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Abstract：
Estuaries are a special transition zone subject to both riverine and marine processes, where environmental issues, e.g. water pollution, eutrophication and hypoxia, have become an increasing cause of concern in recent decades. The vertical transport of water and material is an intrinsic process in estuarine environments, with the atmosphere and seabed being the upper and lower boundaries. However, vertical water renewal in estuaries is not fully understood despite its significance to the estuarine environment being widely recognized. In the present study, the vertical water renewal process in a large estuary is investigated using the concept of water age. A three-dimensional water age model is built based on a hydrodynamic model, in which the age of a water parcel is defined as the time interval since it last touched the air-water interface, and thus indicates the renewal duration from the free surface. Water renewal durations, especially when relatively long, can provide insight into environmental and water quality issues, e.g. a low dissolved oxygen (DO), that can have a significant impact on ecosystem functioning. Results showed that the water age in the Pearl River Estuary (PRE) was characterized by significant spatial distributions and seasonal variations, which depends heavily on the water density stratification, as indicated by the Richardson number. During the wet season, the bottom water age was large at the lower reach of the estuary, up to 8 days, whereas the maximum bottom age during the dry season was ~1 day at the upper reach. Based on the quantification of vertical renewal, a new approach was proposed, and used to successfully evaluating DO depletion. The data and method would benefit for future environmental management, eco-biological restoration and related policy-making, especially when oxygen-based pollution is considered.
摘要：
河口是一个特殊的过渡带，受河流和海洋过程的影响，在环境问题上，例如水污染，富营养化和缺氧，近几十年来日益引起人们的关注。水和物质的垂直运输是河口环境中的固有过程，大气和海床是上下边界。然而,尽管对河口环境的重要性得到了广泛认可，但河口的垂直水更新尚未完全了解。在本研究中,利用水年龄的概念研究了大型河口的垂直水更新过程。基于水动力模型建立了三维水龄模型,其中水包的年龄定义为自上次接触空气-水界面以来的时间间隔，并因此指示从自由表面的更新持续时间。水更新持续时间，尤其是相对较长的时候，可以深入了解环境和水质问题，例如，低溶解氧(DO)，这可能会对生态系统功能产生重大影响。结果表明，珠江口（PRE）的水龄具有明显的空间分布和季节变化特征。这在很大程度上取决于水的密度分层，如理查森号码所示。在雨季，河口下游的底水年龄很大，长达8天,而旱季的最大底龄在上游约为1天。基于纵向更新的量化，提出了一种新的方法，并用于成功评估DO消耗。数据和方法将有利于未来的环境管理，生态生物恢复和相关政策制定，尤其是考虑到氧气污染时。
