Interdisciplinary treatment of agricultural non-point source pollution

With the gradual improvement of the control level of industrial and urban point source pollution, agricultural non-point source pollution has increasingly become the main contribution source of water environment pollution. The academic research on the management and control of agricultural non-point source pollution is mostly concentrated in the field of natural science, with the characteristics of emphasizing monitoring, accounting and engineering measures, ignoring social and economic drivers and weakening policy design. However, these problems which are seldom discussed in the field of natural science are precisely the main direction of social science research. How to give full play to the different advantages of natural science and social science research methods in agricultural non-point source pollution control research, from driving factors to emissions, transport accounting, environmental impact analysis, until the whole process analysis of the effectiveness of policy intervention is a challenging academic proposition.

Nutrient pollution discharge is the key

At present, there are many types of agricultural non-point source pollution control policies in various countries. From the perspective of intervention mode and executive subject, it can be divided into three types: command control type, economic market type and voluntary agreement type. The means of command control are mainly laws, regulations and standards, and the implementation is promoted by the government from top to bottom by means of administrative orders. China's agricultural policy is dominated by growth and income, and there are few laws and regulations on agricultural environmental management. The industry norms and technologies of agricultural environment are often not operational and there are many non-mandatory standards, resulting in weak enforcement. China's peasant households are small in scale, large in number and scattered in distribution, and it is difficult to publicize, implement and supervise policies. Therefore, the management cost of adopting command-control policy is too high. As agricultural non-point source pollution occurs in the process of agricultural production and life, it has the characteristics of wide pollution source area, high dispersion, difficult to collect pipe network and far away from centralized treatment facilities, except for pollution sources in large-scale farming and concentrated residential areas. it is very difficult to adopt engineering measures directly. Therefore, it is more feasible to use economic market-oriented means to guide farmers to adopt an environmentally friendly mode of production from the perspective of economic rationality.

From the analysis of the types of non-point source pollutants, the measured data of most watersheds in China show that according to the equal standard pollution load accounting, the total nitrogen and total phosphorus characterizing nutrient pollution and chemical oxygen demand characterizing reducing organic pollution, among the three most representative agricultural non-point source pollutants, nutrient pollution load accounts for a large proportion (more than 90%). Scientific measurement and control of nutrient pollution discharge is a key link that should be broken through in the research of agricultural non-point source pollution in China. Considering the applicability of policies and the characteristics of pollutants, we are in urgent need of a method platform that can effectively analyze the effectiveness of economic market control policies on nutrient pollution.

NEWS of Natural Science and MAREM of Social Science

The research on non-point source pollution in the field of natural science began in the 1960s, and has gone through the development stages of empirical statistical model, mechanism model and integrated model combined with GIS. At present, the Global Watershed nutrient output Model (NEWS) is a novel method used to simulate the nutrient load and output flux of watershed in the world. NEWS can effectively describe the process of nutrient loss, collection, migration and transformation, interception and export (into the sea), and analyze its environmental impact.

In contrast, the research in the social science field does not care about the physical, chemical and other natural processes of pollutant transport, but focuses on the discussion of the root causes, that is, how socio-economic processes cause the emission of non-point pollutants. and how to design policies and measures to regulate such emissions. However, the peasant household model, which is often used to solve this problem, has some weaknesses, such as lack of complete rationality, lack of heterogeneity, lack of historical environmental data support and so on. MAREM model is a rural environmental management model developed based on experimental economics theory and multi-agent modeling technology. By realizing "artificial villages" and "virtual farmers" in the computer, as well as government, market, enterprises and other subjects, the model simulates the response of farmers to different agricultural policies and the emission of agricultural pollutants under the condition of limited rationality.

Comparing the models involved in agricultural non-point source pollution between NEWS and MAREM, the emphasis of the two is quite different, but they can complement each other. From the generation of agricultural non-point source pollutants to the final environmental consequences, scholars in the fields of social science and natural science persist in the research of the first half and the second half respectively. A simple example is: if we find that the water environment quality of a river basin needs to be improved urgently and put forward the goal of environmental control, then the scheme given by the NEWS model may be a 30% reduction in chemical fertilizer application in the basin. However, how to achieve the reduction of chemical fertilizer application, the NEWS model does not care about and can not analyze. This requires the help of MAREM model to do pre-analysis to tell us what kind of policy design and implementation intensity can achieve this control goal. On the other hand, the research methods of economics are also difficult to tell us how the environmental quality of the river basin will change under different emission levels.

The whole process Analysis of NEWS and MAREM

Can NEWS and MAREM, which seem to be completely different, be effectively integrated to realize the whole process analysis of agricultural non-point source pollution (especially for nutrient pollution)?

One of the most difficult gaps in the integrated application of the two models is that the research scales of the two models are completely different. In terms of spatial scale, NEWS is a basin-level macro-scale model, while MAREM is a village-level micro-scale model. In terms of the scope of pollution sources, NEWS also includes industrial and urban point source pollution, while MAREM only discusses agricultural non-point source emissions, making it difficult to match the data. Therefore, the combination direction of the two must be to realize the soft connection of input and output data, rather than integrate into a unified model.

A simple idea of matching data is to expand the research scope of MAREM to make it have certain urban analysis function. When the "artificial village" of MAREM is regarded as a box, if multiple "artificial villages" are realized at the same time, and towns are designed in the surrounding of the villages, not only the analysis of the interaction between urban and rural areas can be realized, but also the villages can be divided into several plates with different planting and breeding characteristics. However, this expansion method is difficult to operate, and although it achieves the unity of the range of pollution sources, it can not achieve spatial scale matching. In fact, according to the principle that the multi-agent model should not completely reproduce the real world, we should not rely on scale expansion to achieve the leap from individual behavior analysis to watershed spatial scale.

Another idea is to build a bridge between the two methods. System dynamics may be a good choice for this task. The system dynamics model is called the policy simulation laboratory. The model consists of feedback loops between different elements, which is easy to expand, and the application of table functions makes it better to accept the input data of other external models. We only need to establish a basin-scale system dynamics model including agriculture, industry, town and other subsystems, and refine the agricultural part according to the design idea of MAREM. The system dynamics model accepts the simulation results of all kinds of agricultural policies by MAREM, and provides the output data of watershed scale to NEWS. This not only expands the research scale of MAREM, but also overcomes the disadvantage that the system dynamics model is difficult to simulate the new policy which is lack of historical data.

Symbiosis of Farmers' behavior in economy and Ecology

When the problem of methodology is solved, it will return to the policy design itself. When environmental quality and control requirements have been identified, what kind of policies should be selected to achieve this goal, and what types of policies are more likely to be simulated by MAREM?

As mentioned above, the economic market policy may be more in line with the requirements of China's agricultural development and agricultural non-point source pollution control. The basic premise of this kind of policy design is that since the root of the environmental problem is market failure, then the introduction of market mechanism or similar mechanism for economic incentives can be rectified. This kind of policy can be divided into Coase regulation and Pigou regulation. Pigou regulation mainly punishes negative external sexual behavior or rewards positive external sexual behavior by means of taxes and subsidies, so as to internalize externalities. On the other hand, Coase regulation is to realize the internalization of externalities by defining property rights and encouraging the emergence of emissions trading.

Comparing the two kinds of economic market-based means, Coase regulation requires the property rights of pollutable emissions and the implementation of quota trading, which is widely used in industrial point sources. However, in the agricultural non-point source, the groups faced are too large and the social relations are complex, so it is difficult to clarify the sewage property rights of farmers. Relatively speaking, the tax and subsidy methods adopted in Pigou regulation are more practical. Among them, the policies of taxing polluting agricultural production factors and subsidizing farmers who adopt environmentally friendly mode of production have been implemented in European countries. And the means of taxation and subsidies are often used together, and the funds of subsidies can be derived from the corresponding tax income.

Agricultural non-point source pollution control is a realistic and urgent problem in our country. Realizing the symbiosis of farmers' behavior in economy and ecology is the fundamental way to solve the problem of agricultural pollution. The interdisciplinary integration of economics and environmental science may provide a feasible idea for exploring the continuity of policy design, farmers' behavior response, pollutant emission and migration to regional environmental safety analysis.