Study On Transformation Mechanism Of Vadose Zone Water-Groundwater In The Wind-blown Sand Area Of The Ordos Basin

The area of sand blown by wind in the Ordos Basin is rich in energy resources and short of water resources, where groundwater is the main water supply source. Since the rainfall is rare, the evaporation is intensive and the ecological environment is fragile, with the development of economic and energy, the contradiction between water supply and demand and ecological environment is Serious day and day. Hence, it is urgently needed to strengthen groundwater resource development and the research of social economic and ecological environment harmonious development, of which core is to study precipitation-vadose zone water-groundwater transformation mechanism. As a result, it provides water resource guarantee for economic development and the energy base construction and development, and a scientific basis for sustainable development and utilization of groundwater resource.In this paper, We employ the research methods which combine in-situ testing and physical simulation test and numerical simulation, the process of precipitation (evaporation)-vadose zone water-ground water" is the main study objective. The negative pressure, water content, temperature and pressure and other state variables in the vadose zone are observed by experiments, and the variation and correlation among them are analyzed. Then the hydrodynamic field, aerodynamic field and temperature field coupling model are built, the unsaturated soil water flow and interface dynamics are revealed, the gas and temperature variation of the unsaturated zone and the influence on moisture migration are analyzed, the vadose zone water-groundwater transformation mechanism is explored, and the precipitation (evaporation)-vadose zone water-groundwater conversion process is quantitatively simulated.Based on numbers of experiment and theory, the main conclusions are obtained as follows:1. According to the regular pattern of groundwater in study area, it is considered that the vertical alternate model is the main circulation pattern, and the vadose zone is the main channel of the supplies and excretion cyclic patterns. The main recharged source of this pattern is atmospheric precipitation, which is96.01%of the total recharge; Discharge is mainly evaporation, which is62.56%of the total excretion; According to different characteristics of supplies and excretion, the vertical alternate model is divided into four transformation models, which are precipitation supplies-surface water excretion, surface water supplies, precipitation recharge-diving evaporation and rainfall precipitation field irrigation supply.2. According to the text data and numerical simulation results of the comprehensive profile experimental site in situ test, it is considered that the moisture migration rule in vadose zone is different with the difference of vadose lithology. When the vadose lithology is wind-blown sand, it is of the most advantageous to precipitation infiltration; when the lithology of the vadose zone is weathered sandstone, more than0.3m is in evaporation state; when the lithology particles of the vadose zone is fine, it is adverse to precipitation infiltration, and surface soil evaporates strongly. The more complex vadose lithology structure is, the more complex precipitation mechanism is; when the profile lithology of the vadose zone is a single medium, the moisture migration mechanism is easier. In the local climate condition, it is the lithology structure of the vadose zone within the ground below lm that plays a key influence to the vadose zone, the infiltration rate is that wind-blown sand> sandy loam> weathered sandstone, while surface soil evaporation potential opposites. In the meanwhile, because the hold-water ability of wind-blown sand is weak, its evaporation capacity is small, its infiltration rate is fast and other features, the wind-blown sand is the main cause of relatively rich groundwater of this area.3. Through the indoor physical simulation experiment and the vapor heat coupling model the vadose zone, the influence of temperature on moisture migration regulation and phreatic water evaporation regulation is analyzed. It reveals the dynamic process of water content transformation in the typical area of arid and semi-arid area, and it builds the dynamic process of moisture transformation under isothermy and non-isothermal circumstances, which is simulated by vadose moisture migration model. According to the theoretical and experimental analysis, when the surface temperature is more than25℃, ignoring the influence of temperature will cause a greater error when researching the moisture migration of vadose zone and the evaporation and excretion of groundwater; It established the key indicator that surface temperature25℃as temperature affects vadose moisture threshold; It established a proposition that, in the research area, when groundwater depth is less than70cm, with lowering groundwater level decreased, heat island effect would be caused in the surface.4. Through the vadose hydrosphere two-phase sand slot model test and numerical simulation, it analyzes gas migration regularity in the vadose zone and block infiltration moisture mechanism. Hence, it reveals the arid and semi-arid area rainfall condition vadose for water infiltration of gas block function, which mainly performs as two aspects:1) infiltration moisture causes the increase of vadose gas pressure, which causes the reduction of unsaturated permeability; and2) it increases the response time of infiltration moisture. According to the theoretical and experimental simulation study, when precipitation is more than6mm/d, ignoring the influence of gas will cause a greater error when researching the moisture migration of vadose zone and groundwater supply. The precipitation6mm/d is determined as the threshold which gases affect moisture migration in vodase zone. At the same time, vadose gas flow in infiltration under pressure to push a larger value will break by overlying water surface, the gas main to escape the vertical movement primarily. But gas with the passage of moist peak migration to face downward dive in the block due to diving role, gas exist obvious level migration. According to the test and numerical simulation results, based on rainfall condition of water between two phase multi-gas relations and the pressure changes features of profile, the gas in vadose is divided into three areas:Ⅰ steady area; Ⅱ gas pressure changing area; and Ⅲ gas pressure slow-increasing area.5. According to the in-situ test data and indoor physical simulation data, it analyzes soil-air interface and soil-water interface and groundwater kinetic process of the interface. The relationship between water evaporation and soil surface evaporation is determined the water evaporation temperature and the analysis of present in warming period between linear relation in cooling period, positively index;6. The medium characteristic parameters in the Vadose zone determination includes moisture feature parameters and heat conduction coefficient. we designs a nonisothermal pressure vadose parameters of medium to determine the tester, water characteristic parameters and Saturated hydraulic conductivity (Ks). We designs the determination of soil media state of heat conductivity "experiment method, obtained the medium heat conductivity with moisture content, established the relation between new model said elationship KT(θ)=KTs(?),which saturated Heat conductivity and put forward the concept of residual Heat conductivity, and its reliability and sensitivity, solving method is analyzed and verified.7. According to the in-situ test and numerical simulation results, we analyzes precipitation infiltration coefficient and thickness in vadose zone, and get the following conclusions:(1) under the same thickness conditions in vadose zone precipitation coefficient from big to small orders:wind-blown sand>salawusu group> weathered sandstone> sandy loam; and (2) when the lithology structure in vadose zone for wind-blown sand overlying, be helpful for precipitation, therefore its rainfall infiltration coefficient, and when vadose lithology overlying sandy loam knot, precipitation infiltration coefficient is lesser, against water infiltration. Study the buried depth of the change and scuba diving lithology of different groundwater evaporation excretion of the influence of the buried depth, along with depth increased, phreatic water vaporization decreases, reached the limit buried depth is evaporation close to zero, the limits of several lithology buried depth rule for loam (1.6m)> mucky sand (lm)> weathered sandstone (0.8m)> wind-blown sand (0.5m).8. Completing renewability and groundwater renewable resource concept and its connotation, this paper analyzes the groundwater renewable resource characteristics and influencing factors; and put forward a new evaluation of groundwater reproducibility. Based on renewable resources characteristics of groundwater and the influential factors, and according to the specific situation in Ordos Basin, this paper establishes the groundwater of Ordos Basin renewability evaluation index system, and identifies evaluation method. According to the calculation method of renewable resources of groundwater, calculating the renewable resources for groundwater of general water year in the typical area of Subei River Basin is958.02million m3/a, and95%frequency years of groundwater renewable resources is896.63million m3/a. Finally, according to the groundwater renewable ability, it draws the evaluation map of groundwater renewability of Subei River Basin in northern Ordos Basin.