Numerical simulation of the suitable autumn irrigation quota in the Hetao Irrigation District considering the assumption of mobile-immobile water

Abstract: 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.