Study On Bare-soil Evaporation Based On Coupling For Water,Vapor And Heat

Soil evaporation is a process in which mass and heat transfer between soil and atmosphere.It is also a critical component of the global hydrogeologic cycle.About 35 to 40percent of the world's precipitation runs off to rivers and lakes or seeps into the soil,which means that the remaining balance evaporates to the atmosphere.Average global evapotranspiration from land surfaces on Earth is on the order of 0.5 m/yr.This has an important impact on Earth's energy budget,and more than half of the net solar radiation over the land surface is disposed towards evaporation.Soil evaporation has direct effects on hydrogeologic cycle between land and atmosphere and spatial and temporal redistribution of moisture in soil.These in turn influence local meteorological conditions,total surface water runoff,groundwater loss and soil and water management policies with respect to agriculture water supplies.On the base of the couple of water,vapor and heat in soil during evaporating under meteorological conditions,dynamics of evaporating surface developing are studied.Considering some transport mechanisms of vapor in soil,the rules and mechanism of vapor dispersion in soil are analyzed.Considering the nonequilibrium phase change between liquid water and water vapor,the characteristics of soil evaporating under nonequilibrium phase change condition and the rate of phase change are discussed.With coupling model for single-phase free flow and two-phase porous medium soil,the influence rules and mechanisms of meteorological factors,soil parameters and interface conditions on soil evaporating are studied.The main research contents and innovations of this thesis can be summarized as follows:?1?Based on the coupling model for water,vapor and heat in soil,rules of spatial and temporal distribution of water,vapor and heat in soil,and dynamics of evaporating surface developing under meteorological conditions with diurnal fluctuation are analyzed.Two kinds of evaporating surface developing mode are presented.With the theory equations of evaporation characteristic length,the main influence factors and influence rules are determined.The results show that there are two evaporating surface developing modes,which are related with van Genuchten parameter n and evaporation rate E₀ during stage?.With the increase of n of E₀ evaporating surface developing mode can transform between the both.When the n is greater than the larger critical value or is less than the smaller one,the evaporating surface developing mode is not affected by E₀.The evaporation characteristic length L_C is affected by both n and E₀.For small values of n,L_C is equal to viscous characteristic length L_V;for great values of n,L_C is equal to gravitational characteristic length L_G.When L_C depends on both L_G and L_V,the effect of E₀ on L_C becomes more obvious.?2?Considering advection,diffusion and dispersion of vapor in soil,a nonisothermal two-phase porous medium flow model is established,and the transport rules and mechanisms of vapor dispersion are discussed.The results show that during soil evaporating the dispersion coefficient of vapor in soil is affected primarily by evaporation rate,van Genuchten parameter n and saturation dispersity.Both The curves of dispersion coefficient and vapor flux with time are single-peak convex curves,and the time at peaks lies in the transition period from stage?to stage?.The three components of vapor flux are all affected by evaporation rate and n to varying degrees.It is discovered that dispersion mechanism has a restraining effect on diffusion mechanism,but the vapor flux produced by both dispersion and diffusion mechanisms is a little greater than that produced only by diffusion mechanism.This means that dispersion and diffusion mechanisms behave in a complementary fashion.?3?Considering the nonequilibrium phase change of water in soil,based on the nonisothermal two-phase porous medium flow model,an equation for mass fraction of water vapor in the gas phase is added,the transport rules of water,vapor and heat in soil is contrasted with those of equilibrium phase change,and the change mechanisms of phsae change rate are discussed.The results show that the nonequilibrium phase-change-based model predicts a smaller evaporation rate and a longer stage?,and after entering stage?the difference between evaporation rate predicted with nonequilibrium phase-change-based model and equilibrium phase-change-based model becomes smaller with time developing.In stage?evaporation surface has developed into subsurface soil with nonequilibrium phase-change-based model,and the range of evaporation surface is wider than that with equilibrium phase-change-based model.In stage?,when evaporation surface develops to the same depth,the range of evaporation surface is wider than that with equilibrium phase-change-based model.During evaporation process,the curve of phase change rate and time presents a single peak,and the peak value and its corresponding time are related with the depth.With the developing of evaporation surface,both the peak value and its corresponding time decrease obviously especially in the 0?0.02m shallow subsurface soil.?4?A coupling model for free-flow in atmosphere and two-phase porous medium flow is established,and the coupled numerical solving for nine primary variables is realized.The influence rules of main meteorological factors,soil parameters and interface conditions on soil evaporation process are studied.It is provided that although the Beavers-Joseph coefficient?_BJ is influenced by many factors,the evaporation process is hardly influenced by this parameter.During stage?evaporation rate is hardly influenced by permeability,but the duration of stage?decreases with permeability.With great permeability and great relative humidity in atmosphere,during the transition from stage?to stage?,there is liquid water flux at the soil surface.A higher porosity leads to a higher evaporation rate and a longer stage?.At the same saturation of liquid water,the evaporation rate in stage?is hardly influenced by n,but the duration of stage?is influenced by it.For different n,the time entering transition period is different,and the soil temperature at the same time in transition period is obvious.But with time developing in stage?,the soil temperature is hardly influenced by n.