**3.3 Scale of water-salt migration process and its corresponding research techniques**

Although the mechanisms of water and salt transport through the GSPAC system at field scale are considered more comprehensively, the water and salt transport process occurred at an immense scale. The spatial variation of influence factors, especially the measures to regulate soil water and salt changes such as irrigation, drainage, agronomy measures, etc. are carried out on a large scale.

Consequently, the field-scaled model, which is often one-dimensional, cannot simulate large-scale saline water process or make the related evaluation [26]. On the other hand, traditional large-scale hydrological models such as MODFLOW, although they are good at dealing with landscape-scale soil-groundwater interaction and groundwater movement process cannot reflect the small-scale hydrological process neither in saturated zone nor in the unsaturated area due to the lack of small-scale soil hierarchy and detailed structural parameters [27]. Thus, another trend of model development is to develop the coupled models at different scales, such as the model "HYDRUS-MODFLOW" [28] is coupled with HYDRUS-1D model and the groundwater model MODFLOW, which extends the simulation of the movement of soil water and salt under a dynamic groundwater condition to (extend to) the regional scale. The model can stimulate the redistribution process of water and salt both in natural and artificial circumstances. In fact, due to the variability of soil spatial structure and the randomness of various factors affecting water-salt movement, the water-salt transport process has a strong scale-dependent effect and corresponds to the appropriate quantitative techniques and methods in that scale.

Currently, there are effective ways to track the migration of substances in GSPAC systems [29–32], such as isotope, geochemical ions, and rare earth elements. The new Earth Critical Zone study focuses on effectively links between disciplines, scales, and data to achieve the mutual transformation of microscales (soil pores and aggregates), mesoscales (soil profiles, fields, or catena), and macroscales (basins, regions, or global) [33]. It can be spatially interpolated and aggregated according to soil distribution or soil characteristics at landscape-scale according to soil mapping hierarchical system, and then upscaled and downscaled, or it can be transformed on a scale by establishing a relationship between the hierarchical structure of soil models and typical soil processes of different scales. For example, from the mesoscale to the macroscale, "characterization unit regions" can be constructed in combination with topographical changes and land-use methods, thus linking laboratory and field measurements "hydraulic characteristics to watershed scales" ones orderly for spatial scale transformation. On the microscale, soil water and salt movement are mainly influenced by soil structure, soil level, micro-terrain, ion content, soil infiltration, salt leaching, and soil microorganisms. We could quantify the effects of soil and salt effects by soil pore structure, root growth pattern, and water movement, fertilization, soil improvement method, and engineering measures by using X-ray computer tomography, magnetic resonance imaging, and nuclear magnetic resonance, etc. [34–36]. At the mesoscale, the soil water and salt transport and distribution mainly include evaporation, infiltration, lateral seepage, groundwater leakage, and recharge, and is the basic scope of water-salt regulation and ecological environment construction [33]. Geophysical detection techniques such as multi-receiver Electromagnetic Induction (EMI), Electrical Resistance Tomography (ERT), and time-lapse Ground-Penetrating Radar (GPR) are widely used in soil physical properties measurement on scales such as slopes, catchment, and small basins [37, 38]. In recent years, remote sensing technology has been increasingly used in monitoring the physical properties of soil at the macro-scale and in coupling with other methods. At present, it is a significant scientific issue that how to quantify the water-salt migration flux of large-scale farmland system, through irrigation efficiency, soil salt accumulation, and other salt control factors, to build farmland irrigationfertilization-salt control technology mode, and whereby to carry out multi-scale regulation under water-saving and reduced fertilizers in irrigation areas, so that it can achieve not only the efficient use of water resources but also maintain a good environment.
