Ecosystem water use efficiency in an irrigated cropland in the North China Plain

The eddy covariance technique and the cuvette method were used to investigate water use efficiency in an irrigated winter wheat (Triticum asetivum L.)/summer maize (Zea mays L.) rotation system in the North China Plain. The results show that ecosystem water use efficiency (WUEe) changed diurnally and season, ally. Daily maximal WUEe appeared in the morning. WUEe generally peaked in late April in wheat field and in late July/early August in maize field. From 2003 to 2006, seasonal mean WUEe was 6.7-7.4 mg CO2 g(-1) H2O for wheat and 8.4-12.1 mg CO2 g(-1) H2O for maize. WUEe was much lower than canopy water use efficiency (WUEc) under small leaf area index (LAI) but very close to WUEc under large LAI. With the increase in LAI, WUEe enlarged rapidly under low LAI but slowly when LAI was higher than one. WUEe was greater on the cloudy days than on the sunny days. Under the same solar radiation, WUEe was higher in the morning than in the afternoon. The ratio of internal to ambient CO2 partial pressure (Ci/Ca) decreased significantly with the increase in photosynthetically active radiation (PAR) when PAR was lower than the critical values (around 500 and 1000 mu mol m(-2) s(-1) for wheat and maize, respectively). Beyond critical PAR, C-i/C-a was approximately constant at 0.69 for wheat and 0.42 for maize Therefore., when LAI and solar radiation was large enough, WUEe has negative correlation with vapor pressure deficit in both of irrigated wheat and maize fields. (C) 2009 Elsevier B.V. All rights reserved.