1. Department of Physical Geography, Utrecht University, P.O. Box 80125, NL-3508 TC Utrecht, The Netherlands, Soil Physics, Ecohydrology and Groundwater Management Group, Wageningen University, P.O. Box 47, NL-6700 AA Wageningen, The Netherlands, Groundwater and Subsurface Department, Deltares, P.O. Box 85467, NL-3508 AL Utrecht, The Netherlands, Sorbisense A/S, P.O. Box 10, DK-8830 Tjele, Denmark, and TNO Geological Survey of The Netherlands, P.O. Box 80015, 3508 TA, Utrecht, The Netherlands