Classifying irrigation water use efficiency in Hetao Irrigation District using SPA-VFS coupling model

Water Use Efficiency (WUE) can serve as an integrated functional indicator to guide the modernization construction of irrigation districts. Therefore, an accurate WUE classification can make a great contribution to the targeted improvement measures, particularly to the main controlling factors. Set Pair Analysis theory (SPA) and Variable Fuzzy Set theory (VFS) are widely used in water resources carrying capacity assessment, water quality assessment and environmental assessment. However, the classification and evaluation of water use efficiency in large-scale irrigation districts have the characteristics of many influencing factors and dynamic change of evaluation index classification standards. Both Set Pair Analysis theory and Variable Fuzzy Set theory have certain shortcomings in their applications. The SPA-VFS coupling model can realize the complementary advantages of variable fuzzy theory and set pair analysis, and make up for the shortcomings of the above two evaluations. Taking the Hetao Irrigation District as the research area, a WUE evaluation index system was constructed here from the aspects of water use level, engineering condition, agronomic measures, management level, planting structure, and ecological environment. The water efficiency of the irrigation system was divided into five grades: high, higher, medium, lower, and low, according to the current planning, standards, and the development status in large-scale irrigation areas in China. The graded standard of each index was determined according to the evaluation grade. An improved entropy method was used to assess the weight of each index. A SPA-VFS coupling model was selected for the WUE classification evaluation in the study area. Results showed that the WUE level was Ⅲ in the study area using the SPA-VFS coupling model, in good consistent with the VFS and SPA model. The SPA-VFS coupling model was verified to be feasible for the grade in the irrigation area. The stability range of the level eigenvalue was 3.12-3.17 in the SPA-VFS coupling model, whereas, that was 3.00-3.33 in the VFS model under different parameter combinations, indicating a significantly lower stability range of the level eigenvalue in the SPA-VFS coupling model than that in the VFS model. It inferred that the SPA-VFS coupling model was more suitable for accurate grade evaluation of large-scale irrigation areas. Consequently, a strong recommendation was made to subsequently strengthen the daily management of the supporting projects in the canal system and the drainage ditch in the Hetao Irrigation District. It was also highly demanding to timely repair, transform, and update the supporting projects beyond the normal operation. An autumn irrigation system was necessary to deepen the reform, in order to reduce appropriately the autumn irrigation water quota, and then the water consumption for irrigation per unit area. Many efforts, whether engineering, chemical, or biological measures, can be made to enhance the soil quality of saline-alkali land according to the degree of saline-alkali, and to reduce the share of salinization regions for a better ecological environment of irrigated areas. An emphasis should be laid on the lining of field channels, the promotion of efficient water-saving, the transformation of field water saving, and the information construction of irrigation areas.