Establishment and application of integrated agricultural enviro-economic benefit model

Abstract: Agricultural economy development and environment protection are often contradictory and closely linked in today's world. For example, fertilizer application in some extent can increase crop production. On the other hand, it also may cause non-point source pollution and aggravate water pollution. In order to evaluate the environmental and economic benefits of agricultural activities objectively and scientifically, an integrated agricultural enviro-economic model was established in this study. The maximum of crop net profit calculated by agricultural input and output is the target function in this model, and the limit condition is the nitrogen loads released to the surface water body and/or groundwater. Thus how to calculate the crop yield and nitrogen leaching is the key issue. The Weishan irrigation district is the fifth irrigation district in China, which is irrigated by the Yellow River. More than 90% of the area is farmland, with rotation of winter wheat and summer maize. As a case study, the Weishan irrigation district was selected for model application because of its homogeneous planting and measurable irrigation. A calibrated and validated Hydrus-1D model was employed to simulate the nitrogen leaching, root uptake of nitrogen and crop yields in several scenarios with different irrigation and fertilization. And then the regression equations were reached between crop yield/nitrogen leaching and the amounts of irrigation and fertilization to get the values of coefficients in the integrated model. Finally, the integrated model was solved by using the optimization toolboxes in Matlab software, and optimal irrigation and fertilization schemes were obtained under different controls of nitrogen leaching. According to the scenario simulations by Hydrus-1D model, nitrogen leaching had a positive correlation with both irrigation and fertilization amounts. Crop yield had a peak value with some optimal irrigation, while no peak value appeared with the change of fertilization for the model's inner deficiency. Based on the analysis and discussion, the economic benefit would increase as a result of the easing control of pollution (i.e., allowable nitrogen leaching increase), but the growth rate would slow down, that is, the marginal benefit of agricultural development was diminishing at the expense of environment benefit. In the study area, the net income of crops planting was reached the maximum when nitrogen leaching controlled under 500 kg/hm2 with irrigation of 240 mm and fertilization of 732 kg/hm2. If more strict control of nitrogen leaching is expected, fertilization should be decreased, which may reduce the farmer's income. In this way, government can provide compensation to farmers for encouraging more environment-friendly agricultural production. By using the integrated enviro-economic model, the environmental compensation was about 913.5 Yuan/hm2 if cutting the allowable nitrogen leaching from 500 to 450 kg/hm2. Moreover, scientific guidance of irrigation and fertilization can offer best economic and environmental benefits. For example, if the local actual irrigation of 300 mm and fertilization of 610 kg/hm2 changed with irrigation of 240 mm and fertilization of 458 kg/hm2, the farmer's income would increase 478 Yuan/hm2, and nitrogen leaching would decrease 123 kg/hm2 at the same time, which may reduce agricultural non-point source pollution remarkably. Although there are some uncertainties in the estimation, this model can be used to provide references for the policy making of agricultural environmental protection and compensation.