Study On Water And Salt Transport And Salt Redistribution Among Different Land Types In Hetao Irrigation District

Hetao Irrigation District(HID)in Inner Mongolia is one of the three the largest irrigation districts in China.With the implementation of water-saving project,the amount of diverted water and drained water has been greatly reduced,which has been reduced by more than 20%and 50%,respectively.Great changes have taken place in the soil and groundwater system of the irrigation district.The salt retained in the irrigation area cannot be drained out irrigation district,so it can only be redistributed in the irrigation district.There are many different land types in the HID,which includes cultivated land,wasteland,sand dune and lake.The salt stored in the irrigation district is mainly redistributed among these land types,which are also the significant places for salt control in the future.In this paper,the typical land types in the HID(cultivated land-wasteland-lake system,sand dune-wasteland-lake system)are selected as the research objects to analyze the hydrological process and salt redistribution mechanism.The main research contents and conclusions are as follows:1.Based on two-terminal element mixed model,water and salt balance model,soil water dynamics and solution dynamics methods,water and salt transport in cultivated land-waste land-lake system was analyzed.The results showed that during the irrigation period,82%of the irrigation water was stored in 1m soil,18%of the irrigation water recharged the groundwater of cultivated land through leakage.76%of the irrigation water of canal system contributed to the groundwater of cultivated land through underground lateral runoff.The average contribution rate of irrigation water and rainfall to cultivated land groundwater was94%and 6%,respectively.The average contribution rates of cultivated land groundwater and rainfall to wasteland groundwater was 71%and 29%,respectively.The average contribution rate of wasteland groundwater and rainfall to lake was 43%and 57%,respectively.The amount of water contributed by canal system irrigation water to cultivated land groundwater through underground lateral runoff was basically transferred to wasteland groundwater.Lake deficits 631.2mm?706.3mm.The average salinity of cultivated land groundwater increased by 861kg/hm~2.The average salinity of cultivated land groundwater transport to wasteland was 3232kg/hm~2,and wasteland groundwater transport to lake was3140kg/hm~2.In the whole growth period,the salt in soil of 1 m cultivated land still accumulated 56%.The salt was not completely discharged after autumn irrigation,and the desalination rate was 44%.There was slight salt accumulation in deep soil layer.Accumulated salt was 871 kg/hm~2.The accumulated salt was 58%in 1 m wasteland soil and the desalination rate was 62%after autumn irrigation.The salt content of wasteland showed a slight desalination trend throughout the year,with a desalination rate of 3870 kg/hm~2.2.Hydrus-1D model is used to simulate the water and salt transport process in cultivated land-wasteland-lake system,and the model parameters are calibrated and validated by measured soil water and salt data.The results showed,on the 5th day after irrigation,soil water content in the 0–20 cm and 20–60cm soil layers of cultivated land increased by27%?41%and 14%?20%,and in the 60–100 cm soil layer of wasteland increased by12%?15%,whereas the increase of soil water content of lake boundary was small.On the87th day after irrigation,decreased soil water content in the 0–60 cm soil layer of cultivated land was more 11%?14%and 24%?44%than that of wasteland and lake boundary,respectively.In the whole growth period,the deep percolation of cultivated land was34%?40%of the total applied water(rainfall and irrigation).The capillary rise of cultivated land,wasteland,and lake boundary was 28%,36%?46%,and 67%?77%of each of their evapotranspiration amounts,respectively.The salt accumulation in the top soil layer of cultivated land,wasteland,and the lake boundary was 13%,37%,and 48%,and in the deep soil layer was 34%,15%,and 13%,respectively.The salt storages in the 1 m soil layer of cultivated land,wasteland,and the lake boundary average increased by 19%,27%,and 37%,respectively.Soil salt content in the top soil layer of wasteland and lake boundary,as well as in the deep soil layer of cultivated land should be decreased to control salinization.The salt of capillary rise in the lake boundary was three times that of the wasteland.3.Three methods,i.e.,water-table fluctuation(WTF),soil hydrodynamics,and solute dynamics,were combined to build a water and salt balance model to reveal the relationship of water and salt transport in sand dune-wasteland-lake systems.The results showed that during the growth period,transport direction of the groundwater was from sand dune to lake,and during the autumn irrigation and the early freezing period,transport direction of the groundwater was from lake to sand dune.Groundwater salt dynamics were affected by transport path of goundwater.The vertical salt transports of groundwater at the sand dune,sand dune-wasteland junction and wasteland sites were 648kg/hm~2?575kg/hm~2?357kg/hm~2,respectively.The horizontal salt transports of groundwater from sand dune to sand dune-wasteland junction,from sand dune-wasteland junction to wasteland and from wasteland to lake were 481kg/hm~2,222 kg/hm~2,104 kg/hm~2.4.Hydrus-1D model is used to simulate the water and salt transport process in sand dune-wasteland-lake system,and the model parameters are calibrated and validated by measured soil water and salt data.The results showed,The soil water balance of wasteland and sand dune-wasteland junction could be kept during the whole year,but sand dune was in the state of water deficiency.The evapotranspiration of wasteland was twice than that of sand dune,and the evapotranspiration of sand dune-wasteland junction was between them.The groundwater recharge of wasteland is 3?5 times as much as that of sand dune.The salt accumulation rates at 0-100cm soil layers of sand dune,sand dune-wasteland junction and wasteland were 34%?51%,14%?17%and 25%,respectively,during growth period.After autumn irrigation,the salt accumulation rates of sand dune and sand dune-wasteland junction were 47%?59%and 3?6%respectively,however the desalination rate of wasteland was0.7?5%.The sand dune and sand dune-wasteland junction were in the salt accumulation state,and the wasteland was in the desalination state after autumn irrigation.