A comparison of canopy evapotranspiration between perennial rhizomatous grasses and Zea mays

Perennial rhizomatous C4 grasses are currently considered one of the most promising vegetation types to accommodate a cellulosic feedstock based liquid fuel economy. The current focus on using these vegetation types as a source of renewable fuel has sparked numerous concerns associated with environmental impacts. Of particular interest is the impact that altering the composition of vegetation at the landscape scale would have on local and regional hydrological cycles. We hypothesize that evapotranspiration, ET, will be higher for perennial grasses relative to maize as a result higher leaf area, higher above-ground biomass and prolonged growing seasons. To test this hypothesis, a technique in which ET is estimated as the residual in the energy balance equation from measurements of net radiation and sensible and latent heat fluxes was employed. Measurements were made during the 2007 growing season for three replicate plots of the perennial rhizomatous grasses Miscanthus giganteus and Panicum virgatum, as well as for Zea mays planted at the University of Illinois South Farms. When averaged across the entire growing season, ET for M. giganteus was double relative to Z. mays, and 130% of P. virgatum ET. When compared over the periods in which all three species experienced mature and closed canopies (from day of year 200 to 250), M. giganteus still showed higher rates of ET compared with Z. mays, however, the increase was only ~15%. We conclude that ET associated with perennial alternative energy crops are higher relative to annual row crop; with most ET disparity, particularly for P. virgatum, being driven by phenology, quicker canopy closure and a prolonged growing season. Physiological rates of ET were highest for M. giganteus, followed by Z. mays, followed P. virgatum. Differences in phenology were more important than those of physiology for ET overshadowing effects from increased biomass associated with M. giganteus and/or a physiological difference between these species does not contribute significantly to an overall effect. Changes in ET can influence the hydrologic cycle in manners different than traditional row crops and the implications of these results include feedbacks on climate, temperature, precipitation, and altered runoff patterns.