Parameters Sensitivity Research Based On QUAL2K Water Quality Model

Pollutants into the river will dilute, diffuse,settle,degrade under the effect of river water power, the simulation and prediction of this process should establish the corresponding water quality response model according to the characteristics of river hydrology and characteristics of pollutants so that describe the transformation and migration of pollutants in water with time and space. Parameter setting and optimization is an essential part in the application of water quality model.But at present the international commonly used water quality model such as QUAL2K, WASP, MIKE, OTIS, most of the structure is complex and have many parameters, and many of the parameters are difficult to obtain, so it is difficult to use in developing countries and regions. Therefore, it is needed to consider the effect of the interaction between the parameters and the parameters on the uncertainty of the model output and the contribution of the model. Using sensitivity analysis to identify the important parameters of the output response in the model to reduce the the obtain work of parameters in the model and improve the check accuracy.In this paper, the mass transfer process of QUAL2K water quality model is as the breakthrough point., combined with the specific situation of river pollution in China, Finally, the sensitivity analysis of DO, ion ammonia nitrogen, CBODu, river water temperature as the research object was carried out, through the theoretical analysis get the relevance of indicators in the model. Combine the local sensitivity analysis method and global sensitivity analysis method so that understand the sensitivity of parameters of QUAL2K water quality comprehensively and systematically. Through sensitivity studies, the following conclusions are obtained:For the concentration of DO, the high sensitivity parameter is the temperature correction coefficient of the reaeration, and the sensitivity parameters are river reaeration coefficient r, nitrification of ammonia nitrogen temperature correction coefficient, Organic carbon oxidation coefficient, CBODf oxidation temperature correction coefficient, the Manning coefficient of the reach, nitrification of ammonia consumption coefficient of oxygen and ammonia nitrogen nitrification rate coefficient, CBODf oxidation rate, river water depth section coefficient a, river flow velocity section coefficient a, and have the following sequence: S(θa)> S(r)> S(θna) S(roc)> S(θdc)> S(n)> S(Kna)> S(ron)S(kdc)> S(a)> S(a) ;medium sensitive parameter:S(Cgb)>S(qON)>S(krb)>S(KLb)>S(Ksocf)>S(Ksona)>S(kdt). From the analysis of the sensitivity of the results can be seen:the effect of DO on the main process of reaeration in rivers, CBOD oxidation and nitrification of ammonia, bottom algal photosynthesis also has certain influence on oxygen on DO increase, But the effect of respiration on DO was relatively small.For the concentration of ammonia nitrogen in river, the high sensitive parameter is nitrification of ammonia nitrogen temperature correction coefficient; sensitive parameters are river source flow,river water depth section coefficient a,B point source sewage outfall flow,river flow velocity section coefficient a,nitrification rate coefficient of ammonia nitrogen,the medium sensitive parameters are hydrolysis rate of organic nitrogen,average concentration of ion ammonia nitrogen of B point source outfalls,temperature correction coefficient of the reaeration. And havefollowing order: S(θna)> S(Q0)> S(α)> S(QB)> S(a)> S(Kna)> S(khn)> S(CB[NH4-N))> S(θa) In the QUAL2K water quality model, the main effect of NH4 +-N are organic nitrogen hydrolysis and nitrification of ammonia process, but the bottom of algae and phytoplankton death, excretion, photosynthesis (NH4+as the matrix of nitrogen absorption) and phytoplankton, bottom algae growth on nitrogen absorption of NH4+ N effects is very small.In QUAL2K model, sensitive parameter for the CBODu (total carbonaceous BOD) concentration are organic carbon oxidation coefficient,CBODf Oxidation rate,river source flow,reaeration temperature correction coefficient,and have order: S(roc)> S(kdc)> S(Q0)> S(θa);the medium sensitive parameters are B point source sewage outfall flow, the maximum growth rate of bottom algae, Ksocf, nitrification temperature correction coefficient, manning coefficient of river, bottom algal respiration rate, bottom algae death rate,detritus dissolution rate, B point source CBODs concentration. And medium sensitive parameters have the following order: S(QB)> S(Cgb)> S(Ksocf)> S(θna)S(n)> S(krb)> S(kdb)> S(kdt)> S(CB(CBODs)) By comparing the effects of CBODs and CBODf oxidation rate on the concentration of CBODu, it can be seen that the effect of CBODs oxidation rate on theconcentration of CBODu is limited.Atmospheric turbidity coefficient and B point source outfalls average water temperature are sensitive parameters for water temperature.the medium sensitive parameters are atmospheric transmission coefficient, atmospheric temperature,river source flow.have the following order:S(nfac)> S(TB)> S(atc)> S(Tair)> S(Q0) Photosynthetic active radiation, background extinction coefficient, linear chlorophyll extinction coefficient alpha, nonlinear chlorophyll extinction coefficient alpha, ISS extinction coefficient, detritus extinction coefficient, sedimentary layer effective thickness, sediment thermal diffusion coefficient almost had no effect of river water temperature. The results show that the water temperature is more affected by the heat exchange on the surface of the water than the heat transfer between the bottom sediments and the water.Through the global sensitivity analysis on the medium sensitive and non sensitive parameters, application of QUAL2K model in the simulation of DO and CBODu two indicators, in the process of calibration and calibration of the model, a number of medium sensitive parameters can have big influence on the results, so the medium sensitivity parameters can not be ignored in the parameter calibration and calibration. In the global sensitivity analysis of the NH4+ -N insensitive parameters, the influence of the insensitive parameters on the NH4+-N concentration can be ignored.Based on the global sensitivity analysis of meteorological parameters, when domestic scholars use QUAL2K water model to predict the average temperature of a rive r, the atmospheric temperature in the meteorological parameter is a parameter that must be considered.when it is mainly used for the prediction of water quality, you can ignore the effects of meteorological factors on water quality.