Water, heat fluxes and water use efficiency measurement and modeling above a farmland in the North China Plain

Net radiation (R<inf>n</inf>), water vapor flux (LE), sensible heat flux (H<inf>s</inf>) and soil heat flux (G) were measured above a summer maize field with the eddy-covariance technique, simulation and analysis of water, heat fluxes and crop water use efficiency were made with the RZ-SHAW model at the same time in this study. The results revealed significant diurnal and seasonal variability of water vapor flux for summer maize. Most part of R<inf>n</inf> was consumed by the evapotranspiration of the summer maize. The proportion of water vapor flux to net radiation ((LE/R<inf>n</inf>) increased with the crop development and peaked around milk-filling stage with a value of 60%, a slightly lower than that obtained by the RZ-SHAW model. Daily evapotranspiration estimated by the model agreed with the results measured with the eddy-covariance technique, indices of agreement (IA) for hourly water vapor fluxes simulated and measured were above 0.75, root mean square errors (RMSE) were no more than 1.0. Diurnal patterns of H<inf>s</inf> showed the shape of inverted "U" shifted to the forenoon with a maximum value around 11:30 (Beijing time), while LE exhibited an inverted "V" with a maximum value at around 13:00, about an hour later than H<inf>s</inf>. Diurnal change of CO <inf>2</inf> showed an asymmetrical "V" curve and its maximal rates occurred at about 11:30. Variations of water use efficiency during the phonological stages of the summer maize showed a rapid increase with the photosynthetic photon flux density (PPFD) after sunrise, a state of equilibrium around 10:00 followed a decrease. Maximum values of water use efficiency were 24.3, and its average value ranged from 7.6 to 10.3 g kg^-1. Copyright by Science in China Press 2005