Hydrologic Impacts of Converting Grassland to Managed Forestland in Uruguay

Over 500,000 hectares of grassland have been converted to managed forestland in Uruguay since} 990. This study was initiated to determine the hydrologic and water quality impacts of changing land use from grassland (pasture) to pine plantation in Uruguay. Two adjacent watersheds located on the El Cerro ranch in the Tacuarembo River basin were selected for a paired watershed study. Outflow rates and water table depths are continuously measured on each watershed Rairifall and meteorological conditions are also measured continuously on the site. During the initial pretreatment period (July OJ, 2000 through June 2003) both watersheds remained in pasture. One watershed (}07 ha) was planted with loblolly pine (pinus teada L.) in July 2003, while the other (69 ha) remained in pasture. Data collected during the past 48 month period (July OJ, 2003 through June 2007) represent the first four years of the treatment period. Significant changes in water yield were not observed during the first three years of the treatment period, but water yield reductions were observed during the fourth year. Most of the reductions were observed during a wet period that occurred after a prolonged dry period Reductions in water yield occurred during storm flow events. Changes were not observed in the base flow from the watersheds. Peak flow rates from the forested land were only 25% of those observed before planting and the times to peaks were increased by 26 minutes. Data collection will continue through the growth cycle of the trees.