Modeling The Soil Water Flow Process Under Evaporation- Coupling Capillary, Film And Vapor Flow

The unsaturated zone in soil acts as a link between the surface system and the groundwater system and plays a crucial role in water and solute cycling, supporting the terrestrial ecosystem. For its importance, there are more and more works focusing on the soil water and solute transport process, among which modeling has turned out to be a much more powerful tool comparing to field observation, However, commonly used soil hydraulic models that account for capillary forces often do not perform well at low water contents when water flow may occurs in thin liquid films or in vapor form. In arid and semi-arid places, the soil surface usually under low moisture conditions due to strong evaporation, so it is necessary to extent the soil hydraulic model to low water contents. In this paper, we try to develop a new theoretical model that accounts for film flow, after which, we try to couple film, capillary and vapor flows together to describe the water flow process from saturation to zero water content, then we plan to test the model performance in field using observed water content and water potential data. The mail conclusions are as follows:1. A new theoretical and mathematically simple soil hydraulic model that accounts for film flow was formulated based on the film thickness function that accounts for matric potential. Inverse modeling of an evaporation experiment showed that the new model provides a good description of water flow at low water contents.2. Based on the model structure proposed by Fredlund and Xing (1994), the new film model was coupled with capillary based model to describe the hydraulic properties from saturation to zero water content. The new soil hydraulic model is mathematically continues and easier to use. Model testing of 11 soils from the literature and 4 soils from evaporation experiments all showed good results from saturation to very dry conditions. Modeling results showed, in general, the larger partical size the higher film saturation conductivity.3. Coupling the new capillary-film model with an existing vapor formula can describe the entire water flow process from saturation to oven-dry, including liquid flow and vapor flow. Modeling results of a sandy soil show that neglecting film flow can cause a significant underestimate of total water loss while vapor flow is shown to be less important for the short sand column. The C model (capillary) and the CF model (capillary and film) reproduce 89.4% and 99.9% of the total evaporation flux, respectively while the vapor flow only accounts for about 0.1% of the total evaporation flux. The simulation results of an evaporation scenario for two different soils indicate that film flow is much more important for high sandy soil than loam while vapor flow is not important for both soils. However, all these analyses are based on modeling results, there are still a lot of uncertainties exist.4. Tengger desert and Huazaizi site are chosen to test the model performance in field. Modeling results show the new model that accounts for both capillary and film flow can provide a good description of the water flow in both sites while the capillary based model show poor results. For the high sandy soil in Tengger desert, film flow can be the dominant process in the surface soil while in the Huazaizi site with loam soil, film flow becomes important when water content is less than about 0.08.5. A new method for calculating actual evaporation from potential evaporation in moisture limited soil was developed. The new model only introduces one parameter, which accounts for the proportion of the’stage-1’area in the total area after a rainfall event. Modeling testing in Huazaizi and New Mexico sites both shows good results ignoring the magnitude of the precipitation and the difference of soil properties.