Identification Of Critical Source Areas And Delineation Of Management Subzones Of Non-point Source Pollution In Jing River Basin

The identification of critical source areas and the division of management partitions based on non-point source pollution assessment are helpful to improve the efficiency and quality of water environment management in the basin,and realize the fine management of the basin.In this paper,a water and soil assessment tool(SWAT)was used to assess the dynamics of non-point source pollution in Jinghe River Basin from 1967 to 1990 and from2000 to 2020.Four methods,including comprehensive pollution index method,were used to identify the critical source areas in each period,and k-means clustering analysis was used to divide different watershed management zones.The rationality of the partitioning results was verified by single factor analysis of variance.The results are as follows:(1)After the comprehensive control of water and soil conservation measures in Jinghe River Basin,the problem of soil and water nutrient loss has been improved significantly since 2000.From 1967 to 1990,the annual average sediment yield in the basin was 47.07t/hm~2/a,while from 2000 to 2020,it decreased to 22.01 t/hm~2/a,a decrease of 53.24%.It has been reduced to mild erosion.The spatial and temporal patterns of total phosphorus and sediment yield are similar.During 2000-2020,the total phosphorus pollution load is 0.17kg/hm~2/a,which is about 92.1%lower than that in the previous period(1967-1990).However,the problem of nutrient loss still needs continuous attention:although the areas with the most severe total nitrogen pollution(15-18 kg/hm~2/a)have been treated,the pollution intensity is still relatively serious and has a tendency to spread over a large area.(2)The non-point source pollution load in Jinghe River Basin was mainly affected by water yield,precipitation and land use type.From 1967 to 1990,the correlation coefficients of water yield,sediment yield and total nitrogen were 0.64 and 0.40,respectively,and the correlation coefficients of total phosphorus and sediment yield were 0.75.Due to the uneven distribution of precipitation in the basin during the year,the non-point source pollution changed greatly within the year.From May to September,the sediment yield,total nitrogen and total phosphorus load were 90%,80%and 85%higher than the annual load,respectively.At the same time,land use type affected the emission intensity of non-point source pollution in Jinghe River Basin,and the output intensity of different land types in two periods was cultivated land>grassland>forest land.Therefore,the emission control of cultivated land pollution should be strengthened.(3)The comprehensive pollution index method can effectively identify the critical source areas of non-point source pollution coupling and can be defined as an index affecting the management zoning of Jinghe River Basin.The changes of critical source areas in the two periods are mainly reflected in the spatial location shift and pollution load reduction.During 1967-1990,the 20 and 24 western sub-basins at the headwaters of Jinghe River were the key sources of pollution,and the average load of sediment yield,total nitrogen and total phosphorus were 71.16 t/hm~2/a,12.50 kg/hm~2/a and 2.57 kg/hm~2/a,respectively.From 2000 to 2020,the key source area has been transferred to the middle and upper reaches of the basin,the No.7 and No.12 subbasins,and the three pollution loads have been reduced to 42.69 t/hm~2/a,10.73 kg/hm~2/a,and 0.24 kg/hm~2/a.(4)Watershed management zoning is influenced by multiple factors.Based on multiple factors such as land use,hydrological conditions,elevation and slope,and comprehensive pollution index,the watershed is divided into four different management zones:Northern loess hilly gully ecological restoration area(high slope,less woodland,is the most prone to soil erosion in the whole basin),central and western pollution control area(the most serious non-point source pollution area),central and eastern forest ecological conservation area(less affected by human activities,the highest vegetation coverage),southeastern pollution control area(flat terrain,agriculture and economy are relatively developed).The p values of annual rainfall,water yield,sediment yield,organic nitrogen and organophosphate in different hydrological years were all less than 0.05,suggesting that there were significant differences among different zones,which proved the rationality and scientificity of watershed management zoning.(5)Effective and differentiated watershed management can be achieved by adopting targeted measures on the scale of management zones.From 2000 to 2020,the sediment yield in the northern loess hilly gully ecological restoration area is much higher than the average level of the basin.Therefore,attention should be paid to the construction of ecological protection forest in this area,and the planting structure and vegetation type should be adjusted continuously.In key pollution control areas in central and western China,source control and pollution control projects should go hand in hand,especially in critical source areas No.7 and No.12,measures should be implemented to control the amount of chemical fertilizers and pesticides,build terraces and set up vegetation filtration zones.The ecological conservation area of forest land in the middle and eastern part of the river basin is the least polluted area,which has important ecological functions and should be established as an ecological nature reserve.In the southeast pollution control area,a grass waterway project can be set up to prevent and control non-point source pollution under heavy rainfall from May to September,and special tourism agriculture can also be developed.