Thermally-Driven Exchange Flows in Regions of Vegetation

0509658H。水温的空间异质性，从而密度，将驱动水平流动，在地表水体的不同化学区域之间携带通量。水温的空间梯度可能是由于水深的不同或透光率的不同而引起的。例如，空间上均匀的热流导致浅层区域内的温度比相邻的深层区域内的温度上升和下降得更快。现在人们已经认识到，由此产生的由浮力驱动的沿海和远洋带之间的交换在形成湖泊尺度的化学和生态方面起着重要作用。新兴和沉水水生植被的存在是沿海地区的一个共同特征，但只有少数研究考虑了植被对沿海-远洋交换的影响。因为植被提供了很大的阻力，它可以改变热驱动的交换电流。事实上，提案中提出的一项规模分析显示了植被如何控制汇率。此外，通过部分或完全遮盖水柱区域，新兴植被和淹没植被的存在都可以产生温度梯度，推动植被区域和开阔水域之间的流动。由于植被带内通常存在着独特的生物化学环境，这些交换流动为地表水的生物和化学不同区域之间提供了重要的水动力联系。要全面评估这些流动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。拟议的项目将提供这种预测能力。设计了一系列实验室实验，观察了开阔水域和不同形态植被水体之间热驱动交换流动的形成和大小。速度场将使用数字粒子成像测速仪(DPIV)进行测量。紧急植被和沉水植被都将被考虑。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。拟议的工作结果将使研究人员和湖泊管理者能够更好地评估滨海地带的变化，例如由于土地开发，可能如何影响滨海和远洋地区之间的水动力联系，从而可能影响湖泊规模的营养物质平衡和生态。

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.