Differences in ecosystem water-use efficiency among the typical croplands

Water use efficiency (WUE) is an important parameter to assess agricultural production and the reasonable utilization of water resources. Especially in the context of changing hydrological environment, more attention need to be paid on how to use limited water resource to improve crop yield for ensuring food security. Based on 33 site-years of flux measurements over 10 cropland sites using the eddy covariance (EC) technique, the study systematically evaluated the large differences in seasonal and interannual variations of gross primary productivity (GPP), evapotranspiration (ET) and ecosystem WUE across the four crops worldwide including soybean, maize, winter wheat and paddy rice. The lengths of the growing seasons across the main crops extracted from time-series MODIS NDVI data, implied that the longest growth period in winter wheat and the shortest growing season in paddy rice field. Further analyses suggest that maize cropland has the strongest ecosystem WUE with 2.48 ± 0.69 g C kg−1 H2O, followed by winter wheat (2.00 ± 0.39 g C kg−1 H2O), soybean (1.92 ± 0.52 g C kg−1 H2O), and paddy rice (1.88 ± 0.63 g C kg−1 H2O). Meanwhile, the variability in ecosystem WUE exhibited apparent seasonality, and peaked together with GPP in the most active summertime. A series of biotic and abiotic factors affected the GPP as well as WUE variability. Given the complicated interactions among these environmental factors, this study revealed the great potential to remotely retrieve the WUE variability using time-series MODIS NDVI data over large areas. Ecosystem WUE of the C4 crop -maize was obviously higher than the other three C3 crops. Engineering C4 feature into C3 crops may be a feasible way to increase photosynthesis and yield.