The southern United States is a potential leader in producing biofuels from intensively managed, short rotation (8–12 years) woody crops such as southern pines, and native and non-native hardwoods. However, their accelerated development under intensive management has raised concerns that fast-growing bioenergy crops could reduce streamflow and recharge to groundwater. In this study, we characterize and compare the partitioning of precipitation into interception, transpiration, throughfall, infiltration, and soil evaporation for 12 and 13-year-old, intensively managed loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) stands at the Department of Energy Savannah River Site in New Ellenton, South Carolina. Three replicate plots of each species were instrumented with sap flow probes, box lysimeters, integrated temperature and soil moisture probes, precipitation gauges, and throughfall gauges to estimate components of the total water balance. Three replicate plots of non-vegetated bare soil were paired with forest plots. Hydrologic budgets were monitored for two years.
Throughfall was similar between pine and sweetgum during the growing season; lower in pine during the winter due to its evergreen canopy; and similar at the annual scale. Bare soil evaporation was an order of magnitude higher compared to that of the forest floor. Soil evaporation was twice as high below the deciduous sweetgum canopy compared to below the evergreen pine canopy. Soil moisture dynamics followed precipitation patterns, but were also related to tree water use. For example, soil moisture was much lower in sweetgum than pine during the growing season, but remained similar to pine during the dormant season. Sweetgum transpired more than pine during late spring and early summer, while pines transpired more in the winter than sweetgum. Annually however, transpiration was similar in sweetgum and pine. This work suggests that while annual water budgets are similar between these species, sweetgum uses more water than pine during the summer, a time when regional water deficits typically begin to develop. This work provides insight regarding the hydrologic implications of bioenergy crop expansion in the southeastern US for loblolly and sweetgum.