灌溉农田水汽氢氧同位素组成特征研究初探

灌溉农田生态系统水汽变化能够直接反映其耗水状况。本研究应用基于高频实时气态稳定同位素分析仪的箱式法测定水汽氢氧同位素特征, 结合涡度相关技术所测定的农田水汽通量结果, 探讨不同高度大气水汽在不同时段的稳定同位素组成特征。研究结果表明: 对于不同的时间尺度, 生态系统边界层水汽稳定同位素组成在连续几天内的变化有波动, 日变化比较稳定, 小时尺度波动很小; 水汽稳定同位素的大气背景组成特征表现为日尺度上的组成变化呈稳步富集趋势, 小时尺度上几乎无变化。蒸散水汽重同位素特征随时间推移富集明显, 午夜蒸发和蒸腾水汽同位素组成特征相似, 清晨蒸腾水汽比蒸发水汽轻, 午间蒸腾水汽比蒸发水汽重。对于不同高度的水汽来说, 玻璃箱内蒸散水汽稳定同位素组成要比生态系统边界层2 m 处水汽高。建立了土壤蒸发和植物蒸腾稳定同位素变化特征与蒸散过程的线性关系, 相关系数达到0.87。

Preliminary research on isotopic composition of water vapor in irrigated fields

Improvements in water use efficient and limited groundwater consumption are important for water sustainable resource development. Atmospheric water vapor exchange sufficiently indicates water consumption in irrigated agro-ecosystems. This paper clarified the changes in water vapor isotopic composition with height and time of the day or longer periods. The relationship between the quantity and isotopic composition of evapotranspiration vapor was also determined. The chamber method (which was based on high frequency and real-time water vapor stable isotope analyzer) was used to measure isotopic composition of water vapor at different heights and times of the day/period. Using water vapor flux of crop land measured with Eddy Covariance method, the relationship between isotopic composition and evapotranspiration was also determined. Based on the different time scales, the results showed that water vapor isotopic composition at the ecosystem boundary layers continuously fluctuated during several days. The daily changes were stable with smaller fluctuations at hourly time-scale. There was a steady enrichment in daily changes in isotopic composition under atmospheric background. No obvious trend of change was noted in isotopic composition of the day. With no direct and rapid canopy effect, isotopic composition under atmospheric background was stable at hourly time-scale. There was also an obvious enrichment of chamber isotopic composition of evapotranspiration. Isotopic compositions of evaporation and transpiration were similar for midnight and clearly dissimilar during early morning and noon. Due to isotopic fractionation, isotopic composition of evapotranspiration water was heavier than the vapor at 2 m height. A good linear correlation was noted between the quantity and isotopic composition of evapotranspiration with correlation coefficient of 0.87 and regression equation of y = 53.21x-115.06. The study typified the vitality of evapotranspiration research via the isotope method.