青海湖流域位于青藏高原东北部，是全球气候变化的敏感区和生态脆弱区。近几十年来，在气候变化和人类活动的共同影响下，青海湖水位下降明显，流域内草场退化严重，整个流域面临严峻的生态环境危机。地表能量和蒸散发是生态系统演变的关键驱动因子，对区域乃至全球气候具有重要影响，目前已成为全球气候变化研究的热点问题。青海湖流域内地形复杂，生态系统多样，不同生态系统地表能量收支和蒸散发差异明显。然而目前针对青海湖流域地表能量收支和蒸散发的实验观测研究还很少，还不能很好地理解青海湖流域不同生态系统对气候变化和人类活动的响应。本论文于2010~2014年采用波文比-能量平衡观测系统对比研究了青海湖流域嵩草草甸、金露梅灌丛、芨芨草草原和水柏枝灌丛生态系统的地表能量平衡各分量的动态变化特征；测算了不同生态系统蒸散发并分析其影响因素，确定了植被冠层截留占蒸散发的比例；对比分析了不同生态系统的土壤水分和土壤温度时空异质性以及冻融过程对于地表能量收支和蒸散发的影响。主要结果如下：（1）青海湖流域不同生态系统地表能量收支差异较大。全年显热占净辐射的比例为56~64%，潜热占净辐射的比例为35~45%。嵩草草甸显热占63%，潜热占43%，金露梅灌丛显热占56%，潜热占44%，芨芨草草原显热占64%，显热占38%，水柏枝灌丛显热占61%，潜热占41%。土壤热通量年均值都很小，占净辐射的-6~1%。总辐射、净辐射、土壤热通量和潜热通量年内变化为单峰过程，最大值出现在7、8月；而显热通量为双峰过程，在5月底和9月底左右各有一个峰值。潜热、土壤热通量与净辐射的比值年变化过程为单峰过程，7、8月潜热占净辐射的比例可达60~76%；显热通量与净辐射的比值则为“V”型。四个生态系统的年均波文比都大于1，说明显热是青海湖流域陆地地表能量支出的主要部分。在生长季（5~9月），嵩草草甸、金露梅灌丛和水柏枝灌丛的波文比小于1，潜热为同期地表能量输出的主要部分。波文比值在年内变化过程基本为“U”型。波文比与土壤水分具有负相关性。在生长季，金露梅灌丛土壤水分最高，波文比最低，芨芨草草原土壤水分最低，波文比最高，而嵩草草甸土壤水分和波文比都居中。在同一生态系统，月波文比与土壤水分也有较好的负相关性。（2）2012年7月至2013年6月期间，在青海湖流域中，金露梅灌丛和嵩草草甸的年蒸散发较高，分别为507.9mm和493.2mm，而芨芨草草原的蒸散发较低，为413.7mm。2011年和2012年水柏枝灌丛平均蒸散发为436.1mm。嵩草草甸的蒸散发为降雨量的84%，金露梅灌丛的蒸散发与降雨量大致相同；芨芨草草原和水柏枝灌丛的蒸散发为降雨量的126~134%。蒸散发年内变化为单峰曲线，生长季蒸散发占全年蒸散发的75%以上。冠层截留在嵩草草甸、金露梅灌丛和水柏枝灌丛占到同期蒸散发量的24%、36%和31%，是蒸散发的重要组成部分。青海湖流域高寒生态系统蒸散发的主要影响因素为净辐射（Rn）、土壤热通量（G）、温度要素和土壤水分。在生长季，Rn、G或Rn-G是四个生态系统日蒸散发最重要的影响因素；此外，土壤水分对芨芨草草原蒸散发也有重要影响，温度要素对其他三个生态系统生长季蒸散发也有重要影响。在全年，对嵩草草甸日蒸散发影响最大的要素是Rn-G，其次是水汽压；对金露梅灌丛和水柏枝灌丛日蒸散发影响最大的要素是温度，其次是Rn-G；在芨芨草草原对日蒸散发影响最大的要素是土壤水分，其次是Rn-G。（3）土壤温度和水分主要受到海拔高度、降雨、降雨入渗率、冻融过程、土壤性质的影响，在不同生态系统具有很大的时空异质性。在生长季，嵩草草甸、金露梅灌丛、芨芨草草原和水柏枝灌丛平均土壤水分分别为0.27±0.05 m3 m-3、0.30±0.08 m3 m-3、0.25±0.05 m3 m-3和0.16±0.02 m3 m-3；平均土壤温度分别为8.2±2.0℃、3.8±2.8℃、11.5±1.1℃和10.7±0.7℃。土壤冻融过程对地表能量收支和蒸散发具有重要影响。土壤冻结显著减少土壤中液态水，进而降低空气中水汽含量、蒸散发和潜热通量，使得波文比上升。春季解冻之后土壤液态水含量增加，进而增加空气中水汽含量、蒸散发和潜热通量，使得波文比下降。

Qinghai Lake watershed, located at the northeast of Qinghai-Tibet Plateau, is a sensitive and vulnerable region to global climate change. During recent decades, Qinghai Lake has experienced severe water level decline and native grassland in the watershed has been seriously degenerated due to climate change along with anthropogenic activities. The whole Qinghai Lake watershed is suffering serious ecological and environmental crisis. Surface energy flux and evapotranspiration (ET) play significant roles in local and even global climate and in ecosystem evolution, and have been thought as “hotspots” of global change research. However, observations of water and heat coupling in Qinghai Lake watershed are surprisingly limited. We measured and estimated surface energy flux over four typical ecosystems in Qinghai Lake watershed, including a Kobresia meadow, a Potentilla fruticosa shrub, an Achnatherum Splendens grassland and a Myricaria squamosa shrub using the Bowen ratio energy balance method (BREB) under quality controlling between 2010 and 2014; compared ET in different ecosystems and analyzed their influence factors and quantified the perentages of canopy interception to total ET; measured and analyzed the dymanic of soil temperature and soil water content (SWC) in different ecosystems and highlighted the influence of freeze-thaw circle on surface energy flux and ET. The results indicated that:Energy partitioning varied greatly in different ecosystems. Annual sensible heat occupied 56~64% of net radiation, lowest in Potentilla fruticosa shrub and highest in Achnatherum Splendens grassland. Annual latent heat occupied 35~45% of net radiation, lowest in Achnatherum Splendens grassland and highest in Potentilla fruticosa shrub. Annual mean soil heat flux was quite small, occupying about -6~1% of net radiation. The annual curves of daily total radiation, net radiation, soil heat flux and latent heat flux had single peaks which were in July or August. The annual curves of daily sensible heat flux performed double peak points within a year at the end of May and September. The percentages of latent heat and soil heat flux to net radiation changed following single peak curves, while the percentages of sensible heat flux to net radiation changed following “V” form curves.Annual Bowen ratios of all four ecosystems were larger than 1, which suggested that sensible heat flux was the main partitioning of land surface energy in Qinghai Lake watershed. But Bowen ratios in growing season (from May to September) were lower than 1 except for Achnatherum Splendens grassland, which revealed that latent heat flux dominated surface energy partitioning in growing season at the other three ecosystems. Bowen ratios were relatively higher in winter and spring and lower in summer and autumn, shaping “U” forms within a year. There was a good negative correlation between ecosystem Bowen ratios in the growing season and their SWC at Kobresia meadow, Potentilla fruticosa shrub and Achnatherum Splendens grassland. Similar negative correlations between monthly Bowen ratios and SWC could be found at all four ecosystems.Annual ET from July 2012 to June 2013 of Potentilla fruticosa shrub was as the highest as 507.9mm, and that of Kobresia meadow was 493.2mm, while that of Achnatherum Splendens grassland was 413.7mm. Mean annual ET of Myricaria squamosa shrub was 436.1mm. Annual ET of Kobresia meadow was 16% less than annual rain, that of Potentilla fruticosa shrub was similar to the annual rain; while annual ET of Achnatherum Splendens grassland and Myricaria squamosa shrub were 26~34% larger than annual rain. More than 75% of the annual ET happened in the growing season. Specifically, canopy interception occupied about 24%, 36% and 31% of annual ET at Kobresia meadow, Potentilla fruticosa and Myricaria squamosa shrubs, respectively, showing their importance to ET. Fluctuations of ET at alpine ecosystems of Qinghai Lake watershed were primarily drove by net radiation (Rn), soil heat flux (G), temperature or soil water content (SWC). In the growing season, Rn, G or Rn-G was the most important influence factors on daily ET at all four ecosystems, and other influencing factors were SWC at Achnatherum Splendens grassland and temperature variables at the other three ecosystems. In a whole year, Rn-G and vapor pressure explained most of the variation of daily ET at Kobresia meadow, and temperature variables and Rn-G explained most of the variation of daily ET at Potentilla fruticosa and Myricaria squamosa shrubs, while, SWC and Rn-G explained most of the variation of daily ET at Achnatherum Splendens grassland.Soil temperature and SWC had spatial and temporal variability in different ecosystems due to the differences in elevation, rain, infiltration ratio, freeze-thaw circle and soil property. Growing season soil temperature at Kobresia meadow, Potentilla fruticosa shrubs, Achnatherum Splendens grassland Myricaria squamosa shrubs were 8.2±2.0℃, 3.8±2.8℃, 11.5±1.1℃ and and 10.7±0.7℃, respectively; and SWC were 0.27±0.05 m3 m-3, 0.30±0.08 m3 m-3, 0.25±0.05 m3 m-3 and 0.16±0.02 m3 m-3, respectively.Freeze-thaw cycle had significant influence on surface energy flux and ET. Soil freezing reduced soil liquid water content sharply, and then decreased air humidity, ET and latent heat flux, which led to increase of Bowen ratios, while soil thawing had the contrary results.