Study On The Water Transport In The Vadose Zone Of Calcareous Sand Formation

Underground freshwater is an important factor in the construction of islands and reefs,and it is a key link affecting the ecological process of islands and reefs.The aerated zone is the unsaturated zone between the surface and the diving surface.The capillary water together with atmospheric water,plant water,soil water and groundwater constitutes a complete hydrological ecosystem as well as plays a role in the connection.And it is an important part of the hydrological cycle.There are few studies on the water transport in the aerated zone in the calcareous sandstone formation.Capillary water rise and evaporation are two important processes for the upward migration of water in the aeration zone.On the one hand,there is a capillary zone in the aeration zone,the soil moisture content is high or even close to saturation,and the plant roots can penetrate deeper and through the capillary zone.The groundwater is linked to reduce or even eliminate the pressure of water scarcity.On the other hand,the amount of water that migrates through the capillary belt to the upper part of the soil in the form of water and steam to supplement the water evaporation caused by soil evaporation and plant transpiration.The evaporation of soil water is one of the main processes of soil-atmosphere and energy exchange.It is also an important part of the balance of surface heat and water.The project set up a scientific-level meteorological observatory on the site to obtain the necessary meteorological data,combined with field tests,indoor physical model tests to obtain the soil-water characteristic curve of calcareous sand,capillary water rising law,evaporation law,combined with on-site measured data.As well as theoretical analysis and numerical simulation,the system studies the non-saturated water transport characteristics of calcareous sand,and reveals the macroscopic mechanism of unsaturated water transport in the coral reef islands,which provides a reference for the construction of green ecological islands and reefs.By analyzing the on-site meteorological data,carrying out the indoor pressure plate instrument,capillary water rise height and evaporation test,the following conclusions are obtained:(1)The dry season from February to May is less than 100mm.The annual rainfall is over 2500mm.The annual rainfall is greater than the evaporation which makes it possible to form a freshwater lens.(2)As the content of fine particles increases and the dry density increases,the water holding capacity of calcareous sand is enhanced.However,the effect of dry density on the SWCC is limited,and the shape of the SWCC is relatively uniform under different dry densities.There is a difference in the transition section of the SWCC under different fine particle contents,and the difference in the other stages is not significant.By fitting the discrete points with three models of VG model,FX model and Zhong Fang-jie model,it is found that the Fredlund-Xing model has the best fitting effect.Compared with quartz sand,the residual stage of the calcareous sand dehumidification process is not gentle.As the content of fine particles increases,the slope becomes steeper,indicating that the special inner pores and high salinity of calcareous sand enhance the water retention in the residual stage.(3)The more continuous calcium granules,the better the gradation,the higher the gradation of calcareous sand,and the higher the height of calcareous sand.The smaller the particle size,the higher the capillary water rise height.The longer it takes to stabilize.When the particle size exceeds 0.25 mm,the capillary water rise height and the rising rate have obvious convergence characteristics.As the dry density increases,the fine sand level of calcareous sand increases,and the height of capillary water in the silt-grade calcareous sand decreases.With the increase of the concentration of NaCl,the rise of capillary water in calcareous sand and medium sand decreased in the initial stage and then increased.When the concentration of NaCl was 20g/L,the height of capillary water was the smallest.The height of capillary water under fresh water conditions was the maximum.The height of capillary water in calcareous fine sand generally increases slightly with the increase of seawater content,but the difference is very small which the values are very close.(4)The amount of calcareous sand to evaporate depends on the external climatic conditions and the state of the soil itself.The climatic factors dominate.In the initial stage of evaporation,the amount of soil evaporation depends on the latent heat provided to the outside world,but has little to do with the compactness,gradation and particle size of the soil itself.