Interaction Mechanism Between Subsurface Hydrological Process And Vegetation Ecology In Mu Us Sandy Land

The vegetation ecohydrological system in semi-arid area is characterized by multi-factor restriction,multi-system integration and high complexity,and its evolution characteristics depend on the mutual transformation and coupling among atmosphere,vegetation,soil and groundwater,and it has different feedback and response in different spatial and temporal scales.In particular,vegetation participates in the water and energy cycle,such as accelerating evapotranspiration,reducing soil water storage and stopping deep soil water recharge.However,the characteristic and mechanism of root water uptake of xerads are not clear.Due to the influence of rainfall quantity and water level,the dependence degree of different vegetation on soil water and groundwater in vadose zone is obviously different,resulting in the calculation result of macroscopic empirical root water uptake model is not ideal.The distribution pattern of vegetation and its interaction with soil water and groundwater by root absorption has proved to be a difficult problem.Therefore,it is necessary to explore the interaction mechanism between vegetation and underground hydrological processes in arid regions.In this thesis,the Mu Us Desert was selected as the object and the in-situ test base of water-gas-heat in Ordos Basin as weel as the field monitoring station of Gacha Baozhai Nur in Buzhai,were chosed to research water transport mechanism under the influence of vegetation and its mutual feed-back mechanism with groundwater,combined with field investigation,in-situ monitoring and numerical simulation.Firstly,the relationship between the distribution pattern of vegetation and soil water and salt and groundwater level was investigated by field investigation;The distribution characteristics of root biomass and density of typical vegetation were obtained.Secondly,the water consumption of different vegetation in different growth stages was analyzed through continuous in-situ monitoring of individual vegetation;The root water consumption of typical vegetation(Salix psammophila)and the contribution of water and groundwater to evapotranspiration were quantified;The dynamic process of water transport in the vadose zone under the influence of root water uptake was revealed.The dynamics of water exchange in the atmosphere-vegetation-soil water-groundwater continuous system was study based on mass and energy conservation principle;The water exchange of soil-root interface was preliminarily revealed,and the water consumption mechanism under water stress was analyzed,and the root water uptake model of Salix psammophila root was established by the inverse algorithm.Then the model was used to describe the effect of root water uptake process on rainfall infiltration and groundwater recharge.Finally,a Km-scale soil water transport model was established to reveal the influence of different vegetation zones on the underground hydrological process.The results show that:1)The root density distribution of Artemisia sphaerocephala accorded with lognormal distribution function with burial depth.The distribution of root density of Psammophila psammophila accorded with the function of exponential-lognormal composite distribution.The root depth of Artemisia sphaerocephala is shallow,which only consumes vadose water and does not utilize groundwater.However,the root system of Salix psammophila intercepts the infiltration rainfall,which leads to the groundwater can not be replenished and causes the groundwater level to drop.2)Under the shallow water depth(about 0.5 m)scenario,the total evapotranspiration of aeolian sand bare land is 0.99～1.04 times of the total rainfall,while the evapotranspiration of Salix psammophylla land is 1.12～1.17 times of the total rainfall.Under the deep water depth(about 2.5 m),the total evapotranspiration of bare land was 0.66～0.78 times of the rainfall,and the soil water storage increased.However,the total evapotranspiration of Salix psammophylla land was 1.53～1.69 times of the rainfall,which was significantly higher than the total rainfall.It means that rainfall cannot offset the soil water deficit caused by vegetation water consumption and soil water storage decreases.3)Salix psammophila uses a mixture of groundwater and soil water at the same time,and forms an adaptive water consumption pattern in different habitats(vadose zone thickness,groundwater depth,etc.)and growth stages.When the water level was in the root zone or slightly lower than the root zone,the root system could directly absorb water from the saturated zone.When the water depth exceeds the root depth,vegetation develops deep roots and uses hydraulic redistribution to make full use of the deep soil water,and turns to shallow soil water supplemented by rainfall in the rainy season.This water consumption pattern ensures its tolerance to soil water stress.4)According to the measured soil moisture,the root water uptake distribution of Salix was obtained by the inverse method,which conforms to the index-lognormal compound function.This method covers the whole growth period without the root density distribution,and guarantees the integrity of the root water uptake processes,but relies on higher requirements on the soil hydraulic parameters.5)When the water depth is less than 1.5 m,the thickness of the vadose zone decreases with the further rise of the water level,the oxygen content in the soil decreases,the root respiration hypoxia leads to the decrease of plant root activity,and the vegetation becomes more and more exposed to oxygen stress.When the water depth is greater than 1.5 m,the vegetation stress index decreases with the increase of water level,and vegetation begins to be stressed by soil water.When the water level is greater than 5 m,groundwater cannot be directly used by vegetation,and vegetation is under severe stress.6)By constructing a two-dimensional variable saturated groundwater flow numerical model for the lake-dune ecosystem,it can be found that the grass within shallow water level is the main cause loss of soil water and groundwater,and shrubs within deep water level cause vadose water deficit mainly,due to the root depth is less than water table depth.Based on the above research techniques and results,a regulation strategy was proposed to balance vegetation water consumption and deep soil water supply,that is,the distance between Salix psammophila should be greater than 5 times of crown width.At the same time,the groundwater and ecological monitoring methods and technologies in arid regions are put forward to provide support for groundwater and ecological environment management decisions in arid regions.