Thermally-Driven Exchange Flows in Regions of Vegetation

0509658 h。水温的空间异质性，以及因此而产生的密度，将驱动在表层水体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不同或光照穿透的不同引起的。例如，空间上均匀的热通量使浅层区域内的温度比邻近的深层区域内的温度上升和下降得更快。现在人们普遍认识到，由此产生的浮力驱动的沿海和远洋带之间的交换在确定湖泊尺度的化学和生态学方面起着重要作用。涌现和沉水植被的存在是沿海地区的共同特征，但只有少数研究考虑了植被对沿海-中海交换的影响。由于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显示了植被如何控制交换速率。此外，通过部分或完全遮蔽水柱区域，出现和淹没植被的存在可以产生温度梯度，从而驱动植被区域和开放水域之间的流动。由于不同的生物化学环境通常存在于植被带内，这些交换流在生物和化学上不同的地表水区域之间提供了重要的水动力联系。为了充分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。拟议中的项目将提供这种预测能力。设计了一系列的室内实验，以观察开阔水域和不同形态植被水域之间热驱动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，热穿透的深度将使用水色剂来阻挡更深入的穿透波长。研究结果将使研究人员和湖泊管理者能够更好地评估沿海地区的变化，例如由于土地开发，如何影响沿海和中上层地区之间的水动力联系，从而可能影响湖泊尺度的营养收支和生态。

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.