[en] The aim of the study was to develop a 3D-model that calculates the overall water balance components of Olkiluoto Island in the present-day condition utilizing the existing extensive data sets available. The model links the unsaturated and saturated soil water in the overburden and groundwater in bedrock to a continuous pressure system. The parameterization of land use and vegetation was done in such a way that the model can later on be used for description of the past evolution of the overburden hydrology at the site and overburden's hydrological evolution in the future. Measured groundwater level in overburden tubes, pressure heads in shallow bedrock holes, snow depth, soil temperature, frost depth and discharge measurements were used in assessing the performance of the models in the calibration period (01.05.2001- 31.12.2005). Computed groundwater level variation can be characterized by variables ΔH_MEAS and ΔH_COMP, which are the difference between maximum and minimum measured and computed groundwater level value during the calibration period. Average ΔH_MEAS for all tubes located in fine-textured till soil was 1.99 m and the corresponding computed value ΔH_COMP was 1.83 m. Average ΔH_MEAS for all tubes located in sandy till soil was 2.12 m and the corresponding computed value ΔH_COMP was 1.93 m. The computed results indicate that in future studies it is necessary to divide the two most important soil types into several subclasses. In the present study the uncertainty and sensitivity analysis was carried out through a parameter uncertainty framework known as GLUE. According to the uncertainty analysis the average yearly runoff was around 175 mm a^-1 and 50 % confidence limits were 155 and 195 mm a^-1. Measured average yearly runoff during the calibration period was 190 mm a^-1. Average yearly evapotranspiration estimate was 310 mm a^-1 and the 50 % confidence limits were 290 and 330 mm a^-1. Average value for recharge through the bedrock system was 1.7 % from the long term yearly precipitation and 50 % confidence interval was 1.5 - 1.9 %. These values imply that average recharge is around 10 mm a^-1. (orig.)