The aim of this study was to develop, adjust and test two different methods for the estimation of drainage area and potential wetness in ridged fields. The methods were applied to a gridded digital elevation model (DEM) of a potato field in central Sweden. Elevation data were registered by a global positioning system (GPS) receiver. A high-precision real time kinematic (RTK) system was used to record positions both horizontally and vertically. The study can be divided into four parts. First, we present the two different methods of estimating the drainage area and potential wetness. The potential wetness index used is ln(a/tan β), which is directly related to drainage area. Second, we constructed a high-accuracy DEM based on GPS-measured elevation points. We then modelled the GPS errors and the sampling errors in the DEM. Finally, we used a Monte Carlo simulation to test differences between the two methods of estimating the drainage area and potential wetness. The wetness index for the new method, which takes ridges into account, differs both statistically and visually from the first method, which does not incorporate ridges. The new method, incorporating ridges, is thus strongly recommended for hydrological modelling in ridged agricultural fields. © 2005 Elsevier B.V. All rights reserved.