温带草地蒸散发及波文比观测与比较:涡动相关及波文比系统

Comparison of Evapotranspiration and Bowen Ratio Measured by Eddy Correlation and Bowen Ratio System in a Temperate Grassland

蒸散发的精确观测及估算是水资源管理及水文遥感验证的基础。该文利用已有的涡动塔观测数据集,配合波文比系统进行了同步观测,对二者观测的结果进行了对比分析。结果表明:涡动相关法和波文比方法估算的蒸散发均值及季节变动分别为298.73±128.58 W·m-2和272.37±116.87 W·m-2,整体而言这两种方法所观测的蒸散发较为一致,决定系数R2=0.75,平均相对误差小于6%。较涡动相关法,波文比方法估算潜热较低。生长季内,植被在生长初期及割草期出现明显的差别,表明波文比系统在植被生长旺盛期测量潜热效果更为稳定。两种方法估算的波文比具有相似的日变化及季节变动趋势。波文比存在明显的日变化及季节变动,其变动均值及标准差为0.21±0.42;其日变化与太阳辐射相关度较强(R2变动为0.41~0.52),其季节变化与叶面积指数(LAI)具有较高的非线性相关性(R2=0.65),而土壤水分对季节变动影响较小。该研究表明:在温带湿润草地生态系统,植被的生长很大程度上调节着该系统能量的分配。以上研究结果对波文比系统的实际应用具有一定的参考价值。

A reliable measurement for evapotranspiration and bowen ratio is critical to the development and validation of land surface models and remote sensing algorithms.To evaluate the two methods of estimating evapotranspiration rate and bowen ratio and its controls for a temperate-grassland ecosystem throughout a growing season,a comparative study was made using the methods of Energy Balanced Eddy Covariance（EBEC）and Energy Balanced Bowen-Ratio（BREB）to measure the evapotranspiration and bowen ratio.The results showed that：the average evapotranspiration and standard deviation measured by Eddy Covariance and BREB was 298.73±128.58W·m（-2） and 272.37±116.87W·m（-2）,respectively,the error of BREB relative to Eddy Covariance was less than 6%,and the correlation between their half-hourly total evapotranspiration was significant（R^2=0.75）,suggesting that BREB was in good agreement with Eddy Covariance in the temperate grassland evapotranspiration measurement.Agreement between them demonstrated good performance of two approaches.The results suggested that the majority of daily latent heat fluxes was underestimated by EBBR method during growing season.The larger discrepancy was detected during the initial period and a period after grass cutting,which indicated that EBBR method was more stable during fast growing period.The bowen ratio estimated by EBBR and EBEC method displayed consistent diurnal and seasonal variations.Bowen ratio was mainly controlled by short-wave radiation at a diurnal timescale.Also,leaf area index was the primary controlling factor for bowen ratio at a seasonal timescale.Soil moisture at the study site was a minor factor with the moderately humid conditions.Our results emphasize that energy flow paths（i.e.,latent heat flux）from temperate grasslands to the atmosphere are strongly controlled by the physiological responses of plants to solar radiation on a diurnal timescale and vegetation growth on a seasonal timescale.The above mentioned results provide scientific support for the practical application of the Bowen system.