青藏高原典型高寒草原和草甸蒸散发对植被变化的响应研究

Alpine steppe and alpine meadow are two main land covers in the Tibetan Plateau. While satellite observations have identified dramatic changes in the vegetation leaf area index of these alpine ecosystems over past few decades, the impact of such changes on the surface evapotranspiration remains unclear. This is primarily due to paucity of observations for the two components of evapotranspiration, i.e. soil evaporation and plant transpiration. Here we propose to partition the evapotranspiration components of typical alpine steppe and meadow ecosystems by combining in-situ eddy covariance and micro-lysimeter observations at two field stations, Shuanghu and Naqu, in the Tibetan Plateau. By further investigating the dynamics of both soil evaporation and plant transpiration at different timescales, we aim to elucidate the ratio of transpiration to evapotranspiration and its environmental and biological controls over these two typical alpine ecosystems. Additionally, the Noah-MP land surface model would be used to simulate the evapotranspiration and its components after its roughness length and root water uptake parameterization schemes are locally improved. Afterwards, based on the sensitivity analyses using different leaf area index scenarios as model inputs, we aim to quantitatively show the response of evapotranspiration to the changes of vegetation growth in the typical alpine steppe and meadow ecosystems of Tibetan Plateau. This research would be of help for exploring the mechanisms of vegetation controls on the land surface evapotranspiration over Tibetan Plateau, thereby providing a scientific basis for understanding the interactions between hydrological cycle and alpine ecosystems in the Third Pole region.

青藏高原主要下垫面为高寒草原和高寒草甸。近几十年来，高寒草原和草甸的植被叶面积指数变化颇为显著，然其对地表蒸散发的影响仍不甚明了。显然，缺少详实的蒸散发分离资料是理解植被变化如何影响蒸散发的“瓶颈”。本项目拟基于典型高寒草原(双湖站)和高寒草甸(那曲站)地气相互作用观测台站，利用涡度相关系统与微型蒸渗仪的协同观测以分离蒸散发组分，阐明典型高寒草原和草甸的植被蒸腾和土壤蒸发在不同时间尺度的变化特征，探讨关键环境和生物要素对植被蒸腾占地表蒸散发之比例的影响机理。然后，基于陆面过程模型Noah-MP，改进粗糙度和根系水分传输参数化方案，模拟高寒草原和草甸的蒸散发及组分；并进一步结合不同叶面积指数情景下的模型敏感性实验，揭示高原两种典型下垫面地表蒸散发对植被变化的响应模式。本项目的开展将有助于理解植被变化对青藏高原地表蒸散发的调控机理，为探究第三极地区水循环与生态系统的相互作用机制提供科学依据。

青藏高原主要下垫面为高寒草原和高寒草甸。近几十年来，青藏高原植被状况变化显著，然其对地表蒸散发的影响仍不甚明了。本项目聚焦典型高寒草原和高寒草甸，结合观测和模拟，探索两种典型生态系统蒸散发对植被变化的响应规律，取得了如下成果：.(1) 联合涡度相关系统和微型蒸渗仪以及同位素手段，分别在色林错东岸典型高寒草原和那曲典型高寒草甸，开展了蒸散发组分分离的野外观测，发现高寒草原土壤蒸发随植被覆盖度增大而显著减少，提出了一个新的高寒草原土壤蒸发(Es)与地上生物量(B)的参数化方案：Es =0.89*EXP(−B/102.6)+1.5。那曲高寒草甸的观测表明，夏季的T/ET为0.43，且季节尺度上，土壤水分是影响高寒草甸植被蒸腾占地表蒸散发之比(T/ET)的主导因素，叶面积指数(LAI)次之。.(2) 结合遥感蒸散发模型，揭示了青藏高原高寒草原和草甸蒸散发及其组分的空间格局。在高原西部的高寒草原，T/ET一般小于10%，而在高原东部的部分高寒草甸，T/ET可达60%以上。在多年平均尺度上，地表植被盖度的多寡对青藏高原T/ET的空间格局有显著影响。就生态系统尺度的平均值而言，高寒草原和草甸的T/ET分别为14%和34%。.(3) 揭示了高寒草原和草甸蒸散发及其组分对植被变化的响应规律。当植被变绿时(即LAI增大)，两种生态系统的植被蒸腾都增大、土壤蒸发都减小，且前者增大的速率大于后者减小的速率，导致地表蒸散发亦增大。整体而言，当LAI增大50%时，两种生态系统地表蒸散发的相对变化几乎相等，分别增大5%和7%。但其蒸散发组分对植被变化的响应有所差别：LAI增大50%时，草原的植被蒸腾增大了49%；而草甸的植被蒸腾仅增大35%，即前者更为敏感。相反，高寒草甸的土壤蒸发对植被变化的响应较草原更为敏感。.(4) 发展了适用于资料稀缺区的免校正蒸散发模型，研制并公开共享了涵盖青藏高原在内的中国和全球的1982年以来的陆地蒸散发产品。基于新的蒸散发产品，发现全球陆地蒸散发在1982-2016年期间增大了 2.2%。区域尺度上，发现青藏高原多年平均蒸散发为378 mm yr-1, 相当于0.98万亿吨的水量。1982-2017年期间，青藏高原地表蒸散发以0.8 mm yr-2的趋势显著增大，尤以羌塘内流区为甚，其增大趋势达到了1.4 mm yr-2，指示着青藏高原水循环的加速。