Expansion of ethanol production has led to regional-scale cultivation of cellulosic biofuel crops, such as switchgrass (Panicum virgatum L.), on agriculturally marginal lands. A range of forest-based solutions are also being evaluated, especially in the southeastern U.S. However, there may be unanticipated environmental consequences, including changes in water export when managing forests to accommodate biofuel demands at a regional scale. We used the Soil Water Assessment Tool (SWAT) to simulate effects of regional-scale conversion of loblolly pine (Pinus taeda L.) plantations to switchgrass biofuel production on stream flow in the similar to 5 million ha Tombigbee River Watershed in the southeastern U.S. Greater than 50% of the Tombigbee Watershed is forested, with 20% of the watershed supporting primarily loblolly pine forests. We modified the SWAT model by adding five age classes of loblolly pine trees, to more accurately represent existing forested systems. We found that maximum conversion of loblolly pine to switchgrass, affecting 7% of the watershed, represented an extreme land-use change and resulted in a 4% increase in annual stream flow. The more operationally and economically feasible option of converting young (<= 4 yrs) and old (>= 16 yrs) loblolly pine stands to switchgrass on < 8% slope (2% of the watershed) resulted in a 2% increase in stream flow. Changes in annual stream flow were driven primarily by alterations in evapotranspiration (ET). Seasonal changes in stream flow were attributed to complex interactions among water-balance components of ET, surface flow, and groundwater flow. (C) 2015 Elsevier Ltd. All rights reserved.