Estimation Of Agro-ecosystem Scale Water Use Efficiency With Evapotranspiration Separation

Water use efficiency(WUE)is an important indicator of carbon and water cycle in farmland ecosystems,which can reveal the change pattern of crop carbon and water exchange and water use efficiency,and is important for guiding agricultural irrigation and improving water productivity.In this paper,based on the eddy flux observation of the actual evapotranspiration(ET)of farmland ecosystems,we used hydrogen and oxygen stable isotopes and soil evapotranspiration to separate the ET,and estimated the total primary productivity(GPP)of typical maize farmland ecosystems in Ningxia Yellow Irrigation Area by LUE model and TG model,and calculated WUEET=GPP/ET,WUET=GPP/T and IWUEVPD=(GPP.VPD)/ET,and then analyze the relationship between different water use efficiency and environmental factors such as temperature,vapor pressure difference(VPD),CO2 concentration,Photosynthetically active radiation and soil water content in terms of functional response relationship,correlation and sensitivity.The results of the study are as follows:(1)The variation of actual evapotranspiration of maize agroecosystem during growing season in Ningxia yellow-intake irrigation area is single-humped,and the trend of crop evapotranspiration by hydrogen and oxygen stable isotopes is consistent with the variation of actual evapotranspiration,and the trend of crop evapotranspiration by soil evapotranspiration is the same as that of actual evapotranspiration at seedling stage and late maturity stage,and the opposite at mid-growing stage.Except for the seedling and late maturity stages,crop transpiration was higher than soil evaporation in most of the time,and crop transpiration in agricultural ecosystems was the main part of irrigation zone evapotranspiration.(2)Based on the light energy use(LUE)model and TG model,the total primary productivity of maize agroecosystems in Ningxia Yellow Irrigation Area showed a "single-peak" trend during the growing season,with GPP at a low level of less than 2 g C·m-2·d-1 at the beginning of the reproductive period.The GPP gradually increased during the tassel stage and remained high at the tassel stage,with peaks of 5.92 g C·m-2·d-1and 4.99 g C·m-2·d-1,respectively,and decreased to 1.24 g C·m-2·d-1 and 2.91 g C·m-2·d-1 during the late filling stage.(3)The different water use efficiencies had different characteristics during the growing season.The overall WUET,WUEET and IWUEVPD were all at low levels during the seedling stage of maize,but the inherent water use efficiency IWUEVPD-TG of the TG model showed a higher level in the early stage.The overall trend of all the three water use efficiencies increased when entering the pulling and gestation stage.After the tassel stage,WUET,WUEET and IWUEVPD all remained at high levels.All three water use efficiencies showed a decreasing trend from the late filling stage to the early maturity stage.At late maize maturity,the ecosystem water use efficiency WUEET-LUE and inherent water use efficiency IWUEVPDLUE based on the LUE model continued to maintain a decreasing trend,while the remaining water use efficiencies still showed an increasing trend.(4)Among the five influencing factors,soil moisture content is the most sensitive factor for water use efficiency in agroecosystems,and water use efficiency y is significantly and positively correlated with it,increasing with the increase of soil moisture.This was followed by saturated water vapor pressure difference and air temperature,both of which were significantly negatively correlated with the three water use efficiencies,decreasing with their increase.Photosynthetic effective radiation was negatively correlated with the three types of WUE,but was not a sensitive factor.the correlation and sensitivity of CO2 concentration to WUE were both low.