The Influence Of The Shallow Groundwater Table Decline On The Near Surface Temperature And The Surface Energy Balance In The Mu Us Bottomland

The Mu Us bottomland is located in an arid and semi-arid region and belongs to temperate monsoon climate.In recent years,with the excessive exploitation of groundwater resources,the expansion of animal husbandry and the growth of the population,the region is facing problems such as the groundwater table decline,degradation of the ecological environment and soil desertification.The study of the water table depth decline in the Mu Us bottomland is a a beneficial supplement to the law of groundwater environment and climate evolution.,and the research results has important scientific and practical significance for the development and utilization of groundwater resources and the protection of the hydro-ecological environment in the region.In this paper,the profile from vegetation canopy to groundwater in The Mu Us bottomland is taken as the research object.Based on the basic theories of seepage theory,soil hydrodynamics,soil physics,the in-situ data monitoring,indoor experiments,the mathematical statistical analysis,a numerical simulation methods were used to analyze the variation characteristics of soil moisture and soil temperature and the influencing factors of near-surface temperature and surface energy balance in the study area,and the constructed water and heat coupling model was used to discuss the influence of the water table depth on the near-surface temperature and surface energy balance components at the seasonal and day-night time scales,and determine the depth boundary.The main conclusions are as follows:(1)vegetation can reduce the variation of soil temperature;The daily variation range of ground temperature in typical weather(cloudy and rainy days)is relatively small.When the depth reaches 87 cm,the ground temperature basically remains unchanged;The soil profile temperature shows sinusoidal characteristics with the change of seasons,and the peak values of different depth temperatures lag behind,The depth of 10 cm and 20 cm is 2 and 4 hours behind the depth of 5 cm respectively.The underground depths of 0-40 cm,40-87 cm and 87 cm in the study area can be divided into zones of drastic changes in water potential,seasonal changes in water potential and stable water potential;Meteorological factors,the water table depth and vegetation are important factors affecting soil temperature change in profile.The degree of influence of meteorological factors gradually decreases with the increase of the water table depth;the surface temperature of shallow groundwater decreases in summer and increases in winter.(2)Using SHAW model simulation,the results show that the simulated values of water content,soil temperature and total evaporation at each measuring point are consistent with the change trend of the measured values,and the model basically meets the applicability requirements in a certain error range.(3)Based on the result of SHAW model,the fundamental reason for the influence of the water table depth on the near-surface temperature and surface energy balance is to change the soil moisture content distribution of the profile.When the water table depth declines,net radiation flux and latent heat flux become smaller;sensible heat flux increases decreases in winter,and significantly in summer while soil heat flux may decrease in winter,and increases in summer.(4)The typical rainless days in winter and the typical humid days in summer have small effects on the near surface temperature and surface energy balance,but the difference is that the typical rainless day in winter is due to low solar radiation,while a wet day in summer is due to the moisture required for latent heat evaporation.(5)Under the conditions of sand soil in the study area,the upper boundary of the shallow groundwater affecting the near surface temperature and the surface energy balance is determined to be 0.2m,and the lower boundary is 1.8.