Parameter Variation And Flow Composition Of Hydrological Simulation Swat Model In Hanjiang River Basin

Using hydrological model to simulate watershed hydrology is an effective means and common tool for understanding watershed hydrological cycles and for water resources and flood management.Many parameters are introduced into the hydrological model.Some parameters can be deduced directly by the physical conditions of the basin.However,many parameters need to be calibrated by using hydrological observation data.Because of the existence of various errors(including model structural errors,input data errors and the incompleteness of solving methods),the uncertainty of parameter values can not be avoided either in the estimation or in the calibration.SWAT model is developed by the Agricultural Research Institute of the United States Department of Agriculture.It is a semi-distributed hydrological model for agricultural management based on a large number of experimental data in the United States and database in the United States.It is also one of the most widely used hydrological models in the world,including hydrological simulation in China.Taking the Hanjiang River Basin as the research object,according to the distribution of nine hydrological stations in the basin and precipitation data of 193 rainfall stations in the basin,the daily runoff process of nine hydrological stations from 1981 to 1988 is simulated by SWAT model.The relationship between hydrological parameters of SWAT model and natural geographical conditions,the influence of sub-basin division threshold on the parameters,the variation of the parameters between among nested basins,and the composition of runoff were discussed in this articel.Main research work and achievements were introduced as follows.(1)Daily hydrological simulation of SWAT model in Hanjiang river basin: The Nash coefficients of hydrological simulation in nine sub-basins during the period of calibration and validation are more than 50%,and the errors between simulated and measured runoff(Vol)are within ±20%.The sub-basin with smaller catchment area in the upstreams,NS is lower and error of Vol is higher,while with larger catchment area in the downstreams,NS is higher and error of Vol is lower.For the same basin,different catchment area thresholds are set to generate different river network densities,but their effects on model parameters and hydrological simulation results are small.(2)The parameter variation of SWAT model in Hanjiang river basin:(1)The runoff parameter CN increases and decreases regularly with the change of rainfall.The CN value increases with the increase of rainoff during the flood season,while decreases with the decrease of rainfall in dry season.During the rainy season,the CN value is at a high level and the magnitude of the change is small,so the difference in CN value mainly occurs in the dry season.The CN value differs between the calibration of independent watershed and nested watershed.This is because the parameters obtained from the calibration of the hydrological data at the outlet of the overlapping basin essentially reflect the average hydrological characteristics of the overlapping basin.Using hydrological data at one outlet to calibrate parameters of SWAT,can not reflect the hydrological characteristics of different locations in the basin.(2)According to the recommended parameter calibration range,the main channel permeability coefficient CH?K2 is calibrated,but the calibration value is close to the upper limit of the calibration range,and the larger the permeability coefficient is downstream.Therefore,the calibration range of CH?K2 is enlarged correspondingly.The re-calibration discovers that with the increase of the calibration range of CH?K2,the calibration value of CH?K2 increases correspondingly,as well as,the change of the calibration value of CH?K2 in individual sub-basins tend to be stable,and in most sub-basins varies greatly and irregularly.(3)The Manning roughness coefficient CH?N2 of the Hanjiang River basin simulated by SWAT model is larger than of the actual river channel,and the calibration value of the nested basin is larger than that of the single basin.This is because the SWAT model only uses the main channel of the basin to summarize the confluence of the whole basin.The roughness of the distribution in the basin is collected into the main channel of the basin,and the larger the basin,the more concentrated the collection.Therefore,only by increasing the roughness value of the main channel can the water flow resistance of the whole basin be summarized.(4)According to the principle of water balance,in order to maintain the balance of water flow in the user's output,the SWAT model adds the final amount of water infiltrated into the outflow and flows out of the river.This is the expedient method adopted by SWAT model in pursuit of high NS value.In the absence of channel infiltration,for the hydrological simulation of the Hanjiang River Basin,the NS coefficient is less than 0,the effect is very poor.(5)Because the model takes the measured discharge of hydrological stations on the river course as the standard and uses NS coefficient as the index to calibrate parameters,the final runoff yield of the whole basin is constrained.The change of parameters is reflected in the distribution of runoff composition,as CH?K2 increases,the proportion of surface runoff increases,and the proportion of soil flow and underground runoff below the surface decreases.(6)The NS and Vol change in overlapping basins when different basins are integrated with each other.This is because the calibration target is the discharge at the outlet of different hydrological stations and the area of rainwater harvesting is different,which will form a set of different parameters which are related to the geographic conditions within the basin.The parameter calibration follows the mathematical formula while ignoring the physical meaning,which leads to the different results of the calibration based on different rainfall catchment areas.(3)The flow composition of SWAT model in Hanjiang River Basin:(1)The base flow is related to the topography of the basin.Upstream water is collected to downstream,which results the runoff yield increases of downstream with the increase of base flow.The application of digital filtering method to segment the measured runoff shows that the base flow of downstream increases,which also conforms to the topographic characteristics of the Hanjiang River Basin.However,the hydrological simulation using SWAT model shows that the base flow of upstream is larger than downstream's.This is because the calibration process of the model depends on mathematical calculation,ignoring the interaction of hydrological processes in physical space and time,so it will lead to the lack of physical laws.(2)The base flow process of the SWAT model is smoothe,but when water recession occurs,the base flow rate is higher than the observed runoff,and the base flow ratio(BFI)calculated by the SWAT model during the flood seacon is higher than that by the filtering method,and in dry season is opposite.This is because when the SWAT model performs parameter calibration,multiple parameters will affect each other,and different parameters will have the phenomenon of "different parameters,same effects".