Water Balance in the Guarani Aquifer Outcrop Zone, Brazil

The Guarani Aquifer System (GAS) is the largest transboundary aquifer in South America with an area of 1.2 million km² spread over four countries (Argentina, Brazil, Paraguay, and Uruguay). Despite the importance of the GAS as a major source of fresh water in the region, little is known about its recharge processes. The main recharge is supposed to occur in the outcrop zones of the Botucatu and Pirambóia Formations, representing approximately 10% of the total aquifer area. Aiming to estimate the groundwater recharge, a monitoring network has been deployed in the Ribeirão da Onça watershed, which appears to be representative for the GAS outcrop zones. Water table observation wells have been installed under different land uses (grassland, eucalyptus, sugar cane, and citrus cultures) and monitored since 2004. Direct recharge was estimated using a point-scale method (Water-Table Fluctuation) for each land use, after a careful determination of the specific yield. The average results indicate that the direct recharge strongly depends on the land use: eucalyptus forest (135 mm.y^-1), sugar cane (248 mm.y^-1), citrus (296 mm.y^-1), and grassland (401 mm.y^-1). The main climatological variables (precipitation, temperature, solar radiation, wind velocity) as well as the discharge at the draining creek have also been monitored, allowing the evaluation of the water balance in this watershed. The recharge contribution to the regional groundwater system was estimated to be approximately 3% of the annual average precipitation (1400 mm). The estimated recharge is important information for water management purposes since it can be used as an input parameter for regional groundwater models to predict water table levels and to assess potential future climate changes impacts in the GAS.