Estimating Water Use Efficiency in Bioenergy Ecosystems Using a Process-Based Model

Bioenergy, made available from materials derived from biological sources, has been widely considered as one of the major renewable and sustainable energy sources to enhance energy security and mitigate climate change [Beringer et al., 2011; Fargione et al., 2010]. Food grain is currently the most popularly used biomass feedstock for biofuel production, e.g., maize (Zea Mays L.) grain for bioethanol and soybeans [Glycine max (L.) Merr.] for biodiesel. However, traditional biofuels have many unintended consequences concerning feedstock availability, food security, environmental sustainability, and societal welfare. During the last two decades, maize grain production increased about 65%. The major contribution is from grain yield, with about 1.96% increase annually [FAOSTAT, 2012]. But most of maize grain in the United States is used for human consumption, livestock feed, or other purposes. Only about 30% of grain harvested was used for ethanol production in 2009 [USDA, 2010]. The slowly growing maize production may not be able to support the rapidly increasing biofuel demand. Foodbased feedstocks alone may limit further biofuel expansion, for instance, to reach the 2022 U.S. biofuel target [U.S. Congress, 2007]. In addition, conventional food-based biofuel development can be a threat to food security, due to competitive consumption of cropland, water, and nutrient resources that could otherwise be used for food production [Fargione et al., 2010]. Indirect land use impacts on ecosystem services, such as monoculture and deforestation, also limit further expansion of conventional biofuel development [Fargione et al., 2010; Searchinger et al., 2008]. Second-generation biofuel (or advanced biofuel), produced from various types of biomass, is increasingly recognized as another option for bioenergy development. Among many tested species, switchgrass (Panicum virgatum L.) and Miscanthus (e.g., Miscanthus giganteus) were often studied for its characteristics, adaptation, and environmental impacts in the United States [Fargione et al., 2010; Heaton et al., 2008; McIsaac et al., 2010]. Switchgrass is a perennial, warm-season cellulosic crop native to North America. It is widely distributed over the United States, especially the prairies of the Midwest [Wright and Turhollow, 2010]. Switchgrass was originally used for soil conservation, forage production, and other purposes. It is more recently used as biomass crop for biofuel production and electricity and heat production [McLaughlin and Adams Kszos, 2005; Meyer et al., 2010]. Miscanthus is a genus of several species of perennial grasses mostly native to subtropical and tropical regions of Asia. It was used as biofuel crop in Europe since the 1980s and then introduced to the United States recently [Heaton et al., 2008; Stewart et al., 2009]. Several commonly shared characteristics make switchgrass and Miscanthus favorite choices as biofuel feedstock resources. First of all, they can produce abundant biomass, with much higher production than maize or soybeans. It normally ranges from 5 to 15 Mg dry matter (DM)/ha−1 with maximum production of over 20 Mg DM/ha for switchgrass, and about 20–30 Mg DM/ha with maximum of 60 Mg DM/ha for Miscanthus [Heaton et al., 2008; 30