Inverse Optimization, Calibration and Validation of Simulation Models at the Field Scale J. Šimůnek 1 & J. A. de Vos 2 1 U.S. Salinity Laboratory, USDA-ARS, 450 West Big Springs Road, Riverside, CA 92507, USA 2 DLO Research Institute for Agrobiology and Soil Fertility, P.O. Box 14, 6700 AA Wageningen, The Netherlands Abstract An overview is given of the issues of parameter estimation, model verification, and model validation as applied to field-scale subsurface flow and transport problems. We briefly review inverse optimization methods for estimating soil hydraulic parameters from a variety of field experiments, including tension disc infiltrometry, cone penetrometry, and gravity drainage experiments. An example is presented showing calibration of the numerical HYDRUS-2D model using data of a tile-drainage experiment. The hydraulic characteristics of the layered soil profile at the site were identified based on the joined use of laboratory data, field monitoring data, and the numerical model. A split sampling technique was used to test applicability of the numerical model for this study. Keywords: Simulation models, parameter estimation, calibration, tile-drained field Introduction Computer models based on numerical solutions of the flow and solute transport equations are increasingly being used for a wide range of applications in research and management. Application of the models is enhanced by the ever increasing power of personal computers, and the development of more accurate and numerically stable numerical techniques. Precision of the obtained predictions depends to a large extent upon the accuracy of the available model input parameters, and on a successful description of the actual physical system, including soil heterogeneity and the possible presence of nonequilibrium flow and transport conditions such as preferential flow. Parameters in the soil hydraulic functions characterizing the water retention and permeability properties are the most important input variables for models based on numerical solutions of the variably-saturated flow (Richards) equation. These hydraulic parameters directly influence the mobility of various chemical species through their effect on pore-water flow velocities and water contents. Transport and chemical parameters additionally affect the rate of spreading of chemicals and their distribution between mobile and immobile phases. Water flow and solute transport models are increasingly being applied to natural subsurface systems. While a major purpose of their application is to provide future predictions, they are more often used also to interpret the complex interactions found in laboratory and field data (Steefel & Van Cappellen, 1998). Models are also often used to study the sensitivity of natural systems to selected variables and processes, and to determine the uncertainties in predictions. The results of a sensitivity analysis can provide insight into the relative importance of individual model parameters, and thus help guide the allocation of resources for further laboratory and field investigations (Šimůnek & Valocchi, 2000). 431