陆地生态系统碳水通量贡献区评价综述

综述了通量贡献区研究的基本理论、最新进展、研究热点与难点,旨在促进中国区域碳水通量数据空间代表性的定量评价.通量贡献区是通量观测点上风向的空间代表区域,能够反映代表区域对应下垫面的源区内每一点对观测点的通量贡献权重影响,主要受观测高度、空气动力学粗糙度和大气稳定度等因素的影响.通量贡献区通常随着观测高度的增加、空气动力学粗糙度的降低和大气稳定度的增加而变大,反之则变小.通量贡献区的评价模型包括解析模型、拉格朗日随机模型、大涡模拟和闭合模型四类.通量贡献区的评价结果可以广泛应用于通量数据质量评价、实验设计的指导、与遥感技术结合的区域尺度的总初级生产力的估算、城市CO2通量变化的评估以及能量闭合的评价等研究.最新研究表明,对流边界层的通量贡献区存在负的通量贡献区域；有裸地存在的情况下解析模型通常会低估裸地对观测通量的贡献；与水平地面处的通量贡献区相比,山谷处通量贡献区变小而山脊处的通量贡献区变大.通量贡献区模型研究应进一步考虑大气中的平流效应、湍流的非高斯扩散以及建立冠层内部的通量贡献区模型.解决森林冠层内流场的不均匀性、冠层重叠问题、冠层湍流的不稳定性是建立适合冠层内部通量贡献区模型的前提条件.在理想条件的气体释放验证试验的基础上,需要开展复杂条件下的相关试验.

Flux footprint of carbon dioxide and vapor exchange over the terrestrial ecosystem: a review

The eddy covariance technique is a micrometeorological method to directly measure the exchanges of carbon, water and energy between the vegetation and atmosphere. The spatial resolution of meteorological observation of fluxes can expand from tens of meters to kilometers. The eddy covariance method is most accurate when the contributing area of the fluxes is topographically flat, and vegetation extends uniformly within the footprint area. Currently there are more than 100 eddy covariance flux observation sites in China. Most of them are established in non-ideal conditions such as forest, undulating surface, patchy canopy area. Therefore, it is important to accurately interpret the ecological representativeness of flux data by evaluation of the spatial representativeness of its footprint in China. This paper reviews basic theories of the footprint, along with progress and applications about footprint functions. It discusses the research focus and difficulties when considering the development of footprint. The footprint of a measurement point is the influence of the properties of the upwind source area weighted by the footprint function. The major effects on the dimensions of the flux footprint are measurement height, surface roughness length, and atmospheric stability. Increase in measurement height, decrease in surface roughness, and change in atmospheric stability from unstable to stable would enlarge the footprint size and move the peak contribution away from the instrument point. The opposite is also true. Footprint functions can be classified into four categories: Analytical model, Lagrangian stochastic model, Large eddy simulation, and Closure model. The footprint result can be applied to experimental design and to evaluate the quality of CO₂ flux data, the variation of CO₂ flux in urban areas, surface energy balance closure and gross primary productivity of landscape scales or regional scales combined with remote sensing. The latest research shows that there is a negative footprint zone in the convective boundary layer and that the location of the footprint peak is closer to the tower for convergent surface flow than for horizontally homogeneous flow. This is reversed for divergent surface flow. Atmospheric advection and non-Gaussian diffusion should be taken into account when building footprint functions. It is necessary for footprint functions within forested areas to consider spatial heterogeneity, clumpiness of vegetation, and instationarity of canopy layer turbulence. Analogical experiments should be implemented in complex terrain, based on the tracer release experiment in ideal conditions.