Study On The Functional Zoning Of Urban Land Use And Spatial Distribution Of Non-point Source Pollution In African Lakeside Citie

With the effective control of point source pollution by people,non point source pollution has gradually become the main cause of urban water quality degradation and seriously constrains the sustainable development of cities.The rapid development of urban industrialization and urbanization in Africa does not match the outdated infrastructure,and the problem of urban non-point source pollution is severe,gradually becoming one of the serious challenges that constrain the sustainable development of the region.However,research on urban non-point source pollution and its spatial distribution is still very scarce.Therefore,conducting research on the spatial distribution of non point source pollution in African cities is of great significance for understanding the pollution status of African cities,strengthening urban pollution control,and sustainable development.This study takes Mwanza City,Tanzania as a research case,combines the historical and current characteristics of urban development in Africa,and conducts urban land classification and functional zoning based on multi-source data;Using the output coefficient method,source intensity coefficient method,and multi factor evaluation system,conduct research on the calculation,spatial distribution,and risk assessment of non point source pollution emissions in Mwanza City,and then propose hierarchical control measures.The main results of the study are as follows:（1）Functional area of land use in Mwanza CityIn 2020,the land use structure of Mwanza City was complex,with various land use types interwoven and distributed within the city,forming a complex land use structure type.The official residential area is 6.61 km²,accounting for 1.55%of the total urban area;5 km² of land for public services and facilities,Accounting for 1.17%of the total urban area;16.57 km² of industrial land,Accounting for 3.9%of the total urban area;The commercial land area is 0.75km²,Accounting for 0.18%of the total urban area;The forest area is 22 km²,Accounting for5.2%of the total urban area;Grassland area of 50.17 km²,Accounting for 11.8%of the total urban area;Agricultural land area is 191.77 km²,Accounting for 45.1%of the total urban area;The water body area is 2.97 km²,0.7%of the total urban area;The informal settlement area is56.91 km²,Accounting for 13.4%of the total urban area.Overall,due to issues such as land system and urban planning,urban construction land in Mwanza City is spreading in the form of informal settlements around the urban core area,resulting in an increasingly serious mismatch between its infrastructure and urban construction land.（2）Non point source pollution emissions in Mwanza CityIn 2020,the total nitrogen（TN）and total phosphorus（TP）emissions from non point source pollution in Mwanza City were 446.08 t and 73.57 t,respectively.From the perspective of pollution sources,the contribution rates of TN and TP from different pollution sources are consistent,in the order of population living>surface runoff>industrial pollution.From the perspective of land use,the contribution rates of TN from different land uses are in descending order:informal settlements,agricultural land,industrial land,grasslands,built-up areas（excluding industrial land）,and forest land;The TP contribution rates of different land uses,in descending order,are informal settlements,industrial land,agricultural land,grasslands,urban built-up areas（excluding industrial land）,and forest land.Therefore,informal settlements are the main source of nitrogen and phosphorus emissions in Mwanza City,indicating that in the process of urbanization development in African developing countries,the emissions of non-point source pollution nitrogen and phosphorus are mainly related to population size.（3）Spatial Distribution Characteristics of Non point Source Pollution in Mwanza CityThe unit emission intensity range of TN in Mwanza City is between 0 to 14.9 t·km^-2;The unit emission intensity of TP ranges from 0 to 2.4 t·km^-2.The spatial distribution of unit emission intensity of TN and TP is relatively consistent,showing a decreasing trend from the city center to the surrounding areas.The 15th sub basin located in the city center has the highest nitrogen and phosphorus emission intensity,reaching 6.72 t·km^-2and 1.08 t·km^-2 respectively,making it a key source area for urban non-point source pollution.The high value areas of urban non-point source pollution are mainly located near Mwanza Port along the lake,while the northeast and south of the city are low value areas for nitrogen and phosphorus emissions per unit area due to the distribution of a large amount of agricultural land and grasslands and underdeveloped agricultural production methods.（4）Risk Assessment of Non point Source Pollution in Mwanza CityThe spatial distribution of non point source pollution risk in Mwanza city conforms to the characteristics of non point source output in underdeveloped cities.The risk assessment values for non point source pollution range from 0.006 to 0.878,with an average risk range of 0.114.The spatial characteristics are high in the urban central area and low in the surrounding areas.The overall trend is similar to the distribution of total nitrogen and total phosphorus emission intensity factors.Among them,informal settlements in the urban core area are rampant and densely populated,making them high value areas for non point source pollution,accounting for approximately 16.7%of the total urban area.The overall urban area is mainly low-risk areas,mainly distributed in a strip shape in the eastern part of the city,including sub watersheds such as 5th,6th,7th,11th,20th,19th,and 24th,accounting for approximately 50.3%of the total urban area.