The Simulation Study On The Hydrothermal Transport In Aeration Zone Under The Evaporation Condition

Water is a strategic resource of sustainable development of national economy and society. Accurate quantitative assessment of water resources is quite necessary, especially in the areas where water resources is scanty. Because of the lack of surface water system, scarce rainfall, strong evaporation, higher diurnal temperature and extremely fragile entironment in the arid and semi-arid area in Nothwest China, water resources have become an important role which restricts the local economic development. Researches have shown that vertical exchange is the chief way of water cycle in this area and the main discharge of groundwater is evaporation. Thus, it is vital to accurate evaporation account for local water resources assessment. Experts have made lots of studies on the transformation mechanism of "Precipitation-Surface Water-Aeration Zone Water-Ground Water" and.have obtained great achievements. However, when the temperature of profile changed with season and diurnal,whether this condition could impact evaporation, if it did, how influence degree is ,and how to deal with this problem during the numerical simulation process, these problems need further stydies.Therefore, the further study carried on the law of phreatic evaporation is significant, and on the hydrothermal movement mechanism in the aeration zone.The aim of this paper is to research the law of phreatic evaporation and the factors affect it. Based on the theory of fluid dynamics in porous media and soil moisture thermodynamics, the author made an further analysis of hydrothermal movement law in the aeration zone under the evaporation condition with the physical simulation of the phreatic evaporation by hydrothermal transport simulation equipment designed by ourselves. A hydrothermal coupling model is built based on the water and thermal parameters obtained from the physical experiment. Finally, analyzed the influenc of different factors on phreatic evaporative intensity.From these work, some conclusion and recognition are achieved:1. For acquiring the moisture and thermal parameters, we manufactured kinds of testing instruments, which with higher precision and data obtained from them are reliable; designed a new method of "testing thermal conductivity of soil medium by steady-state method", the relationship between thermal conductivity and water content is obtained, which broke the limitation of only relied the empirical formulas in the past.2. Developed the traditional model to a new model for represent the relationship between thermal conductivity and water content:which endowed the parameters in this model with physical meaning.3. For homogeneous media, phreatic steady evaporation (E) and potential evaporation(E₀)show a good linear relationship. The impact of temperature on water transport is more remarkable when the atmospheric evaporation capacity is feeble rather than the intensive evaporation condition.4. Phreatic evaporation is inhibited when the study profile's temperature is higher on the top and lower on the bottom. On the contrary, when the temperature is lower on the top and higher on the bottom, phreatic evaporation is promoted. The bigger temperature gradient is ,the more intense of the inhibition action or promotion action to evaporation. When the temperature difference of the top-surface and bottom-surface is more than 10℃, we should consider the temperature factor on the water resource calculation in aeration zone.5. Take buried depth of 50cm is the dividing value, when groundwater buried depth is less than this value, phreatic steady evaporative intensity decreases slowly with groundwater buried depth increase and the results of the isothermal model agreed with the hydrothermal coupling model; when groundwater buried depth is more than 50cm, the intensity decreases sharply with groundwater buried depth increased, and the hydrothermal coupling model took 60cm as the limit buried depth of phreatic evaporation,which had perfect agreement with the experiment result while the isothermal model shown limit buried depth was 90cm. Thus,when groundwater buried depth is less than 50cm, the impact of temperature in aeration zone on water transport is remarkable, using the hydrothermal coupling model will be more precise.