1. Federal Institute of Hydrology (BFG), Am Mainzer Tor 1 D-56068 Koblenz, Germany, Swiss Federal Institute for Aquatic Science and Technology (EAWAG), Überlandstrasse 133, CH-8600 Duebendorf, Switzerland, and Max Planck Institute for Polymer Science, Ackermannweg 10, D-55128 Mainz, Germany