Water Quality Responses to Biomass Production in the Tennessee River Basin.

This study aims to evaluate how growing cellulosic bioenergy feedstocks in the Tennessee River Basin (TRB) will influence water quantity and quality. We implemented the Soil and Water Assessment Tool (SWAT) to simulate water quantity and quality for TRB. We developed three innovations to overcome hurdles associated with limited data for model evaluation: (i) we implemented an auto-calibration approach to allow simultaneous calibration against multiple responses, including synthetic response variables; (ii) we identified empirical spatiotemporal datasets to use in our comparison; and (iii) we compared functional patterns in landuse-nutrient relationships between SWAT and empirical data. Comparing monthly SWAT-simulated runoff against USGS data produced satisfactory median Nash-Sutcliffe Efficiencies of 0.83 and 0.72 for calibration and validation periods, respectively. SWAT-simulated water quality responses (sediment, TP, TN, and inorganic N) reproduced the seasonal patterns found in LOADEST data. SWAT-simulated spatial TN yields were significantly correlated with empirical SPARROW estimates. The spatial correlation analyses indicated that SWAT-modeled sedimentation was controlled by topography; and NO₃ and soluble P were highly influenced by land management, particularly the proportion of agricultural lands in a subbasin. Using the calibrated model, we compared simulated water quality between three scenarios: (1) baseline, i.e., the calibrated model with the USDA 2009 Cropland Data Layer; (2) BC1, i.e., the base-case scenario following the USDA baseline projection and demands, extrapolated to 2040 with energy crop yield improved by 1%; and (3) HH3, i.e., high-yield scenario that uses assumptions derived from the high-yield workshops with crop yield improved by 3%. Median projected nutrient and sediment loadings showed decreases in TRB. Median sediment loadings decreased by 1.4%. Median nitrate and phosphorus loadings decreased by 50% and 9.5%, respectively. Preliminary results suggest that TRB holds promise for producing cellulosic feedstocks that enhance water quality.