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Long-term prediction of regional groundwater and salt dynamics is important for the agricultural sustainability in Hetao Irrigation District, where the water resources supply and demand contradiction is prominent. The mulched drip irrigation with the well-canal conjunctive irrigation will be applied to save water and control soil salinity. In this study, the improved mass balance model SaltMod by a loosely coupled strategy was used to simulate the long-term groundwater and salt dynamics under current situation and future situation under well-canal conjunctive irrigation with mulched drip irrigation to evaluate the agrucultural sustainability. In the loosely coupled model, 2 SaltMod models were used to simulate the canal-and well- irrigated areas separately. An exchange flux was added in the aquifer mass balance equation to link the canal- and well-irrigated areas. The improved SaltMod could handle with multiple irrigation sources and different groundwater levels in the groundwater aquifer, which makes it possible to be used in the well-canal conjunctive irrigated areas. Then the model was calibrated and validated with 15 years' observation data of the Longsheng well-canal irrigated area in Hetao Irrigation District, Inner Mongolia, China. The data in years between 2002 and 2005 were used as the calibration period and 2006—2016 were used as the validation period. Root mean squared error (RMSE) and relative error (RE) were used to evaluate the results. The results showed that the improved model performed well for calculating the groundwater depth and soil salinity for both canal-and well- irrigated areas. Then the model was used to predict the water flow and soil salinity dynamics in the next 100 years under the current irrigation condition. The results showed that it could sustain a long-term stabilization under the present irrigation practice in the Longsheng well-canal irrigated area. Then the model was applied to simulate the water and salt dynamics with different senarios under the well-canal conjunctive irrigation and the mulched drip irrigation. With the satisfactory result of soil salinity in the canal-irrigated area, the well-irrigated area was the concern, especially the root zone in the well-irrigated area. Different autumn irrigation frequency schemes, irrigation quotas in the crop growing season, groundwater depth in the well irrigation area and groundwater mineralizations were considered to be the impact factors on the soil salinity in the root zone of the well irrigation area. The results showed that, the irrigation quota and groundwater depth had little effects on the variation trend of soil salinity in the root zone of well irrigation area. The autumn irrigation frequency scheme and groundwater mineralization played very important roles on the variation trend of soil salinity. When the autumn irrigation frequency (AIF) was once every two years and the groundwater mineralization increased to 2.0 g/L, the root zone soil salinity would still be less than 3.0 g/kg after 100 years. It would exceed 3.0 g/kg after 60 years when the groundwater mineralization increased to 2.5 g/L. Considering the groundwater mineralization of 90% well-canal irrigated area was less than 2.0 g/L, we recommended that when exploiting groundwater for irrigation in Hetao Irrigation District, it was necessary to apply the water from Yellow River for autumn irrigation to wash out the soil salt. The autumn irrigation frequency was once every two years and the autumn irrigation quota was 180 mm. [ABSTRACT FROM AUTHOR]

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]