Advancing the SWAT model to simulate perennial bioenergy crops: A case study on switchgrass growth

Although the Soil and Water Assessment Tool (SWAT) model has been widely used to assess the environmental impacts of growing perennial grasses for bioenergy production, its utility is limited by not explicitly accounting for shoot and root biomass development. In this study, we integrated the DAYCENT model's grass growth algorithms into SWAT (SWAT-GRASSD) and further modified it by considering the impact of leaf area index (LAI) on potential biomass production (SWAT-GRASSM). Based on testing at eight sites in the US Midwest, we found that SWAT-GRASSM generally outperformed SWAT and SWAT-GRASSD in simulating switchgrass biomass yield and the seasonal development of LAI. Additionally, SWAT-GRASSM can more realistically represent root development, which is key for the allocation of accumulated biomass and nutrients between aboveground and belowground biomass pools. These improvements are critical for credible assessment of agronomic and environmental impacts of growing perennial grasses for biomass production.