大兴安岭地区林下入射辐射对积雪和冻土的影响研究

In forest areas, snow and permafrost are not only controlled by the climate, but also influenced by vegetation conditions through altering radiation balance, sensible/latent heat flux, soil moisture, and soil hydro-thermal properties. Incident radiation onto the ground or snow surface has direct and significant influences on the thermal states of snow and permafrost, and its heterogeneity explains much of heterogeneities in snow and permafrost under the forest. Therefore, accurate estimation of incident radiation and its heterogeneity is essential for quantitative analysis and simulation of snow and permafrost conditions under forest canopy. However, an exponential decay method based on the Beer-Lambert law was often used in most state-of-art land surface models to estimate the incident radiation, which cannot capture its heterogeneity under the canopy. In this project, new measuring methods such as hemispherical photography, ground-based laser radar, shadow and light spot analysis based on photography, and thermosensitive cable were used to collect data on canopy structure, incident radiation, snow depth, temperatures of ground surface and permafrost under a deciduous forest in the Da Xing’an Mountains. In combination with a famous simultaneous heat and water model (SHAW), we are trying to: 1) analyze the heterogeneities and their interactions of incident radiation, snow depth, ground surface temperatures and ground temperatures; 2) replace the Beer-Lambert based radiation transfer modular in SHAW with a new developed incident radiation estimation modular based on canopy structure extracted from hemispherical photo, and use the modified model to simulate the influences of forest vegetation and its change on the snow and permafrost under the forest.

除了受气候控制，森林地区的积雪和冻土还受植被影响。森林冠层通过控制林下辐射、感热、潜热、土壤水分及热力学性质而影响积雪和冻土，其中辐射的影响最直接。林下入射辐射的空间异质性是导致积雪和冻土空间差异的主要原因之一，对它进行准确估算是定量分析和模拟林下积雪和冻土的关键。然而，目前多数陆面过程模型使用基于Beer-Lambert定律的指数衰减方法描述冠层的辐射传输，不能捕捉林下入射辐射的空间异质性。本项目拟在东北大兴安岭地区落叶林内，利用鱼眼相机拍照、光斑和阴影拍照分析、激光雷达扫描和温感电缆等新手段观测森林冠层结构、林下辐射、积雪、地表和冻土温度，结合冻土水热(SHAW)：1）分析林下入射辐射、积雪和冻土空间异质性的相互影响及与冠层结构的关系；2）构建基于冠层结构的入射辐射模型，对SHAW模型中原辐射模块进行替换，然后模拟分析森林植被及变化对林下积雪和冻土的影响，为后续评价其环境效应提供基础。

森林冠层通过控制林下辐射、感热和潜热等影响林下积雪和冻土的状态，其中林下入射辐射的空间异质性是导致林下积雪和冻土空间差异的重要原因之一，它是定量分析和模拟林下积雪和冻土状态的关键要素。本项目按照申请书拟定的计划，在东北大兴安岭落叶林样地内，采用鱼眼相机、积雪花杆、地表温度计和多年冻土测温钻孔等工具和手段对森林冠层结构、积雪深度、地表温度和多年冻土温度开展了综合系统的观测，构建了基于冠层结构（鱼眼相片）的林下入射辐射估算模型，研发了基于光线追踪算法的考虑了山体阴影的山区辐射模型，并对寒区分布式冻土水文模型SHAWDHM进行了改进，分析了山区复杂地形条件下山体阴影的时空分布及其对高寒山区冻土水热过程的影响。结果表明，大兴安岭落叶林的冠层对林下入射辐射具有明显的消减作用，在枝繁叶茂的六七月份，林冠茂密的针混林下太阳总辐射的透过率低至0.29，全年平均太阳总辐射透过率仅有0.4左右，具有显著降低地表温度和保护多年冻土的作用。在地形复杂的山区，即采用1km的模型分辨率，与未考虑山体阴影的模型相比，在考虑了山体阴影的情况下，流域平均入射辐射减少14W/m2，蒸散发减少约18mm，4m深处平均的年均地温减小达0.4°C，模型模拟的多年冻土范围比不考虑山体阴影的模型大8%。在地形遮蔽严重的山谷区域，模型模拟的年平均地表入射辐射减少达50W/m2，年平均地温降低达2℃，蒸散发减小达50 mm，积雪期最大延长30天，积雪深度增加最大达10cm，对于山区复杂地形条件下的冻土水热过程的模拟和制图不容忽视。通过本项目的研究，初步回答了大兴安岭落叶林地区森林冠层对林下入射辐射的衰减作用及其对冻土温度的影响，并定量回答了山体阴影对冻土水热过程的影响，对将来定量分析局地因素对冻土水热过程的影响，以及冻土温度和分布的精细化模拟具有重要的意义。同时也为森林地区和山区复杂地形条件下的生态水文过程研究具有一定的支撑作用。