干旱区绿洲-荒漠过渡带能水交换及其组分拆分研究

Oasis-desert ecotone plays a critical and unique role as shelterbelt of ecological function in maintaining oasis stability and development in arid regions of northwestern China. Knowledge of water and energy exchanges is essential to understanding deeply the maintaining mechanisms for vegetation exists in the oasis-desert ecotone. The comprehensive and elaborated experiments on micrometeorology, water and heat fluxes, evapotranspiration, rainfall partition, land-surface temperature, plant physiology and ecology etc. will be synchronously conducted in a representative area of Linze oasis-desert ecotone in the middle parts of Hexi Corridor, northwest China. The energy and water exchanges and their components for vegetation, soil under vegetation, and bare soil sources/sinks on the patch, fetch area, and landscape scales will consequently be quantified using the coupled three-source model in combination with experimental data. The main aims of proposed project are, (1) to reveal the effect of vegetation on processes of radiation transfer/budget, heat exchange/partition, water transport/feedback on different spatial scales; (2) to quantify the relative contributions of precipitation and groundwater to maintaining vegetation stability and local water and energy budgets in the ecotone; (3) to illuminate the energy and water balance conditions under which vegetation can sustain stability of ecotone, as well as the mechanism of environmental controls on energy and water budgets. All these results from the proposed project are expected to improve greatly our knowledge for understanding of water and energy exchanges in the oasis-desert ecotone, which will lead to a new advance in estimating surface-flux aggregation over the sparsely-vegetated surface in arid regions.

西北干旱区绿洲-荒漠过渡带是维护绿洲稳定发展的唯一生态屏障，认识过渡带能量和水分交换过程是深入理解其植被稳定性维持机理的基础。本项目拟选择河西走廊中段临泽绿洲-荒漠过渡带代表性区域为试验场，同步开展微气象、水热通量、蒸散发、降水再分配、表面温度、植物生理生态等综合精细化观测试验，结合水热交换的三源耦合模型原理，研究斑块、风浪区和景观尺度上植被冠层、冠层下方土壤和裸地土壤三个源/汇的能水交换过程并确定各组分之间的相互关系。主要目标是：阐明植被在过渡带辐射传输与平衡、热量交换与分配、水分输送与反馈过程中的作用与尺度效应；确定降水和地下水在维系过渡带植被稳定和能水平衡中相对贡献；揭示绿洲-荒漠过渡带植被稳定性维持的能水平衡条件及其环境调控机制。有望在认识干旱区绿洲-荒漠过渡带的能水交换过程方面取得新的进展。在干旱区异质稀疏植被覆盖条件下水热通量聚集的测算方法上具有创新。

本项目基于微气象、湍流交换、植物蒸腾、土壤蒸发、植被冠层降雨再分配等同步观测试验数据，结合陆地生态系统能水交换物理模型，测算了河西走廊中部临泽绿洲-荒漠过渡带的辐射传输、热量交换、水分输送过程，阐明了该过渡带能水平衡格局及其植物与环境调控机理。主要研究结果包括：①过渡带不同植被覆盖对表面辐射平衡产生了重要影响，梭梭覆盖区净辐射（639 W/m2）显著高于泡泡刺覆盖区（555 W/m2）和裸地（495 W/m2）；②植被覆盖对能量平衡分配格局的调节作用明显，感热交换主导梭梭覆盖区能量交换（占净辐射的50%左右），而泡泡刺覆盖区和裸土区则以土壤热传导为主；③提出了合理估算了研究区大气蒸发力的计算方法，将研究区多年平均潜在蒸发量更新为1200 mm/a左右；④得到了能够合理模拟过渡带三种主要植物气孔导度动态变化的数学表达式，并应用于复杂植被下垫面的表面导度和能水交换的估算；⑤详细测算了过渡带不同植被覆盖对贯穿降雨量、树干茎流和冠层截留蒸发损失的调节作用，大约21%的降雨量被冠层截留蒸发损失；⑥基于不同尺度水量平衡测算结果，发现该过渡带是一个典型的地下水依赖生态系统，地下水提供了其植物水分利用总量的近50%；⑦降水脉冲显著改变了局地辐射和能量平衡分配格局，降雨后短波向下辐射增加是净辐射增加的主要原因，波文比从3.8减小到1.4；⑧揭示了过渡带能水交换过程的环境与植物控制机理，水热交换过程与大气高度耦合（解耦系数平均为0.1），环境和植物对能水交换过程的调控贡献分别为40%和60%左右。本项目在稀疏植被冠层截留蒸发、复杂植被表面导度和异质稀疏植被下垫面能水交换过程的测算方面取得了新进展，研究成果增进了我们对干旱区绿洲-荒漠过渡带能水平衡分配格局及其维持机制的更深认识，为绿洲水土资源合理利用决策和过渡带防护林体系建设与管理提供了关键数据支撑。