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The Hetao Irrigation District is a representative agricultural production base in the arid region of western China. The water resources shortage and soil salinization have induced to rely heavily on the water diversion from the Yellow River. Nowadays, almost 1/3 of diverted water resources have been used for soil salt leaching during the autumn irrigation period, in order to control the soil salinity in the root zone. Furthermore, the amount of autumn irrigation water has been required to significantly cut down, as stricter management of water resources was released in recent years. Therefore, it is necessary to access the soil salt accumulation and leaching processes in the root zone under the suitable autumn irrigation quota. Much effort has been made on the soil water movement and solute transport with experiment and modeling for the Hetao Irrigation District. However, the commonly-used advective-dispersive equation cannot fully consider the anomalous transport in the heterogeneous porous media in practice. In this study, a new soil water movement and solute transport model was developed to predict the soil salt accumulation and leaching in the root zones using the mass balance principle, named UBMOD. The mobile-immobile assumption was adopted to describe anomalous transport. Yonglian Experimental Station was selected as a study area. The data were collected to monitor the soil water movement and salt transport at 13 points in 2008. The immobile soil water content fraction β, the longitudinal dispersion DL, and the first-order mass transfer coefficient between the mobile and immobile region were calibrated and validated using the monitored data, and their values were 0.2, 0.4 m, and 0.001 d-1, respectively. The soil water movement and salt transport were more intense in the mobile region (including the soil salt leaching caused by infiltration and salt accumulation caused by evapotranspiration), whereas, those were slower in the immobile region, when considering the mobile-immobile water. The results showed that the soil salt content significantly varied in the root zone of farmland with time. Specifically, the soil salt content was larger gradually in the root zone, due to the soil evaporation and crop transpiration during the crop growth period. The soil storage index (SSI) was 5.3% during the crop growth period, while it was −21.1% during the autumn irrigation period. There was an outstanding salt leaching of autumn irrigation in the profiles of soil salt content at different times, where the soil salts were leached to the deeper soil layers near 100 mm. Moreover, the leaching was determined during the autumn irrigation period under three hydrogeological years, five/five irrigation quotas during the autumn irrigation/crop growth period, and five concentrations of irrigation water. Correspondingly, there was a larger effect of hydrological year and concentration of irrigation water on the autumn irrigation quota, whereas, there was a smaller effect of irrigation quotas during the crop growth period. An optimal autumn irrigation quota was 120 mm in the study area, and the concentration of irrigation water was better below 1.5 g/L. The finding can provide sound support to the decision-making of the agricultural irrigation district. [ABSTRACT FROM AUTHOR]

To understand the salt movement mechanism in the freezing-thawing soil and the salt transport in the frozen layer, an experiment is carried out in Yonglian experiment station, Hetao Irrigation District, China. The soil temperature, frozen layer depth, water table depth, groundwater salinity and the ion components, soil water content and soil salinity and its ion components during the freezing-thawing period were observed and measured. The data were analyzed to demonstrate the soil salt transport variations in the soil and groundwater as well as its impact factors. The correlation analysis was implemented to calculate the relationship between the soil content and the ion components and the relationship among those ions, and then to obtain the major ions and major salt composition of the soil soluble salts to control soil salt change. The transport mechanism of soil soluble salts in the freezing-thawing soil was discussed by using the binary phase diagram of water-salt system. The experiment results show that the soil temperature change within the depth of 0-1.0m. There are three stages during the freezing-thawing period in the study area as quickly freezing period (from December 6th, 2017 to February thh, 2018), slowly freezing period (from February 4th, 2018 to March 12th, 2018) and thawing period (from March 12th, 2018 to April, 13th, 2018). The water table depth showed the change as quick increase-slow increase-decrease in the 3 stages, while the average groundwater salinity significantly decreased. Na+, Cl– and SO42– had the greatest variability among all the 8 ions both in the groundwater. These demonstrated that the ions change in the groundwater was caused by the advection of ions in the soil solute. The correlation coefficients of Na+, Cl–, SO42– with soil salt content were larger than 0.9, and the correlation coefficients among Na+, Cl–, SO42– were larger than 0.9, which showed the major ion components of soil salt were Na+, Cl– and SO42–, and the major soil soluble salts were sodium sulfate and sodium chloride. The soil salt transport in the frozen layer depended on the initial soil salt gradient and co-saturated points of sodium sulfate and sodium chloride. The max mass fractions of sodium sulfate and sodium chloride were 2.01% and 1.55%, smaller than their co-saturated points. In this case, the salt would accumulate in the frozen layer when the initial soil salt gradient was positive (the salt content increase along depth). Otherwise, the soil salt would decrease in the frozen layer. This work is significant important to illustrate the relationship between the ion components with the soil salt change and the salt accumulation mechanism in the frozen layer. [ABSTRACT FROM AUTHOR]