The filter effect of big reservoirs on dissolved silicate flux decrease in the Yangtze River drainage basin

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根据1960-2000年间长江大通水文站记录的水、沙以及硅、氮、磷等数据,结合1954-2006年间长江流域库容大于10⁸ m³的162座水库的库容、上游径流量、总磷等数据,利用Vollenweider模型分析了水库过滤器效应对入海溶解硅(DSi)通量的影响.结果表明:1)1990s相比1960s,长江入海DSi通量下降了约1.85×10⁶ t/a,减少了大约25.3%;入海DSi通量的下降与流域径流波动、入海悬沙通量下降以及溶解无机氮通量的增加紧密相关.2)流域水库明显改变径流的自然过程,增加径流的滞留时间,流域90%以上的水库对上游径流的滞留时间超过0.05年,水库产生显著的DSi生物过滤器效应.模型计算显示流域大型水库对DSi的累计滞留量可达0.85×10⁶ t/a,占年均入海DSi通量(1990-2000年)5.4×10⁶ t的15.7%,是入海DSi通量减少量(1.85×10⁶ t/a)的45.9%.3)根据保守估计,流域162座大型水库内泥沙累计淤积量达6.75×10⁸ t/a,不仅使悬沙入海通量显著下降,而且造成大量颗粒吸附的外源和内源DSi颗粒沉淀,这对入海DSi通量减少也起到重要贡献.但目前对水库泥沙过滤器的滞留机理并不清楚,需要展开进一步的研究.;Correlation method based on the historical hydrology(1960-2000) of Datong station and famous Vollenweider model based on historical dam capacity, upstream discharge and total phosphorus records(1950-2006) were used to examine the filter effect of big reservoirs on dissolved silicate(DSi) flux decrease in the Yangtze River drainage basin. The historical hydrological data of Datong station showed that the flux of DSi into East China Sea obviously decreased during 1960-2000, and the net decrease flux was 1.85×10⁶ t/a, accounting for 25.3% of annual DSi flux in 1960s, the decreasing process of DSi flux was closely related to the processes of suspend sediment flux decreasing, dissolved inorganic nitrogen(DIN) flux increasing. The statistics result of reservoirs showed that the total capacity of 162 reservoirs reached to 155.4×10⁹ m³, accounting for 16.8% of annual discharge to sea; furthermore, the 54 super-big reservoirs with capacity volume > 5×10⁹ m³ including the Three Gorges Reservior, the Danjiangkou dam could contribute 87% of the total capacity of 162 reservoirs. In addition, the resident time of runoff in reservoirs was more than 0.05 year for > 90% big reservoirs. The result of the Vollenweider model showed that biological and sediment filter effects could occur in big reservoirs. First, the retention of DSi in 162 big reservoirs reached to 0.85×10⁶ t/a, accounted for 45.9% of the DSi net decreasing flux of 1960-2000, and 15.7% of the annual DSi flux into sea of 1990-2000. It suggest the mechanism of biological filter is insufficient for explaining the DSi retention process behind reservoirs, we need to develop new mechanistic models for the retention of DSi by reservoirs. Second, the result of sediment filter showed that retention of sediment in 162 big reservoirs reached to 6.75×10⁸ t/a, far more than the suspend sediment flux into sea of 3.41×10⁸ t/a of 1990-2000, which implicated that other decreasing DSi flux maybe primarily due to the sediment filter effect.