Study On The Characteristics Of Non-point Source Pollution In The Watersheds Above The Hanzhong Section Of The Han River

Water pollution is one of the important problems facing human beings.Our country is in a period of transformation of water pollution,and the contribution rate of non-point source pollution is gradually rising.The Han River is an important water source for the South-to-North Water Diversion Project,and it is also the source of Shaanxi Province's "introducing the Han and Wei Rivers." Its water quality issues are critical.In this paper,the main research areas are runoff plots,Xuejiaba small watershed,and the Hanjiang River above the Hanzhong section,with TN,TP,NH3-N,COD as the main research indicators,and non-point source pollution as the research object,to study different land uses and slopes.The effect of natural factors on runoff/pollution,analysis of the process changes of runoff/pollution in small watersheds during rainfall.And proposed improved runoff segmentation method,improved rainfall difference method,improved runoff difference method,improved output coefficient method,combined with rainfall difference method,runoff difference method,output coefficient method,RENUMA model to estimate watershed 2010-2018,the non-point source pollution load was compared with the actual value to verify the accuracy of the four new methods,clarify the applicability of the eight methods in the Hanjiang River Basin,and evaluate and analyze each method.It also analyzes the sources of non-point source pollution,the study of the characteristics of spatio-temporal distribution and the identification of key source areas in the study watershed.The main conclusions of the paper are as follows:(1)Affected by the soil moisture content,the experimental analysis shows that the grassland yield is larger than the cultivated land,the smaller the slope,the higher the vegetation coverage,the smaller the yield,and the uneven slope will cause the yield to decrease.The outflow concentration of cultivated land is higher than that of forest and grassland,and the smaller the slope,the greater the plant coverage,the better the plant growth,the lower the concentration of pollutants.Land use,slope flatness,and vegetation coverage affect TN load production.The main factors are:vegetation coverage and its growth,soil phosphorus content are the main factors affecting TP load output;land use and slope are the main factors affecting NH3-N load output;slope and slope flatness are The main factors affecting COD load output.(2)The TN concentration in the small watershed exceeded the standard during the rainfall,which was inferior to the fifth category.The outflow first increased and then decreased,and there was a lag.The runoff coefficient of the small watershed is 0.469,the average annual surface runoff is 2.70×106m3,the total annual base flow is 1.04×106m3,the total annual runoff is 3.74×106m3,and the annual runoff modulus is 731,900m3/(a·km2).The TN load flux is consistent with the instantaneous flow,showing a trend of increasing first and then decreasing;the TP load flux is large at the beginning of the rainfall,and then decreases rapidly.It may be that particulate phosphorus enters the river channel under the initial rainfall erosion,resulting in its flux Increase;TN and TP load fluxes under the influence of rainfall are much larger than those during non-flood seasons.(3)The four improved methods are successfully verified.The improved runoff segmentation method reduces the demand for water quality data,and estimates the TN,TP,and COD non-point source pollution loads more accurately.The average annual relative errors are 14.26%,34.80%,and 14.47%,respectively.The improved rainfall difference method and The improved runoff difference method improves the applicable range and accuracy of the model,and estimates the TN and COD non-point source pollution load of the watershed more accurately.The annual average relative errors are 39.94%,34.98%,18.36%,and 15.47%,respectively;the output coefficient is improved.The estimation results of the method are more accurate than other methods.The annual average errors of the TN,NH3-N,and COD non-point source pollution load in the watershed are 11.51%,16.44%,and 15.47%,respectively.(4)Among the other four non-point source pollution load estimation methods,the relational formula of the rainfall difference method fails,and the relational formula of the runoff difference method TN,NH3-N,and COD fails.The annual average relative error of TP is relatively large.It may be that the natural factors considered are too simple,and environmental changes are not considered;the output coefficient method estimates are too large,which may be due to insufficient consideration of factors such as rainfall,slope,topography,river loss,and water degradation;the relative error of the RENUMA model except for 2016(extra dry year)is 22.46%,which can accurately estimate the TN load of the watershed except for extremely dry years.(5)The non-point source pollution load of cultivated land,forest land,swine and rural population is relatively high,and the total proportion reaches 87.30%,89.04%,86.61%,75.64%of TN,TP,NH3-N,COD,which is the research basin The main source of pollution.The TN,TP,NH3-N,and COD loads accounted for 70.2%,68.5%,72.8%,and 68.8%,respectively,during the wet season.The average annual rainfall accounted for 63.7%.