9-01: Cellulosic feedstock supply risks due to drought

The current practice of modeling cellulosic biomass yields based on point values that have been aggregated over space and over time conceal important energy supply risks related to depending on biomass for transportation energy, particularly those related to local drought conditions.  Using switchgrass as a case study, our work quantifies the variability in expected yields over time and space with a switchgrass growth model and historical weather data.  Even with stable, productive states, yields vary from 5 to 20 Mg/ha.  Yields are likely to be reduced with increased temperatures and weather variability induced by climate change.  Thus, variability needs to be a central part of biomass systems modeling so that risks to energy supplies are acknowledged and risk mitigation strategies or contingency plans are considered.  We find that irrigation, a potential risk mitigation strategy, can very often negate the impacts of drought, although system-wide irrigation is an expensive method to stabilize crops (costing $0.10 to $1.90/gallon).  Unless many surplus acres of cellulosic crops are planted, there will be insufficient ethanol to meet the EISA RFS targets 10 to 25% of the time under rain-fed conditions.  We conclude that thinking in terms of yield ranges, not point estimates, is essential in planning a long-term energy system dependent on biomass.