Comparing Switchgrass and High Biomass Sorghum Evapotranspiration and Ecosystem WATER Use Efficiency In The Southern Great Plains Of The United States.

Water and energy flux densities over co-located switchgrass (Panicum virgatum L.) and high biomass sorghum (Sorghum bicolor L. Moench) were measured using eddy covariance system during 2012 growing season. The objective of this study was to compare evapotranspiration (ET) and ecosystem water use efficiency (EWUE, the ratio of photosynthetic CO₂ assimilation to ET) between ecosystems. The magnitude of peak diurnal ET rate (monthly average) ranged from 0.08 (September) to 0.28 mm 30-min^-1 (May) in switchgrass, whereas it ranged between 0.05 (October) and 0.28 mm 30-min^-1 (June) in sorghum. During the growing season, the weekly average of daily ET was 1.0-6.2 mm day^-1 for switchgrass and 0.7-6.7 mm day^-1 for sorghum. Total ET form both ecosystems was similar (487 mm from switchgrass and 465 mm from sorghum field) during the overlapping growing period (May - October).  However, total growing season ET was higher in switchgrass (653 mm, March - October) than in sorghum (465 mm, May - October) due to longer growing season of switchgrass. Monthly gross ecosystem photosynthesis (GEP) showed strong positive linear relationships with ET (R² = 0.88 and 0.91in switchgrass and sorghum, respectively), suggesting a strong linkage between carbon gain and water loss over the season (slopes = 14.1 g CO₂ mm^-1 ET for switchgrass and 13.1 g CO₂ mm^-1 ET for sorghum). The ratio of cumulative GEP to the respective ET yielded monthly EWUE from 6.3 (March) to 16.1 (June) g CO₂ mm^-1 ET in switchgrass, while it ranged from 4.7 (May) to 13.6 (June) g CO₂ mm^-1 ET in sorghum. The ratio of seasonal sums of daily daytime GEP to daily daytime ET provided EWUE of 12.9 and 10.8 g CO₂ mm^-1 ET for switchgrass and sorghum, respectively. The results illustrate production potential of these two important feedstocks in the dry climates of the region.