Nitrogen Non-Point Source Pollution Modeling And Source Identification Based On ArcSWAT

Eutrophication that was caused by excessive nutrient (e.g., nitrogen and phosphorus) has become a global hot issue of common concern. With point source pollution control capabilities advanced, Non-Point Source (NPS) pollution has increase important influence on water quality. To efficiently control NPS pollution in practice, the quantitative knowledge on nitrogen source apportionment is required for the environment managers. Although it has been well know that catchment soil nitrogen pool and atmospheric nitrogen deposition both paly significant roles for nitrogen export to river, rare studies addressed their contributions to river nitrogen loading. In this study, total nitrogen (TN) sources and their relative contributions to river TN loading were identified and quantified for the ChangLe River watershed, a typical agricultural watershed in eastern China. The SWAT (Soil and Water Assessment Tool) model was adopted for NPS nitrogen pollution modeling and nitrogen source apportionment. Water quality and hydrological parameters were monitored along the ChangLe River system monthly during2007-2009. The watershed natural condition (including soil, climate, land use, etc) and pollution sources information were also investigated and collected, The SWAT model was established in the ChangLe River watershed for modeling runoff, sediment and nitrogen fates after the model parameters calibrating and validating procedures. Based on the validated SWAT model, spatial-temporal distributions of NPS TN export to rivers were addressed. The contributions of different Hydrologic Response Units, different landuse types, and different nitrogen sources to river TN loading were quantified, supporting for identifying the NPS TN pollution critical area and period within the watershed. The responses of river TN concentration to changing watershed TN input quantities were further predicted, suggesting the suitable NPS pollution mitigation options. These results are required for decision-makers in watershed water pollution management practice.The main research results and conclusions of the dissertation are included:(1) Monthly river TN load was strongly and positively related with rainfall and river water discharge (p<0.01), respectively, with high TN loads generally occurring in June and August. Current river TN concentrations at the upstream (with average of3.39mgl-1) and downstream (with average of4.08mgl-1) nodes were both failed to meet the target river TN concentrations (e.g.,2.0mgl-1).(2) Estimated area-specific TN export rate varied in29.20-65.34kg hm-2a-1. The southern and eastern areas producing high TN export rates were identified as the critical area for NPS TN pollution in the Changle River watershed. The combination of regional characteristic variables, such as the farmland area percentage, soil N content and stream density (i.e., the ratio between stream length and catchment area) could explain87.66%spatial variability of NPS TN export rates within the watershed by the regression analysis.(3) Annual TN export coefficients (kg ha-1a-1) for the entire ChangLe River watershed followed:upland (79.72)> garden plot (72.73)> residential land (51.97)> paddy field (45.52)> forest (15.19)(p<0.05).(4) Chemical nitrogen fertilizer, soil nitrogen pool and atmospheric nitrogen deposition, which accounted for40%,29%and25%of total TN export load to river, respectively, were identified as the critical sources for river TN pollution. Nitrogen exported from the animal and domestic wastes contributed to the less contributions.(5) The critical sources for NPS TN export to river presented spatial-temporal variations. Natural sources, such as soil nitrogen pool and atmospheric nitrogen deposition, should be targeted as the critical sources for river TN pollution during the rain seasons. Chemical nitrogen fertilizer application should be targeted as the critical sources for river TN pollution during the crop growing season. Chemical nitrogen fertilizer application, soil nitrogen pool and atmospheric nitrogen deposition was the main sources for TN exported from the garden plot, forest and residential land, respectively. However, they were main sources for TN exported both from the upland and paddy field.(6) Without artificial nitrogen inputs to the watershed, TN concentration was predicted as1.89mg l-1in ChangLe River. When total TN input quantity was reduced by60%, river TN concentration would be lower than2mg l-1. (7) Considering the spatial-temporal variability of critical sources for NPS TN export to river the developed watershed NPS TN pollution control scheme is required determining which region in a watershed and which landuse categories in a region at which period, and how deeply the exports should be reduced. Since the nitrogen contained in soils and derived from atmospheric deposition can not be directly reduced in practice and decreasing chemical nitrogen fertilizer application has a potential risk for reducing the crop yield, it is more efficient and feasible to mitigate NPS pollution through improving fertilizer nitrogen utilization efficiencies in farmlands and enhancing regional nitrogen retention capacity (e.g., constructing riparian buffers and bio-carriers) for the ChangLe River watershed.