Research On Seepage Control And Water Saving Of Plain Reservoir In Inland Arid Areas And Its Influence Of Soil Secondary Salinization Of The Downstream

Water is the fundamental resource of the humankind life.Water resources is the important element which promotes the social civilization progress,but the shortage of water resources are one of the major problems all over the world.In order to solve the problem that the amount of water resources is little and is not evenly distributed in Xinjiang which is located in inland arid areas,large numbers of plain reservoirs have been built.Due to their characteristics,plain reservoirs have been found not only causing large amount of leakage and evaporation but also raising the groundwater level around the reservoirs especially the downstream resulting in secondary salinization of soil around the reservoirs.Taking Qiala reservoir as an example,a systematic study of the seepage of the dam body and foundation and its effects on the groundwater level and secondary salinization of soil in the reservoir downstream has been conducted in this article.(1)“Earth-rock dam seepage analysis system based on boundary element method” is self-developed to compute the seepage slope and discharge of seepage per unit width of Qiala reservoir's dam body and foundation with different blanket lengths.The results show that seepage slope behind the dam can satisfy the demand of seepage stability when the blanket length reaches as many as five times of the water depth before the dam.The dam-rear seepage slope and discharge of seepage per unit width decrease with extensions of blanket length before the dam.When the blanket length reaches as many as 22 times and above of water depth before the dam,seepage slope and discharge of seepage per unit width approach constant.Then it will achieve less to reduce the seepage flow by extending the blanket length.(2)When Qiala reservoir is rebuilt,the blanket length before the dam is extended about 22 times of water depth before the dam,resulting in reducing leakage of 12.407 million cubic meters(about 8.2% of usable storage of Qiala reservoir)compared to the previous reservoir and saving valuable water resources.What is more important is that reservoir downstream groundwater level is effectively decreased by further improving the seepage control system,resulting in greatly reducing the scope where downstream soil of plain reservoirs in inland arid areas becomes secondary salinized.(3)The groundwater level of the wild land in Qiala reservoir downstream varies with seasons.The variety is enlarged with the increasing distance from the dam axis.The reservoir water level is the most influenced factor of the groundwater level of the wild land in reservoir downstream,followed by the evaporation capacity.The groundwater level of the farmland in Qiala reservoir downstream also varies with seasons.And the drainage level is the most influenced factor of the farmland in reservoir downstream,and the reservoir water level and evaporation capacity follows.Dam-rear drainage is the key measure to control the groundwater level in reservoir downstream.(4)The soil moisture of the farmland plough layer(0-40cm)in Qiala reservoir downstream varies greatly,while that of the deeper soil layer(40-100cm)varies little.In the vertical direction,the relationship between the soil moisture and groundwater level increases with the soil depths,that is,the closer the soil layer to the groundwater level is,the higher the soil moisture is.(5)The data of monitor points in the farmland in Qiala reservoir downstream between July 27 th 2012and April 18 th 2013 show the basic law of layered soil salinity.Soil salinity is less in 0-20 cm soil layer,increasing in 20-40 cm and 60-80 cm soil layers,decreasing in 40-60 cm and 80-100 cm soil layer.So soil salinity varies much with soil depth.With the condition that drainage and drip irrigation are combined in farmland,the total salinity in 0-60 cm soil layer decreases by 45-49%.(6)The temporal and spatial variability of soil moisture in Qiala reservoir downstream is that the overall variability of wild land soil moisture is large,especially the topsoil.And farmland soil moisture in all layers is weak variability except that in 0-20 cm soil layer with medium variability according to all monitor points.The temporal and spatial variability of soil salinity in Qiala reservoir downstream is that soil salinity in wild land has no relation with soil depth,varying a lot,and according to the monitor points the coefficient of variation is large.The farmland soil salinity varies like M type,the peak values of soil salinity lie at 40 cm and 85 cm.(7)Based on monitoring data of Qiala reservoir between 2012 and 2013,the HYDRUS-1D model is employed to establish one dimensional numerical model about the migration of the soil water and salinity in farmland of plain reservoirs in inland arid areas.The model parameters are verified as reliable and reasonable based on root-mean-square error between simulated and measured data.With the use of meteorological data and irrigation quotas in 2013,the model predicts the migration of water and salinity in soil profiles with different groundwater levels and different groundwater salinity values.According to the predictions,the key measure to avoid accumulating the salt in farmland topsoil of the plain reservoir downstream in inland arid areas is decreasing the groundwater level and salinity values.