Evapotranspiration and Ecosystem Water Use Efficiency of Switchgrass and High Biomass Sorghum

Abstract

Water and energy flux densities of co‐located cellulosic feedstocks, switchgrass (Panicum virgatum L.) and high biomass sorghum (Sorghum bicolor L. Moench), were measured using the eddy covariance technique during the 2012 and 2013 growing seasons to quantify and compare evapotranspiration (ET) and ecosystem water use efficiency (EWUE). Peak growing season ET from the switchgrass field was approximately 6 mm d−1 in both years; whereas, the sorghum ET reached 6.7 in 2012 and 4.8 mm d−1 in 2013. While both species showed similar water use patterns during the active growing period, seasonal cumulative ET was higher in switchgrass due to a longer growing season. Monthly ET integrals were strongly correlated with monthly integrals of gross primary production (GPP) and net ecosystem production (NEP) for both species, suggesting a strong linkage between C gain and water loss over the season. Different methods of EWUE estimations yielded different EWUE values. The ratio of seasonal sums of GPP to ET yielded EWUE of 9.41 to 11.32 and 8.98 to 9.17 g CO2 mm−1 ET in switchgrass and sorghum, respectively. The ratio of seasonal sums of NEP to ET was 2.75 to 2.81 and 2.06 to 2.18 g CO2 mm−1 ET in switchgrass and sorghum, respectively. Slightly larger EWUE in switchgrass than sorghum shows that the switchgrass was more efficient than the sorghum on using water to gain C. Results suggest that larger seasonal C uptake by switchgrass was enough to offset its higher water use due to the longer growing season. Eddy covariance technique can be used to assess bioenergy crops ecosystem water use efficiency. Swithcgrass has higher ET than sorghum due to longer growing season. Greater seasonal carbon uptake by switchgrass than sorghum was enough to offset higher water use.