ModelCARE’2002, 4th International Conference on Calibration and Reliability in Modelling: A Few Steps Closer To Reality Prague, Czech Republic, 17–20 June 2002 ModelCARE’2002 was jointly convened by the Institute of , Engineering Geology and Applied Geophysics, Faculty of Science, of Charles University (Universita Karlova) in Prague, and the International Commission on Groundwater (ICGW), of IAHS. It was supported by IAHR, NGWA, IGWMC and UNESCO. ModelCARE’2002 was also a contribution to UNESCO’s International Hydrological Programme (IHP-VI, 2002–2007), “Water Interactions: Systems at Risk and Social Challenges”. The organizers were Zbynek Hrkal of Charles University (Prague) and Karel Kovar of RIVM (The ). About 215 scientists and engineers met in Prague to demonstrate new ideas and solutions and face the continuing difficulties in developing models that represent groundwater systems, arguably the most difficult of the earth system models because the systems are inaccessible and, being a source of drinking water, very detailed knowledge is demanded.

Model calibration and reliability conference demonstrates progress and needs (a participant’s impression)

A major feature of the conference was the consideration of both deterministic and stochastic methods of investigation. For any given groundwater system, some things about the system are known relatively well and it is most advantageous to represent them in a model definitively, or deterministically. Other things are less clearly known or known very little, and it is important to represent them using stochastic methods to obtain an honest evaluation of what can be said about the system. Thus, both deterministic and stochastic methods are important. Yet progress in the two areas most often proceeds independently, and sometimes antagonistically. An exciting aspect of ModelCARE’2002 conference was the presence of many of the very best modellers from all degrees of the deterministic– stochastic spectrum. The conference started with 1¼ days of talks about stochastic methods presented by the leaders of the field, including Gedeon Dagan of , , Yoram Rubin of the University of California at Berkeley, USA, Peter Indelman of Technion, Israel, David Tartakovsky of Los Alomos National Laboratory, USA, and Aldo Fiori of the University of Rome, Italy. The single plenary session meant that these talks received substantial attention, and the presentations and questions revealed both the mathematical difficulty of stochastic , and the difficulty of conveying its results, which are so crucial to coping with the uncertainty inherent in groundwater modelling. Two clear conclusions presented were: − the importance of dimensionality and accounting for the inherent three-dimensional nature of natural systems, and − the importance of the contrasts in material properties observed in the subsurface. The remainder of the conference was dominated by deterministic methods with some use of regression and sensitivity approaches to address model development, calibration, and uncertainty issues. New challenges in deterministic modelling include models that integrate groundwater and surface processes (Ed Sudicky, University of Waterloo, Canada) and address scale issues through local grid refinement (Steffen Mehl, US Geological Survey, USA). The dynamics and errors of systems with complex processes were investigated by Eileen Poeter (multiphase flow), Christine Shoemaker (anaerobic bioremediation of hydrocarbons) and Linda Abriola (DNAPLs), of respectively the Colorado School of Mines, USA, Cornell University, USA and the University of Michigan, USA. Model calibration by trial and error alone was evaluated using sensitivity analysis and regression by Theo Olsthoorn (Amsterdam Water Supply, The Netherlands), and the estimation of distribution as well as values of recharge was considered by Yung-Fu Lin (University of Wisconsin, USA). Confusion about weighting observations in sensitivity analysis and regression was addressed by Mary Hill (US Geological Survey, USA). An exciting new method for calculating confidence intervals for our pervasively nonlinear models (nonlinear with respect to the parameters) was discussed by Steen Christensen (Aarhus University, Denmark) and Elizabeth Keating (Los Alamos National Laboratory, USA). These are just a few of the many interesting talks, posters, and people that made for a fun and very interesting four days in Prague. Selected papers from oral and poster presentations at ModelCARE’2002 will be published in an IAHS Red Book (Publ. no. 277) in spring 2003. Next meeting The next ModelCARE conference will be in Utrecht, The Netherlands, in summer 2005. Please mark your calendars!

Mary C. Hill US Geological Survey, 3215 Marine St., Boulder, Colorado 80303, USA Karel Kovar National Institute of Public Health and the Environment (RIVM), Bilthoven, The Netherlands