Hydrodynamic Progress Of Bare Soil Evaporation In Semi-arid Mu Us Desert

Evaporation is a major water balance component of the hydrological cycle in arid and semi-arid regions.It plays a considerable role in regulating the distribution of water in the unsaturated zone.Accurately quantifying energy and mass transfer in the process of evaporation is one of the most urgent scientific issues to be solved.Because of temporal as well as spatial constraints to directly measure soil evaporation,the Richards equation is commonly used to model the behavior of water flow in the unsaturated zone.An accurate knowledge of the relationships among matric potential,moisture content,and hydraulic conductivity is required to solve this equation,i.e,the soil water retention function(?)and the hrdraulic conductivity function(?).By far the two most commonly used functions are those proposed by Brooks and Corey and van Genuchten,they both assume a distinct residual water content,,which is asymptotically reached at very high suctions.The residual water content is interpreted as the water held by adsorptive forces or as a mere fitting parameter.In the very dry range,even the adsorptive water finally reaches a value of 0 and those functions have been proved to perform poorly.Attempts should be made to derive new models account for analyzing the soil water characteristics in the full range between saturation and near oven dryness.In this thesis,bare desert land of the Mu Us Desert was selected as the object to research dynamic mechanisms of water evaporation.Firstly,laboratory experiments on the drying process of initially water-saturated soil conlumn with a heat supply above soil surface were conducted.The collected experimental data were used to analyze evaporation characteristic of the Mu Us Desert.A simple analytical tool was proposed for predicting the maximum depth of the water table hydraulically connected to the topmost soil layer(TSL)via continuous capillary liquid pathways.The critical water content(?e)at drying front and the range of water content(?)in the transition zone from liquid to vapour were determined both theoretically and experimentally.Secondly,based on Richards equation,a numerical model coupled equations governing liquid water,water vapor,and heat transport was established to inverse the drying process of laboratory experiments.Based on the calculation results,the inadequacies and solutions of the traditional hydraulic parameters in solving the water transport in even dry soil were discussed.A new water retention model for full range saturation is given by weighted sum of a capillary and an adsorptive saturation term,and the total unsaturated hydraulic conductivity was modified by coupling an existing capillary conductivity and a developed absorptive conductivity.The modified retention and conductivity models were verified with 5 datasets from the literature.Simulated water contents and accumulated evaporation were in good agreement with measured values when the modified model was employed to reproduce the data collected from laboratory experiments mentioned above.Thirdly,an 43-day data set including atmospheric forcing,soil water content,matric potential and temperature were gathered on a bare soil site,characterized by a large diurnal cycle of temperature and with very dry conditions near the surface.Finally,based on the modified hydraulic parameters,a new integrated numerical model that is able to describe the transport of liquid water,water vapor,and heat transport in soil was developed,the performance of the model was evaluated using soil temperature and water content data collected at field site.The model was used to investigate the mechanisms of the water movement and evaporation within the soil under dry conditions.The results show that:(1)The maximum depth of the water table hydraulically connected to the topmost soil layer(TSL)is 30.7 cm.(2)The critical water content(?e)at drying front is 0.055 and the range of water content(?)in the transition zone from liquid to vapour is0.015-0.055.(3)The improved hydraulic parameters of the full range of saturation can more effectively describe the soil water retetion and hydraulic conductivity at low moisture content.The modified model(capillary and film)and traditional model(capillary)reproduce 98.8%and 87.0%of the total evaporation flux,respectively.Based on the fitting results of Mu Us Desert,the critical matric suction for film flow to be dominating is-140 cm.The film flow terminated when suction reaches a value of-3×10~5cm and the drying front defined as the planeatwhichphasetransformationoccursmovesdownwords.The accumulated evaporation will be underestimate about 10%if the film flow was neglected.(4)Based on the modified hydraulic parameters,a new coupled numerical model was developed to describe the transport of liquid water,water vapor,and heat transport in soil synthetically.The developed model provides a useful approach to investigate the mechanisms of the water movement and evaporation within the soil under dry conditions in Mu Us Desert.The results show that:the water flux driven by the matric potential gradient is 2-4 orders of magnitude greater than the temperature-driven moisture flux,thus matric potential gradient is the main driving force of evaporation in the Mu Us Sandy Land.In the first 3 cm of soil,the upward vapour flux induced by large matric potential gradients is found to be the most important transfer term,which can hardly be canceled by downward one induced by temperature gradients.The film flow is dominant between 3-20 cm below the surface,the soil matric potential and the potential gradient are still high,but not enough to promote the phase transition.The film flow is drived by matrix-potential gradient,the direction of the net flux is upward.(5)Compared with the traditional hydraulics parameter model,the improved model based on capillary flow and thin film flow is better describe water movement from full saturation to complete dyness.No additional parameters are introduced in the new model because the critical water content and critical pressure head are calculated from other parameters and it is simple to use and to implement into simulation tools.The results reported in this thesis have accurately described the moisture transport mechanism process of unsaturated zone in typical arid and semi-arid area and have enriched and developed the theories and methods of soil hydrodynamics and related disciplines.