**3.4 Optimal regulation of groundwater benefit zone**

At present, there are a variety of measures for the regulation of water and salt, the core of which is to inhibit salt building-up by reducing soil evaporation (e.g., mulching), to promote salt leaching by improving soil structure (e.g., soil amendments), to block salt building-up by creating salt-isolation layer (e.g., salt-resistant barrier), or to increase soil drainage to facilitate soil salt discharge (e.g. subsurface pipes), and among other ways [39–41]. In general, crop salt thresholds, local soil types, and groundwater conditions need to be taken into account to clarify the applicability of these methods in saline agricultural production. For salinized farmland with shallow groundwater tables, the utilization of groundwater is greatly influenced by the salt accumulation, salt threshold of crop, and salt leaching scheme, so it is essential to clarify the "groundwater benefit zone" and optimize the regulation. Some regulation of water and salt has been made in the Northern Chinese irrigation area, while there was little research based on the simulation of optimization of groundwater [42]. Although some models currently proposed appropriate groundwater levels and irrigation strategies for specific crops [43], it is still challenging to promote and popularize the results due to different soil types, irrigation systems, plant rooting patterns, salt tolerance, groundwater depth, and climatic conditions. In general, to break the limitations of long-term field test and the lack of investigated factors, the technical parameters of water salt regulation can be obtained based on model scenarios analysis and the influence of different factor combinations on the relationship between groundwater table and crop yield can be considered comprehensively. At present, the water-salt transport model of the GSPAC system is applied to predict the trend of water-salt dynamics and the concentration of salt. The response of crop growth to changes in soil water-salt environment under different irrigation systems and planting patterns is systematically analyzed base on boundary conditions and parameters obtained from various management measures [44].

On this basis, the model scenario analysis can design different combinations of influence factors, to clarify the balance point of water conservation measures and salt leaching, and to establish a plant water supply theory scheme aimed at watersaving and salt control. Thus, the key to regulating groundwater benefit zone can be based on models to construct technical parameters that reflect different regulatory measures. In addition, soil improvement products can be designed based on these technical parameters. For example, we could establish the cause-effect relationship by applying modern analysis means like characterizing the structural morphology, its molecular structure, surface morphology, and performance correlation of the soil water and fertilizer, to carry out component screening-structural regulationfertilization performance determination for material design and optimization, and the optimal technical products for salt-alkali soil water salt regulation. For example, through modern instrumental analysis methods, the structure and morphology of the product are characterized. The relationship between its molecular structure, surface morphology, etc., and soil water and fertilizer storage performance is explored and the structure–function relationship is established. Recently, Swallow and O'Sullivan [45] proposed a new desalination method based on biomimicry of vascular plants, which is to mimic the principle of water absorption of the vascular plant to produce desalination materials. After added to the soil, with the help of natural evaporation, groundwater and soil salt are directly separated the crystallization process. After 30 days of the indoor test, the method can reduce the soil salt content from 8 to 0.8%, and the desalination effect is pronounced. It provides a new technology for saline soil remediation, but it also needs further verification and evaluation in the field.
