Murugesu Sivapalan,* Hubert H. G. Savenije and Gunter Bloschl 1 Department of Civil and Environmental Engineering, Department of Geography, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA 2 Department of Civil and Environmental Engineering, University of Technology Sydney, Broadway, NSW 2007, Australia 3 Department of Water Management, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands 4 Institute for Hydraulic and Water Resources Engineering, Vienna University of Technology, Vienna, Austria