遥感反演土壤蒸发/植被蒸腾二层模型在华北地区的应用

Application of an operational two-layer model for soil evaporation and vegetation transpiration retrievals in North China

利用一种可操作的地表蒸散遥感反演二层模型,以我国华北平原为研究区,选择2004年的3月至6月华北地区主要农作物冬小麦的生长季节作为研究时段,利用MODIS遥感卫星数据,结合地面130多个气象台站的空气温湿度实测数据,实现了土壤蒸发和植被蒸腾的反演。采用国家生态网络禹城综合试验站利用涡度相关系统观测的地表总蒸散半小时平均的数据进行了模型验证,结果表明模型估算的地表可利用能量与地面实测数据的相关系数可以达到0.92,均方差为30.4w.m-2;模型估算的地表总蒸散值与地面实测数据的相关系数为0.85,均方差为21.3 w.m-2,由此证明了模型的可用性。

On the basis of an operational two-layer model for estimating soil evaporation and vegetation transpiration,the vegetation transpiration and the soil evaporation during the rapidly growing season of winter wheat in northern China are retrieved using MODIS satellite data and field measurement data from 137 standard meteorological stations.Pixel Component Arranging and Comparing Algorithm and Layered Energy-separating Algorithm are the key components of the model.The former is used for decomposing surface temperature of mixed pixel into soil temperature and vegetation temperature and the latter is used for calculating Bowen-ratio of soil and vegetation,respectively.A surface energy balance method is used to determine the theoretical boundary lines,namely ＇true wet/cool edge＇ and ＇true dry/warm edge＇,in the trapezoid composed of mixed surface temperature and vegetation fractional cover.The variables required for the model mainly include near surface vapor pressure,air temperature,surface resistance,aerodynamic resistance,fractional vegetation cover,surface temperature and net radiation.Also their retrievals are described in the paper.Terrestrial surface heat fluxes measured by the Eddy Correlation system at Yucheng Agro-ecosystem Station are used to validate the estimated results.It shows that the correlation coefficient between the estimated surface available energy and the measured is 0.92 and the root mean squares difference（RMSD） is 30.4w.m-2,the correlation coefficient between the estimated surface evapotranspiration and the measured value is 0.85 and RMSD is 21.3w.m-2.The above differences are mainly caused by： 1） the scale difference between the field measurement and the MODIS observation;2） the non-closure problem of the surface energy balance from the surface fluxes observations;and 3） the effects of the horizontal and vertical advection on flux measurements.