Biomass can be converted directly into liquid fuels, also called biofuels, that are primarily used for transportation because of their portability and energy density. Worldwide demand for liquid fuels, primarily petroleum-based, is projected to grow (IEA, 2015). Bioethanol is a common type of biofuel in use today that can potentially have low life-cycle greenhouse gas emissions depending on how it is produced (Searchinger et al., 2015; Wang et al., 2015). Bioethanol produced from crops via fermentation is currently the most prevalent liquid fuel sourced from biomass. Currently, the United States is the largest bioethanol producer of any country, primarily producing its bioethanol from corn grain-, converting starch to sugar, and then to bioethanol via fermentation (RFA, 2022). Brazil is the second largest producer, driving its bioethanol from the sugar in sugar cane (Chum et al., 2011). Bioethanol is thus a potential option to supply transportation fuels in a low-carbon future. However, bioethanol production from sugar and starch is land-use intensive—i.e., the area of land used to grow feedstock crops per unit of bioethanol produced is large (Melillo et al., 2009; Zhuang et al., 2013; Song et al., 2016).

The production of bioethanol from cellulosic feedstocks, which are non-food based and include crop residues, wood residues, dedicated energy crops, and industrial and other wastes (rather than just from simple sugars and starch), has the potential to decrease bioethanol’s land-use intensity. In the recent decade, the number of cellulosic bioethanol plants in the U.S. has increased (Brown et al., 2015), rising from 76 kL in 2012 to 38 × 10³ kL in 2017 and after that decreasing to about 7.8 × 10³ kL in 2020 (EPA, 2022). In 2021, bioethanol production further decreased to 2.2 × 10³ kL (RFA, 2022). The overall U. S. ethanol production by corn grain starch accounts for 93.8%, cellulosic biomass accounts for 3.9%, and beverages, food waste, non-corn grain starch, and sugar account for the rest 2.3% (RFA, 2022). According to Energy Independence and Security Act (EISA, 2007), the annual production of renewable fuels of 136 billion liters, including 79 billion liters of cellulosic or advanced biofuels, is expected to be produced in the U.S. by 2022. The annual U.S. production of bioethanol, primarily produced from corn starch in the U.S, rose to 57 billion liters in 2021 (USDA, 2022) (Supplementary Figure S1), which fulfilled the required conventional biofuel target set forth by the EISA (2007). However, the U.S. fell short of the cellulosic or advanced biofuel target of 79 billion liters. The U.S. to meet its goals would depend on increased production of crop yields, such as of corn (Zea mays), and the conversion efficiencies of cellulosic feedstock, such as the perennial grasses Miscanthus (Miscanthus × giganteus Greef et Deu), and two cultivars of switchgrass (Panicum viragatum), namely, upland (Cave-in-Rock) and lowland (Alamo), to bioethanol.

Multiple studies point to Miscanthus’s high maximum biomass yield compared to switchgrasses and corn (Jain et al., 2010; Song et al., 2016). Observation-based estimates for Miscanthus were on average 22 Mg/ha/yr compared to 10 Mg/ha/yr for switchgrass (Heaton et al., 2008; Myers et al., 2012). One study which reports side-by-side trial results in central Illinois showed that Miscanthus produced 60 percent more biomass than corn (Dohleman and Long, 2009). Several crop productivity modeling studies have estimated the spatial variations for bioenergy feedstock for Miscanthus and switchgrasses in the United States. Using the process-based crop-growth land surface model employed here (see Section 2.1), Song et al. (2015, 2016) estimated the average annual yield ranges between 2–25 Mg/ha/yr for Miscanthus, 2–15 Mg/ha/yr for Cave-in-Rock (which has high yields over the same land region as Miscanthus), and 4–17 Mg/ha/yr for Alamo. Most other modeling studies estimated the highest yield values for these bioenergy crops consistent with these estimates (Thomson et al., 2009; Jager et al., 2010; Behrman et al., 2013; Zhuang et al., 2013), but there were a few exceptions. For example, model-based values reported by Miguez et al. (2011) and VanLoocke et al. (2012) were as high as 40.5 Mg/ha/yr and 36 Mg/ha/yr for Miscanthus, and 20 Mg/ha/yr and 16 Mg/ha/yr for switchgrass, respectively.

Few published studies compared the bioethanol yields produced from bioenergy crops’ biomass with bioethanol yields produced from corn grain and stover in the United States. Zhuang et al. (2013) compared modeled potential bioethanol yield of corn, switchgrass and Miscanthus limited to regions where corn is currently grown and found that Miscanthus had higher bioethanol yield.

The aim of this study is twofold: 1) to calculate the spatial distribution of the potential bioethanol yield (per unit land area) of cellulosic bioethanol plants, with feedstocks including corn grain and stover, and perennial bioenergy grasses, including Miscanthus, Cave-in-Rock, and Alamo, across the Central and Eastern U.S; and 2) to calculate the effects of growing corn and perennial bioenergy grasses on surface hydrological components and the impacts of extreme drought and heat on bioethanol yield. To accomplishes these aims, the calculations are performed using a crop growth land surface model, Integrated Science Assessment Model (ISAM) along with estimates of harvest fractions and biomass-to-bioethanol conversion factors. This study is part of a larger study on the Climate-induced Extremes on the Food, Energy, Water Systems (C-FEWS) and the Role of Engineered and Natural Infrastructure (Vörösmarty et al., 2022) that examines food and bioenergy crop yields in the U.S. Midwest and Northeast for a range of different model simulations and outcomes.

The ISAM estimated potential annual bioethanol yield per hectare is shown for the different crops across the Central and Eastern U.S. landscape in Figure 2.

Despite the higher conversion factor of corn grain biomass yield to bioethanol yield (Table 1), it has a comparable bioethanol yield to that of the other energy crops because the biomass yield of corn grain is generally smaller than the three bioenergy crops studied here. Adding the assumed harvest of 30% of corn stover with an assumed bioethanol conversion factor equal to that of other cellulosic feedstocks (see Table 1) increases the overall bioethanol yield of corn (grain plus 30% of stover harvested, Figure 2F).

The estimated maximum bioethanol yield, defined as the area of a crop with the highest ethanol yield in a grid cell, is shown in Figure 3. The potential bioethanol yield of each bioenergy peaks in different locations from north to south, with Cave-in-Rock in parts of northeast, followed by Miscanthus and corn, and Alamo furthest south (Figure 3), overlapping parts of the current corn growing land region (Figure 3).

Miscanthus and Alamo have the highest maximum potential ethanol yield (about 8.5 kL/ha/yr) when comparing yields in the study region, but yields drop off more steeply than corn or Cave-in-Rock (Figure 4), suggesting that a limited number of grids cells in the study region can produce high bioethanol yeild from these two energy crops. The calculated highest potential yield for Cave-in-rock is about 5.7 kL/ha/yr, but the spatial variation of Cave-in-rock yield across grid cells is relatively low. It has the potential to grow on 10%–15% more grid cells than the two other bioenergy crops, particularly in the Northeast and parts of the upper Midwest (Figure 2). Corn’s (grain plus 30% of stover harvested) highest calculated maximum potential ethanol yield is about 5.6 kL/ha/yr. Across grid cells, corn’s potential yield variations are lower than the other crops, and corn can grow on more grid cells than the other three bioenergy crops. It is estimated that corn produces less bioethanol yield than any of the bioenergy grasses in 65% of grid cells in the study region, implying that growing bioenergy grasses can enhance the bioethanol yield in these areas.

Yield is one fundamental factor influencing the desirability of biofuels and associated crops, but it is not the only factor. Crops differ in their consumption of water and the need for added N fertilizer and associated N runoff (Heaton et al., 2008; Hickman et al., 2010; McIsaac et al., 2010; Smith et al., 2013; Zhuang et al., 2013; Song et al., 2016). Growing bioenergy crops can also reduce the annual runoff and N leaching due to higher evapotranspiration (Figure 5) than corn and biological N fixation by Miscanthus (Song et al., 2016). In addition, it is suggested energy crops require less fertilizer input than corn because they recycle nutrients to belowground roots and rhizomes before the dry shoots are harvested in late fall and early winter (Cadoux et al., 2012).

Figure 5 shows the distribution of 1980–2019 mean evapotranspiration (ET), runoff (=R_surface + R_{sub_surface}, where R_surface stands for surface and R_{sub_surface}, sub-surface runoff), and N leaching from simulations for corn (grain plus 30% of stover harvested) and three bioenergy crops. As a whole, corn’s ET is lower (average around 700 mm/yr), whereas runoff is higher (305 mm/yr), than for three bioenergy crops (ET: 740–800 mm/yr, runoff: 161–215 mm/yr). This is because a longer growing season enables root systems to grow deeper and accumulate more biomass than corn, suggesting that the growing bioenergy crops can reduce regional water availability for human use, such as thermoelectric production and irrigation. As for nitrogen leaching, our study results show lower N leaching due to growing bioenergy crops (0.88–2.80 g N/m²/yr) compared to corn (2.58 g N/m²/yr), mainly due to modest amounts of N fertilizer for Miscanthus and switchgrasses compared to corn (Song et al., 2016).

Climate change and extremes are also expected to impact crop yields not only for food crops but also for bioenergy crops. The extremely dry and hot weather conditions, for example, in the 2012 spring and summer led to devastating crop damage in much of the Midwest (Elliott et al., 2018). We investigate this extreme year’s impact on bioethanol yield based on the aggregate metric from the C-FEWS framework (Eq. (1), Vörösmarty et al., 2022) (Appendix A).

Figure 6 shows the expected negative impact of the 2012 drought on corn-based ethanol yields in the Midwest. In current corn-harvested areas (Supplementary Figure S2), growing bioenergy crops attenuates the negative responses to the 2012 drought and heat impact, revealing bioenergy grasses are more resistant to climate variability than corn, because bioenergy grasses being grown perennially and their deep roots can extract water from the deeper moist soil layers and dampen the extreme drought and heat impacts. However, in general, growing bioenergy grasses in current corn-harvested areas over Midwest results in lower bioethanol yields than corn because bioenergy crops are not adapted to winter hardiness in the upper Midwest (i.e., Wisconsin, Minnesota, and west, north, and northeast Iowa), bioenergy grasses were damaged by extreme cold conditions, frost killing or/and over-winter injury (Figure 2).

Yield is one fundamental factor influencing the desirability of biofuels and associated crops, but it is not the only factor. The cost of growing and converting crops to bioethanol also differs between crops (NRC, 2011). While the conversion of corn to bioethanol would divert potential food stocks to ethanol, greater demand for corn could also result in greater production of feed by-products (e.g., distiller’s grains and solids). In contrast, the energy grasses might compete for high-productivity agricultural land (Searchinger et al., 2015). Bioethanol is one potential pathway to lower greenhouse gas emissions from transportation. However, the growth of crops and conversion to bioethanol can result in life-cycle greenhouse gas emissions which differ between crops (Searchinger et al., 2015; Song et al., 2015; Wang et al., 2015), conversion technologies (Edwards et al., 2014), and the potential use of carbon capture and storage for process emissions (Kheshgi and Prince, 2005; Luckow et al., 2010). Finally, the analysis provided here assumes current estimates of crop growth and conversion; future advances in crop and conversion technology could improve the prospects for bioethanol from each of the crops (Edmonds et al., 1996; NRC, 2011), and future climate change can also affect the growth of each crop and its consequent bioethanol yield in different ways.

The prospect of producing bioethanol, particularly from high-yield energy grasses, has motivated both a broad set of studies of biofuel’s role in addressing climate change as well as regulations to encourage their use. That prospect is better informed by estimates of the potential bioethanol yield from the energy crops being envisioned—the objective of this study—than by estimates of biomass yield. This study examines the potential bioethanol yield of corn and three energy crops (Miscanthus, Alamo, and Cave-in-Rock switchgrass) in the Central and Eastern United States. To accomplish this, a state-of-the-art crop growth model is used to estimate the biomass yield for the period 1980–2019, which is then coupled with estimates of crop-to-bioethanol conversion efficiency from recent practice and bioethanol plant design to find the following key conclusions: Each of crop considered has regions where that crop has the highest potential bioethanol yield which is mapped in Figure 3. The high-bioethanol yield regions, particularly in the southern and eastern States of the Midwest, quantified in this study tend to be where corn is currently grown well. The two energy crops cannot grow in the western parts of the Great Plains due to extremely low precipitation and poor soil texture, and in the upper part of the Midwest, north central, northeastern, and northern New England due to extreme cold conditions (Figure 2). In other regions of the study area, the bioenergy crop yields are in between the high-yielding and zero-yielding yield values. Although most bioenergy crops have the maximum potential bioethanol yields across the Midwest region, their harvest areas for high-bioethanol yields are limited, resulting in lower bioethanol yields than corn in the region. A choice to move towards energy crops (as opposed to corn alone) would likely be motivated by a balance of other factors, including economics, life-cycle emissions, competition with food production, and land use, water and climate extremes, N runoff, and the pace of crop and technology improvement into the future. We find that growing bioenergy crops improve the water quality by reducing N leaching but exacerbate water stress by reducing runoff (Figure 5). In addition, bioenergy grasses maintain stably high productivity under drought and heatwaves in the year 2012 over the Midwest. Estimating other factors is beyond the scope of this study, but their estimates would, nevertheless, be partly dependent on potential bioethanol yield.