Effects of Eucalyptus plantations on streamflow in Brazil: Moving beyond the water use debate

Eucalyptus plantations play an important role in the Brazilian economy, and they are commonly associated with their high water use, which may have negative impacts on water availability for other users. As short rotation Eucalyptus plantations expand in South America, there is an urgent need to understand their effects on streamflow. This study analyzes water use by Eucalyptus plantations in Brazil based on streamflow and precipitation data from 19 instrumented catchments in which Eucalyptus plantations occupy > 50% of land area and where records extend from 2 to 15 years (120 years of data). The annual water yield (Q) was compared proportionally to the water availability (P) at each catchment location. Additionally, water use was analyzed in relation to plantation age classes. Finally, a relationship between the observed annual relative water yield (Q:P) and the aridity index (AI) was established by an exponential model, which was used to understand the possible effects of forest plantation areas at the municipality level in Brazil. The main findings of the paper are as follows: (1) Evapotranspiration (ET) exceeded 80% of precipitation for most observations (71%) and exceeded 90% of precipitation for 50% of the annual observations (60 years of data); (2) streamflow represented 5-11% of precipitation for plantations aged 1-7 years, with a slight but not significant decrease for older (6- and 7-yr-old) plantations; (3) for the 19 sites, the annual average runoff ratio declined exponentially with increasing values of the aridity index (PET/P), and the 4 sites with aridity indices > 0.76 had a water yield ratio < 0.1; (4) 40% of Eucalyptus sites in Brazil had aridity indices > 0.76, implying that these areas have runoff ratios lower than 10% of precipitation. Considering these results, a threshold of aridity index = 0.76 is proposed to identify regions where forest management should be more focused to consider water availability for other users. A diagram showing different conditions of water availability and planted area of Eucalyptus in Brazilian municipalities is presented to identify regions requiring forest management changes to balance wood production and water ecosystem services. Considering more critical regions, we propose and discuss a framework for understanding the cumulative effects of natural conditions and management in an attempt to further the debate on water use by Eucalyptus plantations by discussing thresholds of water availability and alternatives to avoid effects and conflicts.