Coupled Transport Of Water-vapor-heat And Estimation Of Phreatic Evaporation In The Vadose Zone Of Mu Us Bottomland

The tidal land of Mu Us Lake basin belongs to the arid and semi-arid area,and there is a big difference between precipitation and evaporation.The water to maintain the ecological environment comes from the water replenished by precipitation,so the vertical water exchange is the main form of regional water cycle.In recent years,energy production,agricultural irrigation and domestic use of water have caused ecosystem problems in the area,such as overextraction of groundwater,reduction of lake area and reduction of river flow.It is an important way to improve water resources management and relieve water resources pressure to study the process of water vapor heat transport and estimation of phreatic evaporation in the vadose zone.The research results have important theoretical value and practical significance for understanding the water and energy balance in arid and semi-arid regions.Based on the basic theories of soil hydrodynamics and soil physics,field monitoring,laboratory experiments,numerical models and other research methods and means are adopted in this paper,which takes the vegetation-vade-phreatic profile in Mu Us Lake Basin as the research object.The temporal and spatial characteristics of soil moisture content,soil temperature,soil heat flux and water vapor flux at different depths were analyzed under typical evaporation and rainfall conditions.The law of moisture and heat transport in the vadular zone driven by temperature gradient and matrix potential gradient is discussed.The season of phreatic evaporation,diurnal variation and its influencing factors were further studied.The following results and understandings were obtained:(1)The measured air temperature and soil temperature at different depths from January to December 2018 showed sinusoidal fluctuations,and the peak soil temperature showed a lag phenomenon.Precipitation events mainly affected the redistribution of soil water at a depth of0-40 cm,while evaporation processes mainly affected the redistribution of soil water at a depth of 0-60 cm.The water vapor pressure is positively correlated with temperature and negative pressure.In the shallow soil,the closer to the surface,the more water vapor flux is,while the transport of liquid water in the deep soil is more important than that of water vapor.(2)The water-vapor-thermal coupling model of the aerated zone under vegetation cover was established,and it was found that the simulation effect of water content and soil temperature in the deep layer near the diving surface was better than that in the shallow layer.(3)The temperature gradient water vapor flux and the temperature gradient liquid water flux have the same order of magnitude,similar absolute value,opposite direction of migration,and have the similar change rule.The liquid water fluxes under the matrix potential gradient are all positive.The steam water movement driven by the matrix potential mainly occurs on the surface,and the steam water flux is small and can be ignored.In the period of 17:00-00:00,the water flux is vapor water flux and liquid water flux with temperature gradient.From 01:00 to16:00,the liquid water flux is mainly the matrix potential gradient.With the increase of the buried depth of water level,both the conducted heat flux and the latent heat flux of water vapor diffusion decrease.(4)Diving evaporation intensity summer(June-August)> autumn(September-October)>spring(March-May);The phreatic evaporation in the daytime under vegetation cover is greater than that in the night,while that in the bare land is greater than that in the day.There is little difference between the model calculation and the diurnal water level fluctuation method to estimate the phreatic evaporation,and the two methods can be mutually verified.(5)Temperature gradient,vegetation cover condition and depth of water level are the main factors affecting phreatic evaporation.