土壤热通量定量化及其对地表能量闭合的影响机制研究

The energy balances at most surface-atmosphere flux research sites remains unclosed, and the flux determination accuracy is limited. The soil surface heat flux is an important component of the surface energy balance equation. Most previous studies on soil surface heat flux used the heat flux plate or harmonic analysis methods, where the water vapor transport was disrupted or the thermal properties calculation was incorrect. The interior vapor transport beneath the soil surface was ignored. In this work, the dynamics of drying front and the evaporation rate in the soil will be analyzed based on the soil physics method. Then the soil sensible heat flux at layers with no water evaporation is determined. Combining it to the soil heat storage above this soil layer, the ground sensible heat flux calculation method will be established. Thus, the sensible heat flux and latent heat flux in the ground heat flux will be divided. Incorporating the ground sensible heat flux of this study into the turbulent fluxes from the eddy covariance measurement, the surface energy balance closure will be improved finally. Moreover, soil evaporation from the heat pulse method will be incorporated into the evapotranspiration data from the eddy covariance measurement, the evapotranspiration decomposition method will be illustrated. Our methods are useful to the surface energy balance ratio improvement and the gas exchange model in the soil surface.

地表能量不闭合问题是生态系统通量观测中普遍存在的现象，严重制约了通量数据的精度。地面热通量是能量平衡方程的重要组成部分，过去的研究大都使用热通量板法或者谐波法进行测定，但会阻碍水汽运动或者错误估计土壤热参数，且均忽略了土壤内部潜热传输。本项目拟从土壤物理学角度出发，通过深入探讨土壤内部的干燥锋推进动态，分析潜热蒸发位置与数值对地面热通量计算的影响；在此基础上，将无相变土层的感热通量与其上部的土壤分层热储量相结合，研究建立获取地面感热通量的新方法，实现地面热通量中的感热通量组分和潜热通量组分分解。将本研究发展的地面感热通量新计算方法，与野外通量观测数据相结合，可实现地表能量闭合度的改进。同时，依据涡度相关技术获取蒸散量，将基于热示踪法获取的土壤蒸发量引入，探讨将蒸散分解为蒸发和蒸腾的可靠性。研究结果不仅可以改进通量观测的能量闭合度，还有助于深入认识陆面地气交换过程。

土壤热通量是地表能量闭合的重要组分，但长期以来，各种生态系统通量观测的能量闭合难题一直存在。本研究基于梯度法首先对土壤分层热流通量进行了定量化，获取了土体内部的分层蒸发量和水汽流动态。以无相变深度处的土壤表观热通量测定值为基础，将其与该深度至地面间的热储量相结合，发展获取了一种排除潜热通量的地面纯传导热通量计算法。该方法避免了传统热通量板法对液水与水汽流场的阻断，同时通过对土体内部潜热通量的定量化，实现了地面热通量中传导组分与潜热组分的分离，可避免能量闭合计算时蒸发潜热的重复计算。结合半干旱草原涡度相关通量数据的计算结果表明，通量板法和新方法的白天能量闭合度分别为79.3%和87.7%，新方法显著改进了能量闭合度的计算。在此基础上，研究对土壤热传输过程中的液水流传热与水汽扩散传热贡献率进行了定量化。在控制土壤孔隙内水汽密度的水吸力研究方面，研究构建了土壤热导率与水吸力在中低含水量区域内的定量关系；而在高含水量区域，率先报道了并不存在一致的热导率-水吸力恒定关系，提出了土壤热导率-水吸力关系在该区域强烈依赖于土壤质地与矿物组成。本项目的研究成果可为各类型生态系统通量观测的能量闭合度计算与改进提供一个新方法，并为进一步地深化理解土壤水热耦合运移过程机理提供理论基础和参考。