 The Guandu River hydrographic region provides water to the Rio de Janeiro metropolitan area. This region has been affected by water transfers and low levels of effluent treatment, which have caused changes in the dynamics of its water purification ecosystem service. To analyze these changes, a methodology was developed consisting of reviewing studies of ecosystem flows, building a database, classifying impacts on water purification in hydrological planning units from a similarity dendrogram, analyzing indicators related to water demands and availability of these units, and developing an ecosystem flow diagram of water purification. The analysis revealed that the worst situation of maintenance of water purification is in the western part of the region, and the demands for it require more than double the volume of withdrawn water to supply the local population and industry. This article was authored by Fabio Miranda Gomez Xavier, Rachel Barty Prado, and Elaine Cristina Cardoso Fidalgo. We are article.tv. Links in the description below.