| Land-atmosphere interface is one of the most important boundaries in earth critical zone,as it combines the atmosphere-soil and groundwater.Evaporation process plays a major role in water-heat transfer at land-atmosphere interface.Liquid water transfer as vapor with solar heating and the turbulence process,then escape from the surface by wind and vapor deficit.Except for climate variables,soil evaporation is also influenced by texture,water level,water availability and other elements.Accurate estimation of soil evaporation is significant for hydrogeology cycle in Northwestern regions.However,accurate measurement on actual evaporation is difficult,as water vapor is hard to capture.A commonly used approach in many applications is to estimate saturated bare soil evaporation（potential evaporation rate）first,then deduce actual evaporation rate as a fraction（the fraction is influenced by water level,resistance etc.）of potential evaporation rate.Nevertheless,saturated bare soil evaporation rate is rarely measured,evaporation rate from water were regard to be the same as saturated bare soil evaporation rate,they have all been used as a starting point to estimate actual evaporation.Besides this,this simplification hypothesis that potential evaporation rate is controlled by climate variables instead of soil properties,and can be calculated through empirical or semi-empirical formulas.Whether these simplifications adequate need to be verified.Current research status that it’s not adequate to make this simplification,using a correction coefficient for the conversion of evaporation from saturated bare soil and water is lack of scientific basis.At present there is little research systematically justify differences of these two kinds of evaporation,and combining with the long-term groundwater field measure of potential evaporation mechanism research is scarce.In order to investigate this simplification,an unique lysimeter experiment were set at Chang’an University,water and environment experiment field in Weishui campus.Evaporation rate from saturated bare gravel,coarse sand and fine sands were measured at a high temporal resolution,together with climate variables,soil temperatures and soil heat flux.Evaporation differences between three textures and water were compared.Differences on water heat transfer at land-atmosphere,water-atmosphere interfaces were compared and explained from the physical point of view,the following conclusions can be drawn:（1）Lysimeter results show that,evaporation of sands when water table is zero PEs is different from water evaporation PEw,there values are different over yearly scale,seasonal scale and temporal dynamics.Thus the correction factor cannot be used for simple conversion between PEs and PEw.PE is different over different textures,yearly results show that,PEfine is higher than PEcoarseand PEgravel over yearly scale.The PE differences were more obvious at daytime during spring and summer.Result show that,PE differences are more pronounced between PEs and PEw.PEs is higher than PEw on yearly scale,with PEs higher than PEwat day but has a smaller value at night.The peak value of PEw is lagging behind PEfine.This research filled the gap in the comparison of evaporation between saturated bare soil and water surface in northwest China,and updated the hypothesis that evaporation between saturated bare soil and water surface are similar.（2）The heat transfer model is established to simulate the difference of heat transfer dynamics between saturated bare soil（solid-liquid two-phase medium）and water（single-phase fluid）to identify three key factors affecting the underlying surface heat transfer and temperature variation on profile.They are,thermal parameters,heat transfer mechanism and boundary conditions.Relevant conclusions have obtained the basis for understanding the evaporation mechanism over different underlying surfaces.（3）The flux terms,including net radiation Rn,sensible heat flux H,total ground heat flux G,heat storage variations N are calculated based on measured radiation terms,albedo,soil heat flux,temperatures and meteorology elements.The energy balance equation is applied to distinguish PE differences over saturated bare soils and water,and investigating the energy transfer dynamics between soil-atmosphere and water-atmosphere interfaces.Results show that,“available energy Rn-G or N”determine the PE differences.These results are quantitatively explained with detailed calculations of the surface energy balance.（4）The vapor transfer process between saturated bare soils and water can be explained based on Fick’s first law of diffusion and boundary layer theory.PE differences over different textures and water and be quantitatively explained,it is found that vapor gradient and viscous boundary layer thickness related to different surface temperature,as well as different character lengths explain the PE differences.Parameters in calculating boundary layer thickness are modified to improve the accuracy of water vapor diffusion model when estimating PE.（5）Based on long term in-situ observation results,the applicability and reliability of four widely used PE estimation models are evaluated.The PE rates of saturated bare soil and water were calculated by energy balance method,the Penman-Monteith（PM）,the Priestly-Taylor（PT）and the FAO-56 Penman-Monteith equations were evaluated.Results show that the well performance of these PE-estimation models based on accurate estimation of flux terms in the energy balance equation.With considering the“available energy”PM and PT method can reproduce the PE differences over soil textures quite well.（6）The effect of using different PE on calculating actual evaporation Ea at different water depths were tested using the hydrological model Hydro Geo Sphere.Results show that using PEs and PEw as upper boundary conditions,the absolute value of revealed Ea differences are up to 18%.Which indicates that the selection of potential evaporation has a significant impact on the estimation of actual evaporation of bare soil. |
