Abstract：
The complex hydrological conditions caused by the backwater effect at the confluence inevitably modify the geochemical processes of elements. However, there is still a lack of comprehensive understanding regarding the precise transformation mechanisms of nutrients in large river systems. This study aimed to investigate the hydrodynamic characteristics and their impact on phosphorus transfer in the lower Han River, which is influenced by backwater from the Yangtze River (the largest river in China). By establishing a hydrodynamic-water quality model, we have determined that the discharge ratio (the ratio of flow between the Han River discharge and the Yangtze River discharge) can be utilized as a representative indicator of the backwater effect from the Yangtze River on the Han River. Three distinct patterns were identified in this study: mixing, backwater, and intrusion. The corresponding discharge ratio values were categorized as >0.08, 0.01∼0.08, and <0.01 respectively. Additionally, the extent of the backwater zone was determined, revealing that the length of the backwater zone increased from 50 km (XG) to 100 km (FS) as the discharge ratio decreased from 0.08 to 0.01. Furthermore, it was observed that the water level at the confluence rose from 2.52 m to 6.83 m in accordance with these changes in discharge ratio values. The migration pattern of phosphorus primarily involved the settling and retention of particulate phosphorus, particularly the labile particulate organic phosphorus (LOP) and dissolved organic phosphorus (DOP). When the confluent patterns became the intrusion pattern, the backwater zone expanded to 150 m (XT), causing a 10.40 m increase in water level at the confluence. An intrusion zone formed, and its phosphorus concentrations were same as Yangtze River\'s. Above the intrusion area, a backwater region formed and its concentrations of LOP and DOP decreased, while the concentration of PO43- increased due to the release from resuspended particles. This release was induced by higher velocity of bottom water brought about by the water exchange of two rivers. The discharge ratio of 0.01-0.08 resulted in the sedimentation of LOP and DOP, causing the lower Han River to act as a \"sink\" for phosphorus, potentially exacerbating phosphorus pollution. Higher discharge ratios in spring led to phosphorus release from sediment, increasing dissolved phosphorus concentrations and raising the risk of algal blooms in the lower Han River. These findings have significant implications for larger rivers worldwide and provide insights into strategies for ecological management and prevention of algal blooms.
摘要：
汇合处的回水效应引起的复杂水文条件不可避免地改变了元素的地球化学过程。然而,对于大型河流系统中养分的精确转化机制仍然缺乏全面的了解。本研究旨在研究汉江下游的水动力特征及其对磷迁移的影响。受长江（中国最大的河流）的回水影响。通过建立水动力-水质模型,我们已经确定，流量比（汉江流量与长江流量之间的流量比）可以用作长江对汉江的回水效应的代表性指标。在这项研究中确定了三种不同的模式：混合，回水,和入侵。相应的放电比值分别为>0.08、0.01～0.08和<0.01。此外,确定了回水区的范围，结果表明，随着排放比从0.08降低到0.01，回水区的长度从50km（XG）增加到100km（FS）。此外，据观察，根据排放比率值的这些变化，汇合处的水位从2.52米上升到6.83米。磷的迁移模式主要涉及颗粒磷的沉降和保留，特别是不稳定颗粒有机磷(LOP)和溶解有机磷(DOP)。当汇合模式变成入侵模式时，回水区扩大到150米(XT)，导致汇合处水位上升10.40米。形成了一个侵入区，其磷浓度与长江相同。在入侵区上方，形成了一个回水区，其LOP和DOP浓度下降，而PO43-的浓度由于从重悬浮颗粒的释放而增加。这种释放是由两条河流的水交换带来的底部水的更高速度引起的。0.01-0.08的排放比导致LOP和DOP的沉积，导致汉江下游充当磷的“水槽”，可能加剧磷污染。春季较高的排放率导致沉积物中磷的释放，增加溶解磷浓度，增加汉江下游藻华的风险。这些发现对全球较大的河流具有重要意义，并为生态管理和预防藻华提供了见解。
