Effect Of NaCl Crust On Water Evaporation In Aeolian Sand Soil

Water resources are the most important natural resources in the world,but a series of problems have been presented in the world,such as serious pollution,uneven distribution,poor water quality and intense evaporation.Among them,the arid area has attracted the attention of Chinese and foreign scholars because of its unique water resource problems,such as high salinity,intense evaporation and water shortage.In nature,salt particles will form with sand soil particle cementing salt crust with special physical and chemical properties,it can not only accelerate soil salinization,the increase of the surface reflect sunlight,aggravate natural biological salt stress,etc.,also a strong influence on sand soil evaporation,the impact of these by salt junction belt especially in arid or semi-arid region.At present,there have been some domestic studies on the effect of salt crust on soil moisture evaporation,but most of them are based on the macroscopic scale such as crust coverage degree to study the effect of salt crust on soil moisture evaporation.However,there is still insufficient research on the influence of crystal structure and growth mode on soil moisture evaporation at the microscopic scale.This paper attempts to explore the influence mechanism of salt crystals on soil moisture evaporation from a micro point of view,so as to provide a certain theoretical basis for desert management,water utilization and ecological restoration.In this paper,two kinds of different particle sizes(100-200?m and 500-800?m)and 5mol/L NaCl aqueous solutions were selected to conduct soil column evaporation test in the mobile aeolian sand soil in Ruoqiang region of Xinjiang.The influence of NaCl salt crust on water evaporation in aeolian sand soil was explored through the test.The main conclusions are as follows:1)The NaCl salt crust formed under the two particle sizes inhibited the evaporation of brine as a whole,but in the initial stage of evaporation(Within the first48 hours),the evaporation group with particle size of 100-200?m showed a temporary promotion effect on evaporation.However,the evaporation with the particle size of500-800?m can inhibit evaporation throughout the process.2)The covering direction of salt crystals or "salt nodules" in the evaporation group with the grain size of 100-200?m diffused and grew from the marginal region to the central region,forming Crusty crust,while the covering direction of the evaporation group with the grain size of 500-800?m diffused and grew from the central region to the marginal region,forming Patchy crust;On the lateral scale,the salt crystal layers formed by the two evaporation groups are both inhibiting the evaporation of brine.3)The salt crust thickness on the surface of aeolian sand soil is not uniform.The salt crust thickness of 100-200?m evaporation group is the thickest,and the central zone is similar to the ring zone.The salt crust thickness of 500-800?m evaporation group is the central zone,and the edge zone is similar to the ring zone.In terms of stratified salt concentration,the salt concentration of the upper layer is the highest under the two particle sizes,and the difference between the middle and lower layers is not large.The evaporation rate of brine does not decrease with the increase of salt layer thickness,but a relatively stable evaporation rate.4)During the evaporation process,the "salt hair" appeared in both particle sizes.The "salt hair" appeared in the 100-200?m evaporation group on the 9th day,while the "salt hair" appeared in the 500-800?m evaporation group on the 3rd day.The salt hairs grow both horizontally and vertically;The size and length of the "salt hair" in the 100-200?m particle size group were not as good as those in the 500-800?m particle size group,and the "salt hair" may have an effect on the evaporation of brine.5)The crystal morphology of NaCl salt plays an important role in the process of soil water evaporation.According to the influence of salt crystal morphology on evaporation,three stages can be divided,namely,the dominant inhibition stage of salt crystal coverage,the dominant change of "salt nodules" and the dominant stage of "salt hair".