The area with planted forests increased in the last decades and they have an important role in programs to sequester carbon. However, the effects of eucalyptus plantations on water resources in the Southern Grasslands biome are largely unknown, and we herein address water budget fluxes including green and blue flows. We evaluated green (canopy interception and evapotranspiration) and blue (discharge) water flows in three watersheds; two predominantly covered with Eucalyptus, either in the first years after planting or at the end of the rotation, and one with livestock-grazing grassland. We used field measurements of rainfall, streamflow and throughfall, and estimated canopy interception and evapotranspiration by water balance. Water flows in the monitored watersheds with eucalyptus plantations were influenced by forest development stage. Annual canopy interception and transpiration were always higher in the watersheds with eucalyptus than in the one with grassland, except for the transpiration in the first year after plantation in watershed with young eucalyptus. Increase in evapotranspiration (green water flow) and the consequent decrease in streamflow (blue water flow) should be considered in local water resources management. Studies on catchment hydrology and forest management for improved water use efficiency and streamflow regulation are required, particularly in understudied regions.

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