The effect of different data qualities (different data aggregation levels) on the results of an integrated simulation model on soil water and nitrogen dynamics is investigated for an area of 3 communal districts which were part of a former large scale study on nitrogen losses. Disaggregated data from this regional scale study are compared to the real situation within the regarded area. Results of groundwater recharge and nitrogen leaching indicate that the aggregation of soil map units has a relative small effect compared to the averaging of weather data and to errors of land use. The use of average data derived from long term weather observations lead to a clear underestimation of groundwater recharge. Nitrogen uptake by plants in the regional study is often less than simulated with the real weather data and the observed rotation. Both result in higher nitrogen losses by leaching in the regional study although the water flux is less.