Research On Soil-groundwater Evaporation In The Wind-blown Sand Area Of Ordos Basin

The wind-blown sand area is located in the arid and semi-arid area of Northwestern China,which is an important new energy base in China.The annual rainfall is rare,evaporation is intense,and water resources are scarce.The vadose zone is a key region connecting the transformation of soil water and groundwater,which affects the process of phreatic evaporation and infiltration.Evaporation,as one of the main driving force of soil water transport,is one of the main driving forces of soil moisture transfer and heat transfer.Due to the influence of external conditions and soil medium,its internal dynamic process is extremely complex and nonlinear.Due to the complexity of the dynamic evaporation process,the research on the dynamics of soil-groundwater evaporation is still relatively weak at present.The core is to take soil-groundwater as an integrated system to study the mechanism and influence factors of soil evaporation in saturated-unsaturated zone.These findings will provide scientific basis and practical guidance for the rational allocation and utilization of groundwater resources.Based on the in-situ experiment site in the Henan,Wushenqi of Ordos Basin,this paper mainly analyzes the variation of water and heat transfer in vadose zone by using combined lab-physical simulation and numerical simulation methods,aiming to reveal the complex dynamic evaporation process in the soil.The main conclusions and understandings are as follows:1.Based on the in-situ field experiments,the zonal rule of water and heat transfer is revealed in vadose zone under different rainfall intensity.When the rainfall intensity is less than 1.5 mm/d,only the soil water content at 3 cm depth increases.For the 9.6 mm/d rainfall process,the profile soil water content above 50 cm depth responds to rainfall.Therefore,the larger the previous rainfall was,the larger the reponse range of soil water ciontent.Under the condition of alternating rainfall and evaporation,the soil temperature at 50 cm depth still changes slightly sinusoidal with time,and the soil temperature at 100 cm depth remains basically unchanged.Therefore,it is inferred that there is a significant zonation at 50 cm depth,and that the key area for the heat transfer and transformation of water vapor is from the surface to 50 cm depth.2.The hysteresis response of soil water content and temperature to the measured values at 3 cm depth is studied.Under the continuous evaporation condition,the maximum correlation coefficients of soil temperature of 5 cm,10 cm,50 cm depth to 3 cm depth were all greater than 0.5,while there was little correlation between soil temperature at 100 cm depth and that at 3 cm depth.With 39.8 mm/d rainfall intensity as the excitation source,the hysteresis response time of 5 cm,10 cm,50 cm depth to 3 cm depth below the surface was 0.21 h,0.32 h and 6.91 h,respectively.The lag time for rainfall is very small for the wind-sand found in the near surface layer,so that the rainfall quickly recharges the soil-groundwater.So the soil can store a large amount of water resources,which in turn forms relatively abundant groundwater resources in the area.3.The coupling model of heat,water and vapor under different water table depths is constructed.The soil water distribution in vadose zone under the influence of different water tables is analyzed;it is found that for the water table close to the height of capillary rise,the soil evaporation process is mainly controlled by the atmospheregroundwater,resulting in larger evaporation rate;for the water table much deeper than the capillary rise height,soil evaporation is mainly controlled by soil properties and there are obvious zero-flux isolines in the shallow vadose zone and it prevents the upward movement of soil water from evaporating,which results in rapid decrease of evaporation rate.It is found that there is a critical transformation region of liquid-vapor water around the depth of 20 cm,and its formation leads to the daily maximum evaporation appearing between 10:00 and 12:00,which is not significantly positive relationship with air temperature.4.Based on the calibrated numerical model,the coupling model of water-vaporheat under the influence of fiexed water tables was constructed on an annual scale,It is found that the relationship between soil evaporation and water table depths is an exponential decline function.The extinction depth of wind-sand medium is further determined to be about 105 cm depth,which is about twice the capillary rise height.The conclusion has important reference significance for the local government to manage the soil and water resources in the sand-blown area.