基于氧同位素的玉米农田蒸散发估算和区分

农田蒸散发(evapotranspiration,ET)的估算和区分是土壤-植物-大气连续体中的重要研究内容,是农业水资源高效利用的重要基础。该研究分析了土壤水、蒸发水汽、蒸腾水汽和大气背景混合水汽氧同位素组成分布特征,并采用2种同位素的方法对玉米农田蒸散发进行估算和区分:1)结合Keeling plot和Craig-Gordon模型的同位素方法(Iso-CG);2)基于土壤水同位素守恒和水量平衡的方法(Iso-WB)。结果表明,在玉米生育期内Iso-WB方法与Iso-CG方法所计算的玉米蒸腾比例分别为0.64~0.91和0.52~0.91,平均值分别为0.80和0.78。玉米蒸散发总量在前期、中期和后期均值分别为3.95、5.30和4.98 mm/d。通过比较参数并与前人研究结果对比分析,表明采用Iso-CG方法估算区分ET相对精确,采用Iso-WB方法计算蒸散发要求的测量精度相对较高,计算误差较大。该研究成果不仅为玉米农田制定灌溉制度及提高用水效率提供了理论依据,而且对深入探索氧同位素水文学领域具有重要意义。

Estimation and partition of evapotranspiration of maize farmland based on stable oxygen isotope

Evapotranspiration(ET)and its estimation and partition are important research contents in the soil-plant-atmosphere continuum(SPAC),as it provides abundant information about water and heat transfer and plays an important role in the efficient water use in agriculture.The isotopic technique is considered to be an effective method to study it.Here,the isotopic compositions of soil water,evaporated vapor,transpiration vapor and atmospheric background vapor were measured and 2 isotope-based methods(Iso-CG method,isotope combined with the Keeling plot and Craig-Gordon model;and Iso-WB method,combining isotope conservation and water balance)were applied to estimate and partition ET.The study was conducted in 2015 at Shiyanghe Experimental Station of China Agricultural University,Wuwei City,Gansu Province,China(37°52′N,102°51′E).The results showed the transpiration fraction varied from 0.64 to 0.91 by Iso-WB method and 0.52 to 0.91 by Iso-CG method,with a mean of 0.80 and 0.78(±0.01),respectively,during the entire growing season of maize.However,the results calculated by Iso-WB were overestimated.The mean ET of maize were 3.95,5.30 and 4.98 mm/d(±0.15 mm/d)during the earlier stage,middle stage and later stage.The results of ET showed that the transpiration rate of maize changed greatly with the seasons,showing a small transpiration rate at the early growth stage,and a slow decline in the later period.The ET determined by Iso-CG method was robust,but there could be some deviation in Iso-WB due to its measurement accuracy was relatively high and the calculation error was large.It indicated that it was relatively accurate to estimate and distinguish ET by using the Keeling plot and Criag-Gordon combined chamber system method.Compared with conventional methods,such as combining hydrometric measurements(e.g.,sap flow)to estimate transpiration with other methods(e.g.,weighing lysimeter measurements)to measure evaporation,the stable isotope based method had the advantages of high accuracy and controllability.For example,the calculation accuracy of the 2 isotope methods used in this study was about±0.15 mm/d.The study results not only provide valuable information for determining irrigation scheduling of maize farmland and improving water-use efficiency,but also have important significance for exploring deeply the field of oxygen isotope hydrology.Meanwhile,the isotope-based method has its limitations compared to other methods of scaling up from leaf level to canopy scale.The sources of error in this isotopic based methods study may come from the variability of isotopic signatures,weather conditions and sample size and time.Therefore,it is still a great challenge in ET estimation and partition.There are very few direct methods to measure isotopic composition of plant transpiration and soil evaporation.We expect that future refinement of methods will allow for accurate and continuous measurement of transpiration and evaporation isotopic composition.For some limitations of determining evaporation isotopic composition,we recommend that field experiments pay particular attention to sampling precision at the evaporation front in the surface soil and temperature.