 Zürich, Switzerland

Eawag – Swiss Federal Institute of Aquatic Science and Technology, Überlandstr. 133, 8600 Dübendorf, Switzerland

Eawag – Swiss Federal Institute of Aquatic Science and Technology, Überlandstr. 133, 8600 Dübendorf, Switzerland

Institute of Geophysics, ETH Zürich, Sonneggstr. 5, 8092 Zürich, Switzerland

Institute of Geophysics, University of Lausanne, Amphipole Unil Sorge, 1015 Lausanne, Switzerland

Institute for Environmental Engineering, ETH Zürich, Wolfgang-Pauli-Str. 15, 8093 Zürich, Switzerland

Institute for Environmental Engineering, ETH Zürich, Wolfgang-Pauli-Str. 15, 8093 Zürich, Switzerland

now at: AHEAD, IIE, EPFL-ENAC, Station 2, 1015 Lausanne, Switzerland

Center for Applied Geoscience, University of Tübingen, Sigwartstr. 10, 72076 Tübingen, Germany