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Resources, Conservation and Recycling, Volume 88, July 2014, Pages 67–75.
Nga Thu Do1, Duc Anh Trinh2, Kei Nishida3
- Hanoi University of Science (HUS), Vietnam National University, No. 19, Le Thanh Tong, Hoan Kiem, Hanoi, Viet Nam.
- Institute of Chemistry, Vietnam Academy of Science and Technology (VAST), A18 – 18 Hoang Quoc Viet, Cau Giay, Hanoi, Viet Nam.
- International Research Centre for River Basin Environment (ICRE), University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan.
Abstract
Nitrogen flows impacted by human activities in the Day-Nhue River Basin in northern Vietnam have been modeled using adapted material flow analysis (MFA). This study introduces a modified uncertainty analysis procedure and its importance in MFA. We generated a probability distribution using a Monte Carlo simulation, calculated the nitrogen budget for each process and then evaluated the plausibility under three different criterion sets. The third criterion, with one standard deviation of the budget value as the confidence interval and 68% as the confidence level, could be applied to effectively identify hidden uncertainties in the MFA system. Sensitivity analysis was conducted for revising parameters, followed by the reassessment of the model structure by revising equations or flow regime, if necessary. The number of processes that passed the plausibility test increased from five to nine after reassessment of model uncertainty with a greater model quality. The application of the uncertainty analysis approach to this case study revealed that the reassessment of equations in the aquaculture process largely changed the results for nitrogen flows to environments. The significant differences were identified as increased nitrogen load to the atmosphere and to soil/groundwater (17% and 41%, respectively), and a 58% decrease in nitrogen load to surface water. Thus, modified uncertainty analysis was considered to be an important screening system for ensuring quality of MFA modeling
Significance Statement.
How many models have ever been proposed for environmental pollution analysis? To identify the critical pollution sources of nitrogen in river system, this study focused on a modified material flow analysis (MFA) modelling reinforced by uncertainty analysis procedure. Uncertainty analysis played an important role in evaluating accuracy of MFA model structure and reliability of input data, which were very important in the cases of data scarcity and uncertainty. Quantifiable criteria to test calculational plausibility and prioritised reassessment of parameter, equation and flow regime settings are effective for saving time and expense in developing countries (see the figure). In addition, this study concluded that agricultural processes were the most significant nitrogen sources for the surrounding environments, probably because of excessive application of fertilizers in the target area. The authors followed up the conclusion by restructuring the paddy process built in the modified MFA (Do and Nishida, 2014, Nutr Cycl Agroecosys 100(2): 215-226). The dynamic transport and transformation of three main nitrogen compositions (NO3-N, NH4-N and Organic-N) in the paddy soil were quantitatively represented. The estimated runoff loads for two rice seasons were in the range of measured values reported in previous studies. Thus, a series of the results imply practical usefulness when the model is applied for river basin management. For enhancement of the usefulness, further works are needed in simplification of the model structure, unsteady state calculation of rainfall-runoff process and direct validation of the estimated results with observation-based data.
