Study On The Eco-Hydrological Indicators And Water Source Of Tamarix Laxa In The Mega-dunes Of The Badain Jaran Desert

Tamarix laxa,which were surprisely discovered in the middle and upper part of the mega-dunes in the southeastern part of the Badain Jaran Desert during the scientific investigation of our team,are tall and have a trunk that grow like arbor.Due to the relative height?about100m-250m higher to the lake surface?,it is difficult for them to directly absorb groundwater.In addition,the desert is very drought with annual precipitation below 80mm.In that way,what is the water source for the Tamarix laxa to rely on?How high is the water content of the sand layer?What is the water content difference between the Tamarix laxa distribution area and the bare sand mountains?What is the ecological hydrological indication to the mega-dunes?This thesis takes the discovered Tamarix laxa as the research object,analyzing the spatiotemporal dynamics of water content measured by drying method based on the hydrogen-oxygen stable isotope technology,comparatively analyzing the water content differences between Tamarix distribution and the bare mega-dunes by positioning monitoring the water content,exploring the water type and precipitation infiltration characteristics indicated by the Tamarix distribution through the frequency distribution statistics of water content,discussing the process of water migration and conversion by measuring the temperature changes of different depths of the mega-dunes,and exploring the water source of the Tamarix laxa and mega-dunes by measuring the?¹⁸O and?D values of soil water,stem water and others potential water sources.This thesis improves the desert ecological hydrology and the drought-resistance mechanism of the Tamarix laxa,provides theoretical basis for filtering suitable species for ecological restoration in arid zone.This paper mainly obtained the following research results:?1?The average water soil content of the Tamarix distribution area is 1.2%,which decreases with the increase of the depth.The profile can be divided into a moisture-changing layer?0-60 cm?,an active layer?60-120 cm?and a relatively stable layer?120-200 cm?according to soil water content feature.The difference of soil water content between different levels of the same slope is significant?p<0.05?,and the moisture-changing layer is significantly higher than others.?2?The soil water content of the Tamarix distribution area is higher than the bare mega-dunes,however,the water content fluctuates greatly,and the average water coefficient of variation was0.58,which is 4.46 times that of the bare mega-dunes.The water content of Tamarix distribution area is more sensitive to heavy rainfall events than the bare mega-dune.Under the same precipitation conditions,the infiltration rate of Tamarix distribution area is faster and deeper than the bare mega-dunes.The infiltration depth of heavy precipitation event is limited in the bare mega-dunes?<60cm?,and is difficult to be transformed into gravity water for rapid infiltration.?3?More than 90%of the soil water content is distributed between 0.5%and 3.0%,with the maximum observed value 4.88%.Both are less than the water holding capacity?5%?,indicating that there is no gravity water in the mega-dune.It is difficult to form capillary water except for the foot of mega-dune,as the composition of mega-dune was medium and fine sand,the groundwater had a limited height?<60cm?under the action of capillary force.Therefore,the water type of mega-dune is mainly adsorbed water and gaseous water.?4?The?¹⁸O and?D of the sand water are not only located below the local meteoric water line(LMWL:?D=6.90?¹⁸O+5.43?R²=0.84,n=18?),but are also on the same evaporation line as condensate water and groundwater(EL:?D=3.91?¹⁸O–31.69,R²=0.87,n=185),indicating that precipitation,condensate water and groundwater may be water sources for the mega-dune.The precipitation mainly supplies shallow water?<60cm?,condensate water mainly replenishes surface water?0-15cm?,and groundwater may be replenished to the mega-dune with gaseous water,but it still lack direct evidence.?5?The water of the mega-dune moves up and down driven by the gradient of temperature,and continuously condenses and evaporates during the migration process,moistening the sand layer.In the spring and summer seasons,with strong evaporation,the temperature of the shallow layer is higher than that of the deep layer,which drives the water to migrate downwards,contrary to the evaporation direction,inhibited evaporation and is conducive to save wet sand.In the autumn and winter seasons,with weaker evaporation,the temperature of the deep layer is higher than that of the shallow layer,which drives the water to migrate upwards,which was conducive to form the wet sand layer.This reciprocating movement of sand water content may conducive to the formation and maintenance of wet sand.?6?The?¹⁸O value of stem water is the closest to soil water,indicating it mainly relies on soil water to maintain growth.However,the?¹⁸O value of stem water was significantly different in different months?p<0.05?,indicating it use different sand layer water in different growth stages.In the dry season,the Tamarix laxa uses water from the active and relatively stable layers,and the contribution rate is over 90%,while during the rainy season,it uses water from moisture-changing layer,and contribution rate is 46.7%.The Tamarix laxa can cross over the drought and survive by changing the sand layer of water use.Heavy precipitation events have a significant impact on the water use options.