我国中亚热带人工林水热通量对干旱差异响应的模拟研究

季节性干旱现象在我国中亚热带地区时有发生,为了研究该区域大气-生态系统之间的相互作用关系及其碳水收支状况,2002年起在江西省千烟洲（26.7°N,115.1°E）人工林生态系统建立了通量观测塔。2003年7月该人工林生态系统遭遇了历史上少有的高温少雨天气,本研究应用基于生理生态学过程的EALCO（Ecological Assimilation of Land and Climate Observation）模型及2003和2004年通量观测数据对该生态系统的水热通量进行了模拟,同时分析了干旱胁迫对它们产生的影响。结果显示,模型能够很好的模拟该生态系统的能量通量的日变化,净辐射、显热和潜热通量模拟值与实测值相关系数的平方（R2）及标准差分别为0.99和8.05 W.m-2;0.81和41.02 W.m-2;0.90和31.49 W.m-2,模型可以解释87%的日蒸散量的变化。从模拟结果看,2003年7月下旬（发生较严重干旱胁迫）较2004年同期（干旱程度轻）相比,冠层及土壤水势下降约2倍,植物蒸腾的日变化形式改变,根系吸水滞后冠层蒸腾的时间缩短约半小时,冠层导度下降40%～60%。模拟与观测结果均表明,2003年7月下旬每天正午的波文比大都介于1～2.2,而2004年同期正午的波文比则介于0.2～0.6。EALCO模型通过Ball模型将植物碳水过程耦合在一起,从而可以很好的模拟植物的气孔行为,进而准确的模拟植物水热过程对干旱的响应。土壤水分匮乏对冠层导度的限制是2003年干旱期间冠层潜热通量模拟值下降的根本原因。

Use a simulation model to study the difference response of water and heat flux to drought in mid-subtropical planted forest

Seasonal drought frequently occurred in the mid-subtropical region of China.In situ measurements based on eddy covariance（EC） system showed that canopy latent heat flux was suppressed during the dry seasons at Qianyanzhou（QYZ） site.Drought-induced decrease in canopy latent heat flux is greatly challenged by terrestrial ecosystem models.In this paper,an ecosystem model EALCO（Ecological Assimilation of Climate and Land Observations） was used for simulating the seasonal drought effects on the fluxes of energy and heat by validating against the EC measurements.The results indicated that the EALCO model agreed well with the observed diurnal variations of net radiation（Rn）,sensible（H） and latent（LE） heat fluxes.The values of square of correlation coefficient（R2） between the simulated and observed half-hourly Rn,H,and LE are 0.99,0.81,and 0.90,and root mean square error（RMSE） are 8.05,41.02,and 31.49 W.m-2,respectively.At daily scale,87% of the variance of the observed evapotranspiration（ET） was explained by the model.Model analysis suggested that canopy and soil water potential during the extremely dry period（the last-ten days of July in 2003） were double decreased compared to the same period but with very slight drought in 2004,and the time lag between root water uptake and canopy transpiration during dry period was shortened about half hour,and the simulated canopy conductance decreased 40-60%.Both observed and simulated Bowen ratio at noon time during the dry period in 2003 ranged from 1 to 2.2 but only 0.2 to 0.6 during slight drought periods in 2004.The success of the EALCO model in simulating the response of water and heat fluxes to seasonal drought is attributed from the application of Ball-Berry stomatal conductance model which has high capacity to describe stomatal behavior by coupling plant carbon and water processes in a good manner.The decrease in the simulated canopy latent heat flux is explained by the soil water constraint on the canopy stomatal conductance during the dry period at QYZ site in 2003.