Over the past decade, industrial-scale soy agriculture has become a dominant land use in many parts of the Brazilian Amazon.  In the state of Mato Grosso, conversion of forest or former pasture to soy now rivals deforestation for pasture as the region’s predominant type of land use change.  Such large-scale changes in vegetative cover have been linked to shifts in water balance and runoff patterns. Changes like these to the hydrological functioning of the region could have important consequences for freshwater ecosystem functioning as well as for rainfall generation. We monitored water discharge for one year in nine catchments, each dominated by a single land cover (3 forest, 5 soy, 1 pasture), in the Upper Xingu watershed of Mato Grosso.  We compared total water yields, hydrograph patterns, and the contribution of stormflow to streamflow under soybean agriculture, pasture, and forest.

Results/Conclusions
Water yields were higher in the soy and pasture than in the forested catchments.  Both total and daily water yields in the soy watersheds were more than double those in the forested catchments, with the greatest difference observed during the dry season, when differences in evapotranspiration should be most pronounced.  However, contrary to expectations for higher rates of overland flow generation in soy watersheds due to soil compaction, there was no difference in the contribution of stormflow (< 15% of streamflow) to streams between the three land cover types.  Additionally, average daily water yields remained constant throughout the hydrologic year for all land uses, despite dramatic dry and wet seasons.  High soil permeability in the Oxisols on the plateaus that dominate these watersheds appears to buffer watersheds against increasing rates of stormflows and increased erosion.  However, it may be that consequences of soil compaction from heavy machinery and the legacy of use as pasture will increase with time, particularly under systems where tilling is restricted to the first years following conversion.  Nevertheless, the increase in water yield has the potential to have broad regional effects.  Increasing water yields by more than double will affect the morphology and habitat in stream channels as well as increasing riparian forest saturation.  And, as agricultural conversion continues, the cumulative effect of increased water yields will also alter water balance and water yield downstream for larger rivers and regions.