Interannual water vapor and energy exchange in an irrigated maize-based agroecosystem

In this paper, we present results from 4 years (May 2001–May 2005) of water and energy flux measurements made in a no-till, irrigated maize–soybean rotation system in eastern Nebraska, USA. The peak green leaf area index (LAI) reached 6.0 and 5.5 in maize (2001 and 2003, respectively) and 5.7 and 4.4 in soybean (2002 and 2004, respectively). The dependence of evapotranspiration (ET) on leaf area was consistent with previous studies. There was a nearly linear relationship between the daily ET/ETo (where ETo is the reference evapotranspiration over a grass reference crop) and LAI until a threshold LAI (between 3 and 4). Above this threshold LAI, the ET/ETo was virtually independent of LAI. The cumulative growing season (planting to harvest) evapotranspiration was 544 and 578 mm for maize, and 474 and 430 mm for soybean. The interannual variability in the growing season ET totals correlated very well with the number of days when the LAI was greater than 3. The non-growing season period (harvest to subsequent planting) contributed between 20 and 25% of the annual ET totals for both crops. The maximum canopy surface conductance (Gsmax) was 29 mm s−1 for maize in both years, 41 mm s−1 for soybean in 2002 (peak LAI = 5.7) and 36 mm s−1 for soybean in 2004 (peak LAI = 4.4). The variability in Gsmax was largely explained by the leaf nitrogen concentration, consistent with the literature.