Continuous measurements of CO 2 and water vapor exchanges made in three cropping systems (irrigated continuous maize, irrigated maize-soybean rotation, and rainfed maize-soybean rotation) in eastern Nebraska, USA during 6 years are discussed. Close coupling between seasonal distributions of gross primary production (GPP) and evapotranspiration (ET) were observed in each growing season. Mean growing season totals of GPP in irrigated maize and soybean were 1738114 and 99669 g C m -2, respectively (standard deviation). Corresponding mean values of growing season ET totals were 54527 and 45423 mm, respectively. Irrigation affected GPP and ET similarly, both growing season totals were about 10% higher than those of corresponding rainfed crops. Maize, under both irrigated and rainfed conditions, fixed 74% more carbon than soybean while using only 12-20% more water. The green leaf area index (LAI) explained substantial portions (91% for maize and 90% for soybean) of the variability in GPP PAR (GPP over a narrow range of incident photosynthetically active radiation) and in ET/ET o (71% for maize and 75% for soybean, ET o is the reference evapotranspiration). Water productivity (WP or water use efficiency) is defined here as the ratio of cumulative GPP or above-ground biomass and ET (photosynthetic water productivity=SigmaGPP/SigmaET and biomass water productivity=above-ground biomass/SigmaET). When normalized by ET o, the photosynthetic water productivity (WP ETo) was 18.41.5 g C m -2 for maize and 12.01.0 g C m -2 for soybean. When normalized by ET o, the biomass water productivity (WP ETo) was 27.52.3 g DM m -2 for maize and 14.13.1 g DM m -2 for soybean. Comparisons of these results, among different years of measurement and management practices (continuous vs rotation cropping, irrigated vs rainfed) in this study and those from other locations, indicated the conservative nature of normalized water productivity, as also pointed out by previous investigators.
