1. Institute for Computational and Applied Mathematics; Einsteinstr. 62 Münster D-48149 Germany
2. Institute for Applied Analysis and Numerical Simulation; Pfaffenwaldring 57 Stuttgart D-70569 Germany
3. Institute for Modeling Hydraulic and Environmental Systems; Pfaffenwaldring 61 Stuttgart D-70569 Germany
4. SINTEF ICT; PO Box 124. Blindern Oslo N-0314 Norway