稳定同位素法和涡度-微型蒸渗仪区分玉米田蒸散组分的比较

Comparison of partitioning evapotranspiration composition in maize field using stable isotope and eddy covariance-microlysimeter methods

农田蒸散组分区分研究对理解土壤-植物-大气连续系统水分传输动力学过程与调控机制具有重要意义。利用稳定同位素原位连续观测技术测定了玉米田不同高度大气水汽稳定同位素的动态,结合Keeling Plot方法区分了玉米田蒸散组分动态过程,并与涡度相关系统（eddy covariance system,EC）-微型蒸渗仪（micro-lysimeter,MLS）结合法的区分结果进行了比较。研究表明：降雨后大气水汽稳定同位素组成δv显著降低,不同高度δv与大气饱和水气压差VPD、太阳净辐射Rn呈线性相关,相关系数R均值分别为0.65和0.51;土壤蒸发水汽稳定同位素组成δE存在显著的分馏效应,较低的土壤含水率和大气相对湿度会降低分馏效应;与传统的EC-MLS法的区分结果相比,利用Keeling Plot方法和连续原位监测的水汽稳定同位素信息,可准确估算农田蒸散比,误差仅为-0.02~0.08;试验观测期内即玉米中等覆盖地面后,IS法和EC-MLS确定的作物蒸腾与农田蒸散比的均值分别为0.81和0.78,二者较一致。

To partition evapotranspiration into its components, dynamics of water vapor δ18O at five different heights were monitored continuously in maize field by stable water vapor isotopes analysis system combined with Keeling plot curve, which were compared with the estimations through the eddy covariance system and the micro-lysimeter. The results showed that the stable isotope composition of atmospheric water vapor （6v） in maize field was significantly decreased after rainfalls, and linearly related to vapor pressure deficit （VPD） and solar radiation （Rs） with correlation coefficients （R） of 0.65 and 0.51, respectively. Water vapor stable isotope composition （δE） from soil evaporation has significant isotopic fractionation effects near the soil surface. Low soil water content and atmosphere relative humidity alleviated the isotope fractionation. Compared with the results estimated by the EC-MLS method, the error estimated by IS method was less, about -0.02 - 0.08. The results indicated that the combination of Keeling plot method with in situ continuous measurements of water vapor stable isotope composition accurately separated evapotranspiration in maize field. During the measurement period, i.e., after maize canopy moderately covered the soil, the average ratios of crop transpiration to evapotranspiration determined by 1S and EC-MLS methods were 0.81 and 0.78, respectively.