U.S. Department of Energy (Award# DE-EE0004396) Motivation The U.S. - PowerPoint PPT Presentation

Funding Acknowledgement: Manuscript Accepted The David and Lucile Packard Foundation and the January 22, 2013 U.S. Department of Energy (Award# DE-EE0004396)

Motivation • The U.S. has chosen to pursue a renewable fuel standard mandating 36 billion gallons of ethanol equivalent biofuels (EISA, 2007) – 16 billion gallons from cellulosic feedstocks • Avoid food-vs-fuel concerns associated with grain ethanol • Potential environmental benefits • Candidate feedstocks: Corn stover, perennial grasses, fast-growing tree species – Stover does not require additional land or new crops • 60% greenhouse gas reduction requirement, but no other environmental performance requirements

Previous Work (1 st and 2 nd Gen. Biofuels) • Life cycle assessment and greenhouse gases: Kim and Dale (2008), Kim et al. (2009), Fargione et al. (2008) • Water resource impacts: Tilman et al. (2009), Secchi et al. (2010), Cibin et al. (2011) • Limited amount of work considers economic and environmental dimensions together – Hill et al. (2009) and Searchinger et al. (2008) – ILUC work using GTAP, MIT models • Gap: Cross-media (water and air) environmental impacts and economic cost

Research Questions • Do GHG benefits and biofuels come at the expense of water quality? • How are water quality and GHG emissions from soils related to stover removal? • How are costs of stover collection and pollution related? • Does purely economic optimization to supply biomass result in significantly different environmental outcomes than joint minimization of cost and pollution?

Study location: Wildcat Creek Watershed Cibin, et al. in Hydrological Processes Volume 26, Issue 11, pages 1629-1641, 28 SEP 2011 DOI: 10.1002/hyp.8280 http://onlinelibrary.wiley.com/doi/10.1002/hyp.8280/full#hyp8280-fig-0001

Integrated modeling and optimization framework (Reeling and Gramig, 2012) INPUTS MODELS OUTPUTS Soils, Yield, nitrate, total P, sediment SWAT weather [biomass or pollutant/ha] Baseline GHG flux, CO 2 and N 2 0 DAYCENT [Mg CO 2 -e/ha] + 8 stover removal [$/ha] Practice cost budgets practices Multi-objective Optimization Trade-off frontiers: • Cost-yield • Cost-pollutant loading

Water quality modeling Soil and Water Assessment Tool (SWAT) • Watershed divided into sub-basins • Sub-basins divided into Hydrological Response Units which are ≅ farms • Outputs: – Biomass yields – Nitrate – Total P – Sediment

Greenhouse gas modeling DayCent biogeochemical model • Daily time-step version of the CENTURY model • Soil carbon gain/loss and N 2 0 flux • Field-scale simulations based on soils, weather, crops, mgt (same as SWAT) – Simulations run for 41 soil types representing majority watershed – Area-weighted (by soil type) emissions of each practice on each HRU calculated from results

Economic modeling of practice costs • The cost of each practice is calculated as the cost difference relative to the baseline – Baseline = Conventional tillage corn followed by no-till soybeans • Costs include: tillage, herbicide, nutrient (N) replacement, stover harvest+storage, average transportation, and cost of any yield change • Cost of each practice considered is based on the area of the HRU  Practice costs = ($/ha * total hectares)  Cost of yield change = (Corn price)*( Δ yield/ha)*(total ha)

Stover removal modeling scenarios and costs per hectare Increased annual cost of corn Scenario Crop Rotation What is harvested in each production over baseline c Tillage b Abbreviation a (year1-year2) scenario $/Mg Stover d $/ha CB (Baseline) Corn-Soybean Yes Grain only (no stover removal) $0 No stover removal CB38 Corn-Soybean Yes Grain + 38% stover removal $61.55 $13.71 CB52 Corn-Soybean Yes Grain + 52% stover removal $92.76 $15.06 CC38 Continuous corn Yes Grain + 38% stover removal $194.70 $44.63 CC52 Continuous corn Yes Grain + 52% stover removal $259.47 $43.24 CBNT38 Corn-Soybean None Grain + 38% stover removal $64.79 $14.51 CBNT52 Corn-Soybean None Grain + 52% stover removal $95.42 $15.55 CCNT38 Continuous corn None Grain + 38% stover removal $163.35 $37.24 CCNT52 Continuous corn None Grain + 52% stover removal $228.13 $37.78 Stover removal rates, nutrient replacement rates, and cost assumptions adapted from Brechbill, Tyner and Ileleji (2011)

Simulation results (1995-2009) Average annual pollutant contribution d Average stover Total harvested yield in years Greenhouse gas Scenario biomass with N Total when collected Abbreviation a Nitrate, Sediment, emissions from replacement, Phosphorus, (10yr avg for CB), CO 2 -e e Mg/ha c kg/ha Mg/ha cropland, Mg/ha/yr b kg/ha Mg/ha CB (Baseline) No stover removal 10.07 8.49 1.83 2.23 5.98 CB38 4.490 (2.25) 14.56 8.38 1.74 2.29 6.25 CB52 6.16 (3.08) 16.24 8.24 1.73 2.35 6.40 CC f No stover removal 10.15 10.41 1.71 2.27 6.53 CC38 4.363 14.52 9.47 1.65 2.41 7.49 CC52 6.000 16.15 9.16 1.65 2.51 7.90 CBNT f No stover removal 10.04 8.77 2.39 2.13 5.87 CBNT38 4.465 (2.23) 14.50 8.57 2.23 2.22 6.06 CBNT52 6.136 (3.07) 16.17 8.45 2.19 2.28 6.23 CCNT f No stover removal 10.12 11.32 2.41 2.02 6.22 CCNT38 4.387 14.51 10.16 2.02 2.08 6.94 CCNT52 6.038 16.16 9.76 1.90 2.14 7.28

Optimization: A Heuristic Approach • Normative analysis to inform what is possible given cost and pollution (or stover) criteria • A single solution is an allocation of 1 stover removal practice to each of 922 ‘fields’ in the watershed • Genetic algorithm used to search for solutions – Discrete practices aggregated over watershed – Multiple maxima/minima, so gradient-based approach will not work

Pollution loading (or stover yield) = watershed-average per hectare pollutant load or feedstock yield for each of the four pollutants and the feedstock each stover collection cropping practice (8 scenarios) a vector containing the size data for all of the individual land units in the watershed = load of a pollutant or supply of feedstock j per hectare produced from practice γ k implemented on land unit i

Watershed average cost per hectare of crop production practice = net additional cost per hectare of implementing γ k on land unit i relative to the baseline corn-soybean rotation with no stover collection each stover collection cropping practice (8 scenarios) a vector containing the size data for all of the individual land units in the watershed

Multi-objective optimization: Joint minimization “Minimize the sum of watershed -average per hectare pollutant load (or minus feedstock yield) and the cost per hectare of crop production practices” • Unconstrained optimization • Perform five separate yields a trade-off frontier, optimizations to jointly rather than a single solution minimize… 1. Cost and (-)stover yield 2. Cost and nitrate • Frontier illustrates a 3. Cost and TP range of outcomes 4. Cost and TSS • Useful because can’t 5. Cost and GWP actually dictate what is done…

Optimization results: min cost-max stover Conventional tillage corn-soy w/ 52% removal (CB52) Conventional tillage corn-soy w/ 38% removal (CB38)

Stover Production vs. Stover Production vs. Nitrate Loss Total Phosphorus(TP) Loss Nitrate (kg/ha) TP (kg/ha) Average Stover Production (Mg/ha) Average Stover Production (Mg/ha) Stover Production vs. Stover Production vs. Global Warming Potential (GWP) Total Suspended Sediment (TSS) Loss TSS (Mg/ha) GWP (kg/ha) Average Stover Production (Mg/ha) Average Stover Production (Mg/ha)

Relationship between stover production and practice cost per hectare for all multi-objective optimizations Nitrate minimization overlaps heavily with stover maximization Small range of stover yields spanned by points selected by other optimizations, but very wide range of costs from $63 to $178 per ha along TP and sediment curves

Nitrate-Cost outcomes from different optimizations Nitrate and stover move together, as cost increases Nitrate and GWP always go in “opposite” directions, as cost increases

Combined trade-off frontiers for all cost-pollutant multi-objective optimizations

Take-away points from combined trade-off frontier plot Stover maximization or NO 3 minimization GWP and TSS reduce nitrate minimization loading objectives increase TP loading All objectives that don ’ t minimize TSS, All stover removal increase it scenarios result in higher soil GHG flux than baseline

Conclusions • Maximizing stover results in lower nitrate and TP, but higher sediment and GWP • All stover collection practices considered increase contribution to GWP from crop land • A move toward more continuous corn will lead to larger environmental impacts compared to corn- soybean rotation w/o any stover removal

Next steps and what have you • Stover removal – “Correct” N replacement rate…? – Cover crops or other GHG and/or erosion mitigating practice(s) • Farmers’ behavioral response will not be that selected by optimizations… • Conditions under which perennial crops would be grown to supply cellulosic biomass • Constrained optimization, like economists are supposed to do!