Towards the response of water balance to sugarcane expansion in the Rio Grande Basin, Brazil

Abstract. This study explores the short-, medium- and long-term impacts of expansion of the sugarcane plantation on the water balance of the Rio Grande Basin, Brazil, as estimated by changes in evapotranspiration, soil moisture content and surface runoff calculated by a hydrological model. Twenty years of simulation are made using three different land use scenarios that include the basin area planted with sugarcane in 1993, 2000 and 2007 as estimated from satellite images. Complementary, it is used a scenario for sugarcane plantation defined by the Brazilian Institute for Agricultural Research (EMPRAPA) as all areas suitable for sugarcane cultivation within the Rio Grande Basin. In addition, parameters for sugarcane fields were specifically defined via calibration and validation of the hydrological model for all growth phases based on the annual cycle of sugarcane phenology in the Rio Grande Basin.

According to results from the land use classification of satellite images, the expansion of sugarcane fields mostly replaced pasture lands. Modelling results for short-, medium- and long-term clarify that impacts of this expansion depended not only on the amount of areas planted with sugarcane, but also the type of land use replaced, location of the expansion within the basin and regional soil properties. Largest impacts on the water balance are observed if areas located close to headwaters with low soil water capacity are planted with sugarcane. In case all areas suitable for sugarcane plantation, as defined by EMBRAPA will actually be planted, simulations showed that the annual accumulated values of evapotranspiration increase up to 180% while surface runoff is reduced to 20% of the values calculated using a land use scenario from 1993.