Spatial distribution characteristics of the suitable salt leaching quota in typical irrigation areas of southern Xinjiang based on SHAW model

Soil salinization has been one of the major soil degradation processes threatening the natural ecosystem in the world. Effective utilization of water resources can be used to prevent soil salinization so far. It is of great significance to clarify the spatial distribution characteristics of soil salt leaching quota in the fallow period in the arid region of Northwest China. However, the point-scale salinity leaching quota cannot fully reflect the regional situation, due mainly to the spatial variability of soil salinity and texture. Taking the Alar irrigation district of southern Xinjiang as an example, this study aims to obtain the spatial distribution characteristics of soil salinity and texture using remote sensing images in recent 20 years. Spatial interpolation was also combined to extract the land use data. The hydraulic response unit was then identified with the identical simulation condition using simultaneous heat and water (SHAW) model. An improved distributed model was established to clarify the spatial distribution characteristics of appropriate salt leaching quota under different irrigation modes. The results showed that there was ever-increasing farm land over the past 20 years, in terms of the main land use type and the area in the north of the irrigated areas. An outstanding spatial variability was also found in the soil salinity areas. Specifically, the randomness and variability of the surface layer were higher than those of the deep layer in this case. The soil was mostly at the level of mild salinization in the irrigated areas. In the distribution of soil salt, the soil salt content was larger in the west than in the east, while larger in the south than in the north. The farm land in the north of the irrigation area was mostly formed by the late reclamation of wasteland, indicating the higher content of soil base salt. Meanwhile, the soil salinity was significantly higher than that in the south. Furthermore, the soil texture presented the silt and sandy loam, accounting for about 36.81% and 19.44% of the soil, respectively. As such, the soil salinity and sand content were the main influencing factors on the salt leaching quota. An optimal salt leaching was carried out to fully meet the soil moisture content suitable for the cotton emergence in the next spring. Nevertheless, the high sand content normally led to low soil water retention. Particularly, the smaller quota of winter irrigation cannot provide the soil moisture content that is required by crops in spring. At the same time, only winter irrigation treatment was very necessary to leach the salt into the deeper soil layer using the higher irrigation quota, in order to reduce the soil salt returning. By contrast, only spring irrigation treatment was considered whether the soil condition after irrigation met the hydrothermal and salt conditions required for the cotton seedling emergence. The salt leaching quota of only winter irrigation was generally higher than that of only spring irrigation and winter irrigation + a small amount of spring irrigation. Only spring irrigation treatment was required for the minimum salt leaching quota. But, the salt accumulation resulted in the soil during the growth period during only spring irrigation treatment, due to the limitation of salt leaching depth. The treatment of winter irrigation + a small amount of spring irrigation (300 m3/hm2) was combined with the advantages of the rest of the three irrigation modes (only winter irrigation, only spring irrigation, and winter irrigation + a small amount of spring irrigation). The suitable winter irrigation quota of winter irrigation + a small amount of spring irrigation in the irrigated area was mainly between 1 500 and 2 250 m3/hm2, indicating the most beneficial option for water saving and crop emergence. This finding can also provide a strong reference for the optimal allocation of local water resources in the control of soil salinization.