image: This is the topography of the Bohai Sea (depth in meters) and routine monitoring stations for total nitrogen in 2009. The size of each dot indicates TN concentration. view more
Credit: ©Science China Press
Surface water contamination due to nonpoint source (NPS) pollutants has severely degraded water quality and threatened ecosystems. However, previous research has primarily examined the NPS pollutants in watershed and coastal rivers or soils rather than seawater or bays without assimilation methods, which results in the uncertainty in the model.
During recent years, the Bohai Sea has been one of the most exploited area in the world, and these exploitations have severely exacerbated environmental pollution in the Bohai Sea. Pollution modelling has provided an effective means for environmental evaluation and management. However, due to the uncertainty of control variables, there is still uncertainty in the models. 4D-Var data assimilation is one of the most effective and powerful approaches. Based on the theory of inversion, 4D-Var involves the adjoint method, and has the advantage of assimilating various spatio-temporally distributed observations into numerical models while maintaining dynamical and physical consistency within the model
As a step toward a practical and systematic numerical model, this study introduces the adjoint method and for the first time takes the Bohai Sea as a case study for combining the NPS term and initial condition inversion. The topography of the Bohai Sea and routine monitoring stations are shown in Figure 1.
In an article coauthored by Yongzhi Liu, Youli Shen, Xianqing Lv and Qiang Liu, scholars at the Ocean University of China and Beihai Marine Environmental Monitoring Centre Station of State Oceanic Administration which is published in Science China Earth Sciences, reported their findings: "The adjoint method offers a powerful tool for NPS TN assessment."
They made lots of experiments to improve the model. In gradient related experiments, they found "the steep decent method is better than L-BFGS method in their adjoint model". In ideal experiments, they found their model is capable of inverting the control variables. In practical experiments, they found their model presents an accurate simulation of TN.
They summarized their numerical experiments: "the assimilation of NPS term (ST) is more important than that of initial value (IV). Simulated results are further improved with the adjoint method. Furthermore, NPS TN is essential for TN simulation. The Bohai Sea is severely polluted in three bays"
Furthermore, they found "the adjoint model is not specific to the Bohai Sea and can be generalized to other regions, such as the Beibu Gulf". This research provides preliminary results for NPS pollutant estimation.
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See the article:
Liu Y Z, Shen Y L, Lv X Q, Liu Q. 2017. Numeric modelling and risk assessment of pollutions in the Chinese Bohai Sea. Science China Earth Sciences, 60(8), doi: 10.1007/s11430-016-9062-y.https://link.springer.com/article/10.1007/s11430-016-9062-y