Simulation of the Hydrologic Effects of Afforestation in the Tacuarembó River Basin, Uruguay

The Soil and Water Assessment Tool (SWAT) was used to simulate the hydrology of two small paired catchments in northern Uruguay. The control and treatment catchments (69 and 108 ha, respectively) were monitored for a three-year pretreatment period during which the land use was grassland with livestock grazing. Subsequently, the treatment catchment was planted (57% afforested) with loblolly pine (Pinus taeda). The objectives of the modeling study were to simulate the hydrologic response of the two catchments during the pretreatment period and predict the hydrologic effects of converting the native pasture to pine plantation. SWAT models of the two catchments were calibrated and validated using data measured during the pretreatment period. The model predicted outflows from the catchments reasonably well as compared to observed outflows during the years with above average rainfall (5% to -13% error). Model efficiency (E) for daily outflow volumes was greater than 0.71, indicating a good fit between simulated and observed results. A 33-year continuous simulation was performed on three land uses: grassland with livestock grazing, grassland without grazing, and pine treatment. The conversion of the catchments from the baseline pasture condition with grazing resulted in a predicted reduction in average annual water yield from the catchments of 15% for native grassland without grazing, and 23% for pine trees. A maximum predicted hydrologic effect was estimated by maximizing the model parameter that increases the ability of pine trees to withdraw water from the ground. For this condition, the model predicted a 30% reduction in mean annual water yield from the afforested catchment.