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 (WUE_e) changed diurnally and seasonally. Daily maximal WUE_e appeared in the morning. WUE_e generally peaked in late April in wheat field and in late July/early August in maize field. From 2003 to 2006, seasonal mean WUE_e was 6.7–7.4 mg CO₂ g⁻¹ H₂O for wheat and 8.4–12.1 mg CO₂ g⁻¹ H₂O for maize. WUE_e was much lower than canopy water use efficiency (WUE_c) under small leaf area index (LAI) but very close to WUE_c under large LAI. With the increase in LAI, WUE_e enlarged rapidly under low LAI but slowly when LAI was higher than one. WUE_e was greater on the cloudy days than on the sunny days. Under the same solar radiation, WUE_e was higher in the morning than in the afternoon. The ratio of internal to ambient CO₂ partial pressure (C_i/C_a) decreased significantly with the increase in photosynthetically active radiation (PAR) when PAR was lower than the critical values (around 500 and 1000 μmol 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, WUE_e has negative correlation with vapor pressure deficit in both of irrigated wheat and maize fields.