Thermally-Driven Exchange Flows in Regions of Vegetation

植被区域中热驱动的交换流

• 批准号: 0509658
• 负责人: Heidi Nepf
• 金额: --
• 依托单位: Massachusetts Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-08-01 至 2010-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0509658&HistoricalAwards=false
• 关键词: Thermally; Driven; Exchange; Flows; Regions

0509658H. M. NepfSpatial heterogeneity in water temperature, and thus density, will drive horizontal flowsthat carry fluxes between chemically distinct regions of a surface water body. The spatialgradients in water temperature may arise from differences in water depth or differences inlight penetration. For example, a spatially uniform heat flux causes the temperaturewithin a shallow region to rise and fall more rapidly than an adjacent deep region. It isnow well recognized that the resulting buoyancy-driven exchange between the littoraland pelagic zones plays an important role in setting lake-scale chemistry and ecology.The presence of emergent and submerged aquatic vegetation is a common feature ofthe littoral region, yet only a handful of studies have considered the impact of vegetationon littoral-pelagic exchange. Because vegetation provides significant drag, it can modifythe thermally-driven exchange current. Indeed, a scale analysis presented in the proposalshows how vegetation can control the rates of exchange. In addition, by partially or fullyshading regions of the water column, the presence of both emergent and submergedvegetation can produce temperature gradients that drive flow between the vegetatedregion and the open water. Because a distinct bio-chemical environment often existswithin the vegetated zone, these exchange flows provide an important hydrodynamic linkbetween biologically and chemically distinct regions of surface water.To fully evaluate the impact of these flows on lake-scale chemistry and ecology, onemust be able to predict their frequency, duration and spatial footprint. The proposedproject will provide this predictive ability. A series of laboratory experiments have beendesigned to observe the formation and magnitude of thermally-driven exchange flowsbetween open water and water with vegetation of different morphology. The velocityfield will be measured using digital particle imaging velocimetry (DPIV). Both emergentand submerged vegetation canopies will be considered. Further, the depth of heatpenetration will be varied using water colorant to block more deeply penetratingwavelengths.The results of the proposed work will enable researchers and lake managers to makebetter evaluations of how changes in the littoral zone, e.g. due to land development, mayimpact the hydrodynamic link between the littoral and pelagic regions, and thus mayimpact the lake-scale nutrient budget and ecology.

0509658H M. Nepf水温的空间不均匀性，以及由此产生的密度，将驱动在地表水体的化学性质不同的区域之间携带通量的水平流动。水温的空间梯度可能是由水深的差异或光穿透的差异引起的。例如，空间上均匀的热通量导致浅区域内的温度比邻近的深区域更快地上升和下降。现在人们已经认识到，由此产生的浮力驱动的沿岸和远洋区之间的交换发挥了重要作用，在设置湖泊规模的化学和生态。存在的紧急和淹没的水生植被是一个共同的特点的沿岸的地区，但只有少数研究已经考虑到植被对沿岸-远洋交换的影响。因为植被提供了显著的阻力，它可以改变热驱动的交换电流。事实上，提案中提出的尺度分析表明了植被如何控制交换率。此外，通过部分或完全遮蔽水柱区域，挺水和沉水植被的存在可以产生温度梯度，驱动植被区域和开放水域之间的流动。由于植被带内通常存在着独特的生物化学环境，这些交换流为地表水生物和化学特征不同的区域提供了重要的水动力联系。为了全面评估这些流对湖泊尺度化学和生态的影响，必须能够预测它们的频率、持续时间和空间足迹。建议的项目将提供这种预测能力。本文设计了一系列的室内实验，观测了开放水体与不同形态植被水体之间热交换流的形成和大小。速度场将使用数字粒子成像测速仪（DPIV）进行测量。将考虑紧急和淹没植被冠层。此外，热渗透的深度将使用水着色剂来阻挡更深的穿透波长。拟议工作的结果将使研究人员和湖泊管理者能够更好地评估沿岸的区的变化，例如由于土地开发，可能影响沿岸的和远洋区域之间的水动力联系，从而可能影响湖泊尺度的营养收支和生态。