Improvements Of SWAT Model-Building Procedure And The Simulation Of Non-Point Source Pollution In Hanjiang River Basin

As non-point source pollution is difficult to monitor and control,numerical modeling has become an important way to study on it,and it is valuable to investigate approaches to improve the accuracy of non-point source pollution modeling.The mechanism of non-point source pollution to take shape and transfer is affected by multiple factors including landscape pattern,so investigating the contribution of landscape pattern to non-point source pollution leads to a better understanding of the mechanism of non-point source pollution.Besides,designing and assessing best management practices for controlling watershed non-point source pollution is helpful for water environment protection.In this study,geographical and meteorology data in Hanjiang River Basin were collected to build a non-point source pollution model based on SWAT(Soil and Water Assessment Tool).Approaches to improve modelling procedure were investigated.The calibrated model was used for simulating the pollution of Hanjiang River Basin and studying the relationship between landscape pattern and non-point source pollution.Best management practices were proposed and assessed based on the results of the simulation.Main results are as follows:1.A SWAT model for Hanjiang River Basin was built,with an improved approach for constructing SWAT soil parameters database of regions in China proposed.The approach allows modeller to calculate the bulk density,saturated hydraulic conductivity and available water of both soil matric and total soil.An improved approach for the calibration of baseflow recession constant was proposed,based on a generalized digital filter equation for baseflow separation introduced by this study.2.A multiple-objective-and-multiple-station calibration and validation procedure based on SWAT-CUP program was put forward.According to the modelling results,the coefficients of determination and Nash-Sutcliffe coefficients of streamflow,sediment and ammonia nitrogen were basically over 0.7 with relative error basically less than 8.5%.3.Sankey diagram was used instead of land use transition matrix to express the status and dynamics of landscape pattern,and it was revealed that the urbanization progress in Hanjiang River Basin presented a trend that the urban area was spreading from lower stream to upstream.4.By means of multivariate statistics,regression equations and redundancy analysis were conduct to investigate the relationship between landscape metrics and water quality indicators.The results showed that landscape metrics accounted for 81.7% variation in changing of nonpoint source pollution,where the contribution of landscape configuration metrics were greater than landscape composition metrics.The percentage of forest area was negatively related to non-point source pollution.The patch density and shape index of urban and agricultural land were strongly negatively related to non-point source pollution.Uplands contributed more pollutant than paddy fields.5.The simulation results also showed that the water quality in the Hanjiang River Basin presented a downtrend in the context of current land use management strategies and human activities.The western subbasins were key areas of total nitrogen pollution,with a total nitrogen concentration of 3.91 mg/L.The eastern subbasins were key area of total phosphorous pollution,with a total phosphorous concentration of 2.15 mg/L.Best management practices,such as vegetated filter strips,riparian buffer zones,reforestation and adjusting agrotypes can reduce nitrogen pollution by 4.63%-8.95% and phosphorous pollution by 0.91%-42.41%.