The controlling factors of ecosystem water use efficiency in maize fields under drip and border irrigation systems in Northwest China

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The ecosystem water use efficiency (WUEe) is the critical link between water and carbon cycling in the terrestrial ecosystem. The evaluations of the biophysical factors influencing WUEe could reveal mechanisms of controlling WUEe. In order to save water in agriculture, drip irrigation (DI) has been promoted to replace border irrigation (BI) in northwest China. Different irrigation methods will cause differences in evapotranspiration (ETa), gross primary productivity (GPP), and WUEe. Further research is needed to quantify the contribution of each factor to the differences in ETa, GPP, and WUEe. To separate the different factors controlling ETa, GPP, and WUEe variability between DI and border BI, the 4-year data under DI and BI measured by eddy covariance systems were collected and analyzed. To explore the differences in ETa, GPP, and WUEe under different irrigation methods, Penman-Monteith (P-M) model and stomatal conductance-photosynthetic transpiration coupling (SMPT-SB) model complemented by perturbation analysis were employed. The results showed that the 4-year mean evapotranspiration (ETa) decreased by 8% under DI compared with the value under BI, and the mean gross primary productivity (GPP) under DI was 5% higher than the BI. The WUEe under DI was 14% higher than that under BI. The P-M model and the SMPT-SB model agreed well with ETa and GPP measured on daily time scales, providing confidence in their ability to separate the biological and physical controls of ETa, GPP, and WUEe. Based on the two models, the first-order Taylor series expansions of the total ETa, GPP, and WUEe derivatives were applied to the P-M model and SMPT-SB model and compared with measured differences in ETa, GPP, and WUEe. More than 60% of the difference in ETa caused by the two irrigation methods was attributed to variations in the coupled surface resistance, and drip irrigation mainly achieves water saving by reducing soil evaporation. The difference in maize growth due to irrigation methods was the main reason for the differences in GPP. The differences in vapor pressure deficit and the coupled surface resistance caused by irrigation methods were the main factors causing the difference in WUEe. It could be surmised that drip irrigation increased WUEe in maize fields by changing surface resistance, which mainly reduced soil evaporation.