呼伦湖流域蒸散对区域气候暖干化响应机制及其对湖泊水情影响

Catchment water balance change is the fundamental driving force of lake hydrological regimes in arid and semi-arid regions. Among which the response of evapotranspiration (ET) on the regional climate change and possible influence on water balance is still remain further investigation. Hulun Lake, as the largest freshwater lakes in Northern China, plays an important role in maintaining regional ecological security. The abrupt lake water level decrease has resulted in the continuous deterioration of wetland environments and ecology. The hydrological controversy around Hulun Lake is not only to climate change, but also relates to the regional ecological security. The problem has attracted attentions from different communities. In view of the variation of ET caused by water supply conditions and potential evapotranspiration, and its possible influence on catchment water balance, this study: (1) to reveal the mechanism of how actual ET response to the regional environmental restrict factors like, fast change in soil moisture and large temperature difference between day and night time, by high frequency field observation on ET and other environmental parameters; (2) to correct the remote sensing retrieved ET data based on the field monitoring data, and rebuild the catchment spatial and temporal ET dataset; (3) to reveal the spatial-temporal patterns by EOF method based on the corrected remote sensing retrieved data; and (4) to reveal the impacts on lake hydrology from ET dynamics based on the lake balance and the lake level-volume-area relationship. The study is expected to provide some reference for a better understanding on the ET response to regional climate changes and the study on lake-catchment interactions.

流域水量平衡变化是干旱半干旱区湖泊水情变化的根本原因，而蒸散对区域气候的响应机制及影响是流域水量平衡中有待深入研究的重要方面。呼伦湖作为我国北方第一大淡水湖，在维系区域生态安全方面发挥着重要作用。2000年以来，持续的水位下降导致了湖泊环境和生态急剧退化引起广泛关注。鉴于在流域水量平衡中蒸散变化及其可能影响，本项研究（1）针对供水条件和蒸散能力两个影响流域实际蒸散的关键要素，通过野外实验高频观测，研究在下垫面干湿快速变化、昼夜温差大等条件下蒸散响应机制；（2）利用野外实验观测获取的规律认识，对遥感蒸散反演数据进行修正，获取更能反映流域实际蒸散的长序列时空数据；（3）基于以上数据，利用EOF等统计方法揭示流域蒸散时空变化特征；（4）结合呼伦湖湖盆结构和湖泊水量平衡其它要素变化，揭示流域蒸散变化对呼伦湖水情变化影响。为深入理解流域蒸散对区域气候变化响应机制和干旱区湖泊水情响应提供参考。

针对北方最大淡水湖呼伦湖2000年前后急剧水位变化，考虑到研究区下垫面快速干湿变化和昼夜温差大等自然条件，基于原位观测获取的高频观测数据及其机制性变化的认识，在对遥感反演降水和蒸散进行修订的基础上，结合分布式水文模型和经验函数正交分解等方法，揭示了湖泊极端水情多尺度形成机制。降水模式的时空分析表明：控制呼伦湖流域降水的是东亚季风和西风环流相互作用的两个主要模式。其中模式1高降水中心位于流域两侧山区；而模式2的高降水中心则位于流域中部地势平坦的区域。当降水模式1较强的年份，流域总降水量及径流较丰；而当降水模式2较强的时候，流域总降水和径流则明显偏枯。但2000年后模式强弱变化使流域总降水量减少幅度只有10%左右，远小于同期地表径流量超过50%的降幅。结合降水模式变化对蒸散的影响发现，2000年后，模式1变弱、模式2加强使更多的降水发生在流域中部平原区，形成了不利产流，而有利蒸散的格局，流域蒸散占降水比重显著增加。降水减少加之蒸散增强是导致流域及湖泊水量匮缺根本原因。遥感反演蒸散强度及趋势的特征确认了降水模式转换的蒸散效应，大尺度气压场及水汽输送的变化佐证了降水模式强度转换的动力机制。针对ET/P与湖泊水情变化的相位差异，发现了中高纬多年冻土区湖泊水文响应对温度和降水变化的滞后性。2000年前，温升导致流域冻土不断退化，退化的融水补给了河川径流。随着冻土区快速北移，失去冻土退化融水补给是2000年后呼伦湖发生急剧水位下降重要推动原因。