The main objective is to assess preference heterogeneity related to the spatial distribution of water quality improvements throughout a river basin. In a choice experiment, the river basin's hydrogeographical units and the levels of water quality improvement are included as attributes in the experimental design. Changes in water quality throughout the river basin are visualized with maps and modeled simultaneously in relation to where respondents live, in a random utility model. Not accounting for spatial preference heterogeneity results, in this case study, in an underestimation of welfare when aggregating willingness-to-pay values from subbasins to the river basin as a whole.