For the Atlas, a spatial system of catchments comprising three levels was developed thus allowing a direct comparison of catchments within one level, and a linking one below the other of the catchments in all three levels by means of aggregation or disaggregation.
Large catchments (river basins) usually covering an area of multiple 1000 km² are the top level. The water balance basins including catchments of 100 to 150 km², are a result of subdividing the river basins. The small catchments (30 to 50 km²) make up the lowest level of the spatial system.
