| Recently, water pollution and its resulting harms on the human health has been a common problem worldwide. Non-point source (NPS) pollution has been a main cause for water quality impairment in many countries and regions. The quantitative information of NPS pollution in watershed scale, which is the basis for NPS pollution control in practice, is one of the most focuses in current studies. This study selected the ChangLe River watershed as the study area, which was a representative NPS pollution dominative watershed in southeastern China. The SWAT (Soil and Water Assessment Tool) model was adopted to quantitatively analyze the NPS pollution. Water quality and hydrological parameters were monitored along the ChangLe River system monthly during 2003-2007. The watershed natural condition (including soil, climate, land use, etc) and pollution sources information were also investigated and analyzed. Under the guidance of such theories as soil, ecology, environment, hydrology science, etc, SWAT model for nitrogen and phosphorus dynamic simulations in the watershed was calibrated and validated effectively. Then, the watershed nutrient exports from background and human activities to the river were both estimated to reveal the NPS pollution characteristics in different regions, pollution sources, and periods. The nutrient export coefficients for different pollution sources were obtained. Further more, in-stream nutrient removal capacity and its spatial and temporal variations were both fully addressed using different metrics. According to the river water quality control target, the water pollution control scheme that focused on the watershed nutrient input quantity in different pollution sources was founded. To further improve the river water quality, the approaches that can enhance in-stream nutrient removal capacity were also discussed. The developed theories and methods system on NPS pollution dynamic simulation and its quantitative control in this study provide a demonstration for watershed NPS pollution control in southeastern China.The main research results and conclusions of the dissertation are included:1) Aimed at the characteristics of the typical small watershed in developed area of southeastern China, the SWAT modeling approach system for NPS pollution simulation was established, which included the methods for model databases establishment, the NPS pollution combined in the model, the sensitivity analysis for model parameters, calibration and validation. It provides a referential experience for SWAT model application in similar watershed.2) Established SWAT model in ChangLe River watershed provided satisfactory modeling results on runoff (surface runoff, base flow and total flow), sediment, and nutrients (total nitrogen and total phosphorus) in both calibration and validation procedures, since the Ens and R2 for their modeled and measured values were both higher than 0.6.3) The inputs from fertilizer application, domestic and animal waste were the main sources for nitrogen and phosphorus in the watershed. The TN input quantity from these three sources was 3596.00 t yr-1,1394.47 t yr-1,1563.23 t yr-1, respectively, and for TP was 1770.51 t yr-1,348.62 t yr-1,554.50 t yr-1, respectively.4) Watershed TN and TP export load to the river was 1972.20-2843.99 t TN yr-1 and 71.54-132.07 t TP yr-1, respectively. The TN and TP export load from farm land contributed 76.69%-79% and 88.13%-90.69% of total TN and TP load for the watershed, respectively. The TN and TP export load from sub-watershed MS I occupied 57.84%-63.37% and 50.28%-57.05% of total TN and TP load for the watershed, respectively. However, TN and TP export loads were both increased with river water flow.5) Background TN export coefficients for different land uses ranged from 5.03±0.98 kg ha-1 yr-1-23.26±4.24 kg ha-1 yr-1, with the order as following:dry land > other land> paddy field> forest> garden> residence land. Background TP export coefficients ranged from 0.018±0.008 kg ha-1 yr-1-1.404±0.422 kg ha-1 yr-1, with the order as following:dry land> paddy field> other land> garden> residence land> forest. The background TN and TP export loads accounted 26.76%-34.74% and 14.87%-24.44% for their total export loads for the watershed, respectively. Background TN and TP export loads were also significantly increased with river water flow. 6) The TN export ratio (the ratio between the nutrient export load derived from anthropogenic input and the anthropogenic input quantity) in dry land, paddy field, garden, and residence land was 0.3793-0.4367,0.1491-0.2140,0.2730-0.4727 and 0.2134-0.2942, respectively. For TP, it was 0.0854-0.1754,0.0197-0.0293, 0.0171-0.517, and 0.0256-0.0619, respectively.7) The in-stream removal amounts for TN and TP was 962.58-1186.23 t yr-1 and. 49.68-79.36t yr-1, respectively. More than 50% removal amounts were occurred in the mainstreams. However, removal efficiencies that compared with the total input nutrient load in mainstream were lower than that of three tributaries. The largest removal amounts for TN and TP were appeared in flood season, and the lowest values appeared in dry season.8) There were three groups for current in-stream nutrient removal metrics. RL and RA formed first group, which reflected the amount properties about removal process. The second group included Kx, SL, RE, RLW and Kt, which indicated the efficiency properties about removal process. The third group was Vf, which expressed the inherent removal potential. Metrics in the first group were increased with river water flow, water depth, widths and lengths, while metrics in the second group were in opposite. The Vf, which was the only metric that influenced by water temperature, was increased with water temperature. Both Vf and RA can be used to compare the nutrient removal capacities among different streams or reaches, while others were only suitable for temporal comparisons.9) The water environmental capacity (the largest output loads permitted) for TN and TP was 1068.78-2023.9 t TN yr-1 and 88.97-143.35 t TN yr-1, respectively. To maintain the current water quality,656.60-1540.16 t yr-1 of TN export loads should be reduced, which occupied 27.35%-54.15% of current TN export load. Accordingly, the TN input quantity in the watershed should be reduced 2688.94-4483.46 t yr-1, with TN input quantity in the paddy fields accounting for the largest amount. Contrary, TP still had 10.22-35.24 t yr-1 residual water environmental capacity, which accounted for 8.54%-42.48% of the current TP export load. Thus there was 244.89-1463.85 t yr-1 residual TP input quantity in the watershed. 10) If maintain the current TN input quantity in the watershed, the average TN concentration at the watershed outlet can be decreased 30% of current TN concentration through slowing down 30% of river flow velocity. This provided a simple but efficient approach to improve the river water quality and control NPS pollution.The innovative and distinctive points about the dissertation were:1) The population was centralized in rural habitation area and its domestic wastes were treated inefficiently in China. Thus the daily rural domestic wastes was treated as a nutrient input into the habitation land in this study, which obtained the satisfying modeling results (the coefficients En and R2 were both more than 0.6 for nutrient simulations). This method provided a useful reference to take the rural human living influence into consideration in the SWAT model application at the similar watersheds.2) Background nitrogen and phosphorus export coefficients for different land uses were firstly estimated, which quantitatively explained the results that river water quality could not be improved rapidly and significantly after reducing the watershed nutrient input quantity in a certain proportion. This provided the important information to establish more scientific practices and strategies that aimed at water quality improvement.3) The study systemically explored the physical mechanisms and calculation methods for current in-stream nutrient removal metrics. The metrics were classified by statistical method and clarifies their differences in the responses to the hydrological condition, climate and geological characteristics. The relationships and differences among all metrics were fully addressed and their application limitations were distinguished. These efforts advanced the quantitative methods on in-stream nutrient removal capacity.4) The water pollution control scheme, which combined the watershed nutrient input reduction with river removal capacity enhancement, was founded. It was in favor of mitigating contradictions between water pollution control and economy development, and benefited the implement of the water quality control scheme in practice. This combined water pollution control scheme offered an improved approach to combat the NPS pollution control in watershed scale.
|
PDF Full Text Request