鄱阳湖湿地水汽通量格局及对水位变化的响应研究

Wetland ecosystem is distributed in the transition zone of water environment and land surface, and water and heat transmission are main manner of matter and energy exchange between wetland and its environment. The Poyang Lake wetland consisted of the Poyang Lake and its bottomland, and the bottomland significantly shows seasonal cycles of wet and dry phases due to fluctuation of water level. Evapotranspiration (ET) from the water and land surface is a crucial component of water and energy balance. Study of water and heat exchange pattern in Poyang Lake wetland are important to recognize and understand in the development and evolution of wetland, wetland ecological system, and the distribution and characteristics of regional water balance and climate change. However, limite to the observation and estimation method on tradition, it is difficult to obtain the accurate water evaporation and spatial distribution of wetland evapotranspiration. Therefore, it is unclear the temporal and spatial pattern of ET and the response mechanism of water level changes in the Poyang Lake wetland. In this project, (1) we apply the eddy covariance method to observe the water and heat flux between water and atmospheric in the Poyang Lake, and analyze the seasonal and annual variation of water surface evaporation of the Poyang Lake, and clarify the influence mechanism of climate elements and atmospheric turbulence condition on water evaporation; (2) we estimate the ET over the Poyang Lake region using surface temperature - vegetation index triangle method based on MODIS products data, and discuss the spatial and temporal variation characteristics of evapotranspiration in the Poyang Lake wetland and effect of environmental factors; (3) according to the character of dry-wet alternate frequently in the Poyang Lake wetland and bottomland, we extract exposed area and date of land surface, and research the response of ET on water level changes in the Poyang Lake wetland, and qualitatively evaluate the effect of climate change and water level change on the ET. These studies will enhance to clarity the temporal-spatial pattern of evapotranspiration in the Poyang Lake wetland, and reveal the its response mechanism of water level changes in alternating wet and dry wetland and beach. The project will lay the foundation for the study of wetland water balance and ecological system protection.

湿地是分布于水陆环境过渡地带的生态系统，水热交换是其与外界物质与能量交换的主要方式。鄱阳湖湿地受水位波动的影响，其洲滩呈现显著的季节性干湿交替现象。鄱阳湖湿地的水热通量格局对湿地生态系统的分布与特征、及区域水量平衡和气候变化具有重要的影响。然而，基于传统的观测和估算方法，难以获得水面实际蒸发量和湿地蒸散的空间分布，因此，尚不清楚鄱阳湖湿地蒸散变化的时空格局及对水位变化的响应机制。本项目基于涡度相关通量观测，分析鄱阳湖水面蒸发量的变化规律及影响机制；针对湿地洲滩季节性出露和干湿交替频繁的特点，利用遥感数据基于温度-植被指数三角法反演湿地蒸散，分析鄱阳湖湿地蒸散的时空变化特征及其影响要素；结合水位变化特征及气象要素，定量评价水位和气候变化对蒸散变化的影响分量。阐明鄱阳湖湿地蒸散的时空变化格局，揭示其对湿地洲滩出露和干湿交替变化的响应机制，为湖泊湿地水量平衡和生态系统保护研究奠定基础。

内陆水体的水热通量过程对区域乃至全球的气候变化具有重要的作用。季节性湖泊湿地的水热通量不但受到气象和环境要素的影响，还受到水文要素的影响。然而针对水面积和水位等水文要素对湖泊湿地水热通量如何影响仍不清楚。鄱阳湖是我国第一大淡水湖泊，其水位在年内和年际间波动剧烈，也导致淹没面积的巨大变化。如果不考虑湖泊水位的季节性变化，将给鄱阳湖水热通量的估算带来巨大误差。本研究基于涡度相关通量观测分析了不同时间尺度湖泊水体水热通量特征及其驱动因子，阐明了年际尺度上水文要素对水热通量的影响机制。应用遥感数据反演鄱阳湖湿地蒸散，阐明鄱阳湖湿地蒸散的时空变化格局，定量区分气象和水文要素变化对湖泊湿地蒸散的影响分量。研究结果如下：(1) 鄱阳湖湿地受到水位波动和下垫面变化的影响，湖泊湿地水热通量呈现两种不同的日变化特征，这一变化的阈值发生在水位12.6m水面积比例为40%时。(2)季节性湖泊湿地水热通量的驱动因子具有尺度依赖性，在日尺度和月尺度上，湖泊水体蒸发受到风速等动力学因素和辐射的影响，而在年尺度上，季节性湖泊则主要受到水位或水面积变化的影响。(3) 鄱阳湖蒸散的空间分布呈现湖心高周边低的格局。鄱阳湖湿地水面蒸发与湿地蒸散之差的年际变化，水位变化可以解释其变化的73%，其次是相对湿度，净辐射和风速。鄱阳湖湿地蒸散年内的空间分布与水位和湖泊的淹没面积呈正相关。因此，鄱阳湖的水位降低的趋势将可能减少湖泊蒸发，进而影响区域降水而加剧湖泊流域的水文干旱。