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磷是生命体的必需元素,也是湖库富营养化的关键限制性因子,对浮游动植物、底栖动物群落健康等具有重要作用[1-7]。河流是磷等生源要素的陆海传输通道,长江作为亚洲第一大河流,其磷的时空输移特性对干流和中下游通江湖泊、东海的生态系统结构与功能都具有重要影响[8-11]。
长江从巴塘河口至宜宾“三江口” (金沙江、岷江和长江三江交界处) 为金沙江,宜宾以下称为长江。过去几十年,长江上游和金沙江系列控制性水库的陆续建成投运,极大地改变了天然河流的自然属性和作用过程[12-15]。在河流筑坝形成水库后,上游来水进入库区流速变缓,河流的携沙能力下降,强动力条件下的“河流侵蚀搬运作用”逐渐变成“湖泊沉积作用”[16]。库区水文情势的改变使库内颗粒态磷明显减少,并导致总磷浓度随之下降[17-19]。自2003年三峡大坝建成后,经过了135 m、156 m和175 m试验性蓄水后,于2011年进入高水位正常运用期。已有研究表明,三峡水库对磷存在一定的滞留效应,蓄水试运行阶段,库区干流断面年均总磷在2008年沿程呈明显降低趋势;尤其是2014—2017年金沙江下游溪洛渡和向家坝电站运行后,库区干流断面平均总磷下降45%[20]。
金沙江下游有4个呈串联形式的梯级水电站,从上游往下游依次为乌东德、白鹤滩、溪洛渡和向家坝水电站。向家坝和溪洛渡水电站分别于2012年和2013年投产,乌东德和白鹤滩水电站分别于2020年和2021年投产,其中白鹤滩是装机容量仅次于三峡的世界第二大水电站。梯级水电站拦截了金沙江大量泥沙,对库区水文和泥沙情势产生了较大影响[21-26]。上游来水泥沙量锐减导致三峡入库泥沙随之减少,而由于水体磷与泥沙密切相关[27-28],金沙江4座梯级水电站的运行是否对三峡库区总磷通量产生影响,值得关注与研究。目前,已有研究主要关注向家坝和溪洛渡水电站投产运行后对库区及下游总磷的影响[29-31],而乌东德和白鹤滩投产运行后的影响鲜有报道。
本研究拟通过对2016—2022年三峡库区及主要入库支流和金沙江下游总磷、流量监测成果进行分析,以揭示金沙江下游梯级水电站运行后三峡库区总磷通量的变化,为持续性观测梯级水电站与流域磷迁移的规律奠定基础,为后续评估金沙江下游梯级水电站运行后的水环境效应及流域水质安全保障提供参考。
金沙江梯级水电站运行后三峡库区总磷通量变化分析
Change of total phosphorus flux in the Three Gorges Reservoir area after the operation of the Jinsha river cascade hydropower station
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摘要: 金沙江下游从上往下依次建设有乌东德、白鹤滩、溪洛渡和向家坝4个呈串联形式的梯级水电站,这4座大型水利工程的建设运行对下游水文情势产生了重要影响,并随之影响磷的迁移过程。基于4座梯级水电站的建设运行特点,将研究时间划分为2座电站运行阶段 (2016—2019年) 和4座电站运行阶段 (2020—2022年) 。通过分析2个阶段金沙江下游攀枝花至三峡出库南津关干流段监测站位总磷、流量和通量的变化情况,阐明金沙江下游梯级水电站运行对三峡库区总磷通量的影响程度。除嘉陵江汇入断面外,其余断面在4座电站运行阶段总磷浓度均有所降低。4座水电站的运行使得梯级水电站的调蓄作用进一步加强,但三峡入库流量波动在4座电站运行阶段并未变小,支流汇入对三峡入库流量的影响不容忽视。除沱江和嘉陵江外,其余断面总磷年均通量在4座电站运行阶段均出现一定程度的降低,沱江在2个阶段的总磷年均通量变化不大,嘉陵江在4座电站运行阶段的总磷年均通量较两座电站运行阶段增加了52%。向家坝出库对三峡入库总磷通量的贡献率由2座电站运行阶段的31%降低到4座电站运行阶段的26%,需持续关注金沙江梯级水电站的源汇作用和其对三峡入库总磷的影响。该研究结果可为持续性观测梯级水电站与流域磷迁移的规律提供参考。Abstract: In the downstream of Jinsha River, four cascade hydropower stations were constructed successively, namely Wudongde, Baihetan, Xiluodu and Xiangjiaba. The construction and operation of these four large-scale hydropower projects had an important impact on the downstream hydrological situation, and in turn affected the migration process of phosphorus. Based on the construction and operation characteristics of four cascade hydropower stations, the research period was divided into two hydropower stations operation stage (2016-2019) and hydropower stations operation stage (2020-2022). By analyzing the changes of total phosphorus (TP) concentration, flow and flux at monitoring stations from Panzhihua to Nanjiguan section in the two stages, the influence of cascade hydropower station on TP flux in the Three Gorges reservoir area was illustrated. Except for the Jialing River inlet section, the TP concentration of the other sections decreased during the four hydropower stations operation stage. The operation of the four hydropower stations further strengthened the regulating effect of the cascade hydropower stations, but the fluctuation of the inflow flow of the Three Gorges is not reduced during the four hydropower stations operation stage, which meant that the influence of the inflow flow of the Three Gorges could not be ignored. Except for the Tuojiang River and Jialing River, the average annual TP flux of the other sections decreased to a certain extent during the four hydropower stations operation stage. The average annual TP flux of the Tuo River in the two hydropower stations operation stage did not change significantly, and the average annual TP flux of the Jialing River in the four hydropower stations operation stage increased by 52% compared with that in the two hydropower stations operation stage. The contribution of Xiangjiaba TP discharge to the TP flux of the Three Gorges reservoir decreased from 31% in the two hydropower stations operation stage to 26% in the four hydropower stations operation stage. It is necessary to pay attention to the source and sink effect of the cascade hydropower station on the Jinsha River and its influence on the TP of the Three Gorges Reservoir. This result can provide a reference for continuous observation of the phosphorus migration in Changjiang river basins after the operation of the cascade hydropower stations.
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表 1 金沙江下游4座梯级水电站及三峡水电站的主要指标特点
Table 1. The main index characteristics of four cascade hydropower stations in the lower reaches of Jinsha River and the Three Gorges Hydropower Station
水电站 控制流域
面积/km2多年平均
流量/ (m3·s−1)水库回水
长度/km正常蓄
水位/m多年平均
来沙量/t多年平均
含沙量/ (kg·m−3)开发建设阶段 乌东德 4.06×105 3 830 201 975 — — 已全部投产 白鹤滩 4.30×105 4 170 183 825 — — 2022年底全部投产 溪洛渡 4.54×105 4 570 194 600 — — 安全运行9年 向家坝 4.59×105 4 570 157 380 2.23×108 1.54 安全运行10年 三 峡 1.01×106 13 600 667 175 3.97×108 0.92 安全运行19年 表 2 总磷浓度与浊度相关性分析
Table 2. Correlation analysis between TP and turbidity
相关性指标 龙洞 三块石 朱沱 南津关 回归方程 y = 1 354x − 2.8 y = −247x+17.0 y = 1 791x − 61.8 y = 1 217x − 47.4 R2 0.72 0.06 0.61 0.45 相关系数 0.566** −0.118 0.601** 0.270 -
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