Grasslands Versus Marginal Cropland Conversions for Bioenergy: Effects on the Hydrological Regime.

Switchgrass has attracted attention as a promising second generation biofuel feedstock. Both existing grasslands and marginal croplands have been suggested as targets for conversion to switchgrass, but their production potential and their hydrologic impact is not clear. The objectives of this study were to model switchgrass biomass production on existing grasslands (scenario-I) and marginal croplands (scenario-II) and to evaluate the effects on evapotranspiration (ET) and streamflow. Marginal croplands were defined as those croplands which have severe to very severe limitations for crop production based on the NRCS land capability classification system. The Soil and Water Assessment Tool (SWAT) was applied to the 1063 km² Skeleton Creek watershed in north-central Oklahoma, a watershed dominated by grasslands and winter wheat cropland. The average annual simulated switchgrass yield for both scenarios was 12 Mg ha^-1. Yield variability under scenario-I ranged from 6.1 to 15.3 Mg ha^-1, while under scenario-II the range was from 8.2 to 13.8 Mg ha^-1. Comparison of average annual ET and streamflow between the baseline simulation and scenario-I showed that scenario-I had 5.6% higher average annual ET and 27.7% lower streamflow. Compared to the baseline, scenario-II had only 0.5% higher ET and 3.2% lower streamflow, but some monthly impacts were larger. In this watershed, greater water yield reduction per ton of biomass production was predicted under scenario-I compared with scenario-II. Our results suggest that, from a hydrologic perspective, it may be preferable to convert marginal cropland to switchgrass production rather than converting existing grasslands. Regardless of the land conversion strategy, there would likely be tradeoffs between feedstock production, food production, and water resources.