海南南渡江下游二维水动力及水生态数值模拟

摘要

为了模拟河流水生态系统演变,探讨热带河流浮游植物和浮游动物生物量在流量影响下的沿程变化规律,在求解河流二维浅水方程的基础上,考虑浮游动物、浮游植物、悬浮碎屑、无机氮、无机磷等输运和演变子模型以及各子模型间的耦合作用机制,构建水生态动力数学模型,并将该模型应用于我国热带河流——海南南渡江下游龙塘坝至入海口段(长约28 km).数学模型采用ADI-QUICK格式差分方法对水动力学方程和对流扩散方程进行离散求解,进而驱动各类生物变量输运和演化.通过对河流水面线高程、营养盐含量、浮游生物生物量模拟值与实测值加以对比进行验证.实证结果表明,在生态参数合理率定前提下,忽略河流流量的时间变化,采用丰水期、平水期和枯水期工况下的代表流量加以概化,数值模拟仍能获得局部河段浮游生物生物量模拟值与实测值整体相符合的结果.浮游植物和浮游动物生物量均沿水流方向逐渐降低,但随流量的增大整体上呈降低趋势,丰水期上游浮游生物生物量为下游的1.6倍,枯水期达到9.3倍.受海水入侵、沿程污染排放等因素的影响,枯水期浮游生物生物量模拟值与实测值的偏差远大于丰水期.枯水期河道浮游植物和浮游动物生物量模拟值的最大值分别为77.71和38.56 mmol/m3(均以C计),约为丰水期的3.8倍,说明枯水期水质富营养化风险远高于丰水期.%In order to simulate the evolution of the aquatic ecology system and study the streamwise variation regulations of biomass of phytoplankton and zooplankton in a tropic river,a mathematical model for simulating aquatic ecology dynamics was established.The model solved the two-dimensional shallow water equations (SWEs)and incorporated sub-models for transport and evolution of phytoplankton,zooplankton,suspended clastic,inorganic nitrogen and inorganic phosphorus.Coupling mechanisms among sub-models were also studied.The model was applied to the Lower Reach of Nandujiang River,China,which is 28 km long from Longtang Dam to estuary.ADI-QUICK Finite Difference Scheme was adopted to discretize the hydrodynamic equations and advection-diffusion equations.The resultant flows were then used to drive the transport and evolution of ecology variables of different categories.The mathematical model was verified from water level,nutrient concentration and biomass against field survey data.Verifications showed that,provided with properly calibrated ecology parameters,the biomass distribution in a local reach predicted by the model agreed well with the survey data in general,even with idealized constant boundary conditions.Simulation results showed that the biomass of phytoplankton and zooplankton decreased downstream in the river and decreased with the lowering of discharge.The upstream biomass was 1.6 times that of downstream during flood discharge and 9.3 times that during drought discharge.Due to the neglected influence of seawater intrusion and pollutant discharges along the river,the simulations showed deviations from site measurement during drought period much larger than flood period.The maximum biomass of phytoplankton and zooplankton in drought period were 77.71 and 38.56 mmol/m3 respectively,which were around 3.8 times that of flood period,implying much higher risks of water eutrophication in drought seasons.