Technical Abstract: The conversion of land for biofuel cultivation is expected to increase given concerns about the sustainability of current fossil-fuel supplies. Nonetheless, research into the environmental impacts of biofuel crops, primarily the hydrological impacts of their cultivation, is in its infancy. To investigate such issues, response of a 1649 square kilometers semiarid basin to the incremental substitution of the widely-discussed biofuel candidate switchgrass for native land uses was modelled using the Soil and Water Assessment Tool (SWAT). Median discharges decreased 5.6%-20.6% during the spring and 6.4-31.2% during the summer depending on the quantity of acreage converted. These were associated with the increased spring and summer evaporation of 4.3%-46% and 2.2%-24%, respectively, depending on the quantity of switchgrass biomass produced. The substitution of switchgrass also resulted in larger quantities of water stress days. The authors encourage the exploration of alternative biofuel crops in semiarid areas to mitigate such negative impacts.