Research And Prediction Of Soil Salt Leaching Rules In Luanjing Lrrigation Area, Inner Mongolia

Luanjing, located in the south of a la shan meng area, Inner Mongolia, is a typical arid area, characterized by low and irregular rainfall, high evaporation. The average annual evaporation is 19 times higher than annual precipitation. The agricultural development here must depend on irrigation. Since the establishment of irrigation area in 1994, irrigation water has been diverted from the Yellow River and the main irrigation mode is free flooding irrigation without drainage system. During irrigation period, irrigation water interacts with soil and leaks downward with some salt. The leachate will uplift the level of groundwater continuously and may induce soil secondary salinization.Based on analysis and summary of previous research works, this paper combined field with indoor experiment,studies on the soil salt movements of typical lithologic structure and soil texture with free flooding irrigation. The correlation curve between the accumulation leachate volume and the corresponding salt concentration was determined by fitting experimental data. Combined the fitting results with leachate volume during each irrigation, the paper predicts the leachate concentration.Firstly, the paper collects the hydrogeological data of study areas and analyses construction and material constitution of unsaturated zone, and then selects typical soil sections to collect soil and soil solution samples at different depth, studies their respective salt dynamic variation. The consequence indicates that:The soil is desalting during irrigation with the yellow river. Before irrigation, the soil salt content is high, during early irrigation phrase, salt leaching rate is fast and the leaching amount is large. With increase of irrigation times, salt leaching amount gradually decrease, salt profile basically tends to be stable. The main composition of soil salt transforms from Cl-—Na+gradually to HCO3-—Ca2+. After each irrigation, soil solution concentration increases with depth. And with increase of irrigation times, soil solution concentration of same depth decreases gradually. Irrigation desalting relates to soil original salinity, irrigation amount and lithologic structure of unsaturated zone. The larger the soil original salinity and irrigation amount are, the larger the irrigation desalination is. Double layer structure is much easily desalting than multilayer structure.Secondly, the paper collects soil samples from the place nearby hole 1 and without irrigation. The upper layer of the sampling point is sandy loam, and the lower is sand. In order to study the impact of soil texture on soil salt movement, they are leachinged indoors respectively. The consequence indicates that:The soil desalting ratio relates to soil texture, original salinity and salt composition. During the whole experiment, the accumulative leachate volume of sandy loam is 7.05 PV(pore volume) and its desalting ratio is 71%, while the accumulative leachate volume of sand is 7.01 PV and its desalting ratio is 84%. This show that desalination in sand is easier than that in sandy loam. The desalting ratio sort of their main soluble salt ions are also different. Cationic sort of sandy loam is Na+>K >Mg2+>Ca2+, while that of sand is Na+> Mg2+>Ca2+>K+. This kind of difference causes by their different original cationic composition. Their anionic desalting ratio sort are same, and that is Cl->SO42-> HCO3-. The soil in different layer but belongs to the same texture also has different desalting ratio. When the gap in original salinity between adjacent layers is big, the more salinity is, the larger desalting ration is. When the gap in original salinity between adjacent layers is small, the soil desalting ratio of upper layer is lager than the lower. The salt concentration of their leachate decreases by power function with the increasing of accumulative leachate volume. This tend is coincided with dynamic variation rule of field soil salt. The pH of leachate rised first and then dropped.Thirdly, because the sampling profile of indoor experiment collected nearby hole 1, supposes irrigating it in field as hole 1, and predict its leackage concentration. As they are same at lithologic structure, texture boundary and salt leaching rules, their leackage volume during each irrigation approximately are same. This paper does soil water equilibrium test of hole 1 and obtains the leachate volume during each irrigation. The field irrigation leachate is equivalent to leachate generated during indoor experiment. The correlation curve between the accumulation leachate volume and the corresponding salt concentration was determined by fitting experimental data. Using this curve, we can calculate the leachate concentration of sampling profile corresponding to each accumulation leachate volume. The relation coefficient of each soil are all greater than 0.98, this show that the correlation degree between the accumulation leachate volume and the corresponding salt concentration is high.Finally, the paper provides some control measures of soil salinzation in response to the local utilization situation. Such as modifying irrigation norms and improving irrigation technique, optimizing plant structure and implementing water-saving irrigation system, strengthening the water conservancy project measures and agro-ecological measures, and so on.