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The paper investigates the catchment area extension calculations as a function of the adopted spatial resolution and the effect of these spatial resolutions on the runoff calculations, for a number of catchments. Calculations were done with hypothetical and constant amounts of precipitation over space. Adopted grid size varied from 3000 to 2 m. Area estimate errors were computed in respect to 2 m grid resolution and the SCS method used for excess precipitation calculations. Results have shown that 75% of the catchments have errors of

Grids are a very suitable way to allow hydrologic modelling. The efficiency of a grid based model should be defined in respect to a spatial resolution, i.e. the pixel size that the model can handle with an adequate degree of precision. The size of a pixel defined as a computation unit can introduce significant artefacts on the results, due to surface error estimation plus intrinsic model error. Quantify these influences is the aim of the tests carried out over 100 catchments on the French southeast region. Each of these catchments was defined with one to thousands pixels and the errors computed. Effective rainfall was calculated by means of the SCS method. The intrinsic error is due to the CN aggregation procedure to make adequate the soil and land use source maps to the pixel size adopted for the computations. Results had shown, as expected, that for coarse resolutions the error due to surface estimation is the most important, meanwhile for fine resolutions only the aggregation error is present. The paper contribution stays on the intermediate scales where thresholds of error magnitude could be associated to spatial resolution.