Simulation Of Spatial-Temporal Changes Of Complex Non-Point Source Loads Under Rapid Urbanization

Point source pollution is under better control, and non-point source (NPS) has become the major source of affecting water quality. According to pollution sources, non-point source consists of agricultural and urban non-point source. With the development of rapid urbanization, urban-rural fringe becomes a special transition zone. In the urban-rural fringe, agricultural and urban non-point source often coexist. Compared with single agricultural or urban non-point source, non-point source pollution in urban-rural fringe is more complex. Constructing a new model to solve the difficlut problem about estimation of non-point source loads in urban-rural fringe is of theoretical significance and practical value.In this study, the concept of "complex non-point source pollution" and estimation model are put forward. The complex NPS pollution is identified as the receiving waters pollution caused by various pollutants in agricultural and urban surface runoff together where agriculture land and construction land coexist. Based on the agricutural non-point source pollution model, urban non-point source pollution model, urbanization model, and focal neighborhood methd, the CA-AUNPS model is established. We simulate the spatial-tempral changes of complex non-point source loads in Tangxun watershed; analyze the main impact factors using Self Organized Mapping (SOM), linear correlation, and Multiple Linear Regression (MLR); establish Best Management Practices (BMPs) system. Main research contents and conclusions are as follows:(1) The spatial-temporal changes of land-use in Tangxun watershed were simulated and analyzed by cellular automata (CA) and the land-use degree change model. The CA simulation results showed that construction land increased from10.87to38.84%during1991-2011; farmland firstly increased and then decreased, becoming the dominant land type with50.43%in2001; forest/green land and water experienced rapid decrease. The land-use changes for the next10year show a similar trend to that during2001-2011. The construction land in2020was predicted to increase to58.13%, becoming the dominant land type in the watershed. The simulation results based on land-use degree change model showed that the land-use degree comprehensive index (7) in Tangxun watershed increased during1991-2030, which indicated the land-use degree become higher; the land use degree change (ΔI) per decade were greater than0, which indicated the land-use in watershed is in developing stage; the land use degree change rate (RI) in two periods of1991-2011and2011-2030were22.43%and11.39%respectively, indicating that the urbanization development speed over the next20years is slightly smaller than that in the past20years. (2) A spatial-temporal change model of non-point source (CA-AUNPS) is established by coupling of export empirical model, L-THIA model, and CA model. Based on the actual and simulated land-use situation in2011. the Kappa coefficient in the experiment was0.4970. which indicated that the simulation accuracy of CA was satisfactory. On the other hand, the weights with continuous distribution over the range0-1realized the quantification of NPS features, and the AUNPS model made each cell’s NPS features much more realistic, which indicated that the building of the CA-AUNPS model have the rationality. By contrasting simulation results with reference values, the errors of the TN and TP loads were5.65%and9.11%, respectively. The above analysis revealed that the CA-AUNPS model was effective for spatial-temporal changes simulation of complex non-point source pollution loads. The new model well solved the problem about estimation of complex non-point source pollution loads in urban-rural fringe.(3) The simulation results based on CA-AUNPS model are as follows:In terms of the spatial changes, the TN and TP loads generally showed as rural/urban construction land> wasterland/bare land> farmland> forest/green land, and the high-value areas of NPS pollutant loads has expanded from north to south with the increase of construction land. With regard to temporal changes, during1991-2030, the TN loads firstly decreased then increased, and the TP loads generally displayed an increasing tendency; by2030, the TN and TP loads will increase to390.12and39.40t/a, respectively. By correlation analysis between TN and TP loads in various land types, spatial distribution of TN and TP load is consistent; the spatial correlation coefficient generally showed as wasteland/bare land> forest/green land> rural/urban construction land> farmland. By comparing the simulation results of non-point source pollution loads among agricultural (A), urban (U), and coupling non-point source pattern (C-pattern), the TN and TP loads in C-pattern were higher than those in A-pattern and U-pattern; the change trends of TN and TP loads in C-pattern were consistent with that in A-pattern during1991-2001, and the change trends of TN and TP loads in C-pattern were consistent with that in U-pattern during2001-2030. The main reason is that the farmland is the dominant land type before2001, and agricultural non-point source is the main non-point source form in watershed; the rural/urban construction land is the dominant land type after2001, and urban non-point source is the main non-point source form in watershed. By analyzing the contribution of agricultural or urban non-point source to complex non-point source, in1991, the contribution of agricultural NPS to complex NPS (ωa) were72.49%(TN loads) and65.63%(TP loads); the contribution of urban NPS to complex NPS (ωu) were27.51%(TN loads) and34.38%(TP loads), and the agricultural NPS was dominant; by2030, ωa of TN and TP loads were only8.98%and10.50%, respectively; ωu of TN and TP loads rose to91.02%and89.50%, respectively; and the urban NPS dominated. With the development of urbanization, ωa decreased whereas ωu increased year by year. The CA-AUNPS model well reflected the actual characteristics of complex non-point source in Tangxun watershed.(4) Based on the actual characteristics in Tangxun watershed, the SOM, linear correlation, and MLR were used to analyze the influencing factors on complex non-point source pollution loads. The land-use change and annual rainfall are the main influencing factors on spatial changes of TN and TP loads, and the land-use change, slope, and NDVI are main influencing factors on temporal changes of TN and TP loads. The results showed that TN and TP loads were positively correlated with I and annual rainfall in the temporal changes; the farmland was the main land type affecting the TN loads, and rural/urban construction land was the main land type affecting the TP loads; the TN and TP loads were positively correlated with slope in spatial distribution, and the spatial correlation coefficient showed as wasteland/bare land> forest/green land> farmland> rural/urban construction land; the TN and TP loads were negatively correlated with NDVI in spatial distribution, the spatial correlation coefficient of TN loads and NDVI showed as forest/green land> farmland> rural/urban construction land> wasteland/bare land, and the spatial correlation coefficient of TP loads and NDVI showed as farmland> rural/urban construction land> forest/green land> wasteland/bare land.(5) Based on3S technology and CA-ANNPS model, the BMPs system were established to control and manage the complex non-point pollution in Tangxun watershed. The results indicate that,3of the watershed’s5sub-regions were the key areas for non-point source pollution control, which were located in northwest, northeast, and south-central areas of Tangxun Watershed, respectively. Through combined measures of BMPs system consisting of4subsystems, including the macro management, source control, process curtailment, and end treatment, the total removal rates of TN and TP loads would to be48.30-97.96%and60.60-91.79%, respectively. Under the worst case, after taking BMPs measures, the TN and TP loads in2020were281.87t/a and22.22t/a, with the loads reductions88.19t/a and11.67t/a, respectively; the TN and TP loads in2030were298.18t/a and20.97t/a, with the loads reductions91.94t/a and18.43t/a, respectively. The proposed BMPs system can be effectively used to control complex non-point source pollution in Tangxun Watershed.