Uncertainties in major input variables in water and solute models were quantified and their effects on simulated functional aspects of soil behaviour were studied using basic soil properties from an important soil mapping unit in the Netherlands. The major sources of uncertainty were: (i) spatial variability of basic soil properties such as soil profile composition and soil texture combined with spatial Variability in water-table depths; and (ii) uncertainty associated with the use of pedotransfer functions to predict soil hydraulic functions. The two different sources of uncertainty were quantified and their respective contributions to the explanation of variability in modelling results were statistically evaluated by an analysis of variance. Additionally, regression analysis was used to evaluate the role of spatial variability in water-table depths only. It appeared that uncertainty of pedotransfer functions was an important factor when simulating the functional aspects of soil behaviour 'days with a good workability' and 'days with sufficient aeration'. The combination of variability of basic soil properties and of water-table depths dominated when simulating the functional aspects 'breakthrough characteristics of chloride and of the adsorbing chemicals cadmium and Isoproturon'. With the exception of the simulated functional aspect 'days until 10% chloride breakthrough', the uncertainty in the other simulated functional aspects was largely explained by spatial variability in the evolution of water-table depths. Sampling should therefore mainly aim at adequately describing the variability of basic soil properties including water-table depths if leaching of chemicals is studied, but should also aim at an adequate description of soil hydraulic functions using pedotransfer functions when functional aspects of soil behaviour of a physical nature are studied.