Challenge from the appearance of vadose water within the surface iayer of megadunes, Badain-jaran dune desert, Inner Mongolia
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摘要: 1997-2003年对巴丹吉林沙漠中不同位置共7个高大沙山和1个流动沙丘表层2m内的孔隙水情况用常规方法作了观测,每年均在相近日期进行。孔隙水量随深度而增大,历年观测期间2m附近的体积含水量约3%,达到相应实测最大持水量的约65%或更高,25cm至1m孔隙水δ18O的正值表明它经过了反复补给-蒸发过程。观测了雨后入渗深度及其在沙丘120m坡面上的分布,另由历年短期能量平衡测验获得沙山陆面蒸发和凝结概念。认为当地年降水量和凝结量不足以对沙山2m表层内的孔隙水现象作出解释,疑另有与大尺度地下水系统相关的补给源。Abstract: The vadose water within the surface layer with depth of about 2 m of 7 megadunes and 1 movable dune was observed during camel expeditions of 1997-2003.The relative height of these megadunes ranges from 300 to 400 m with the altitude of 1 300 m-1 488 m a.s.l.Traditional methods are used to probe the vadose water content and the physical features of its eolian deposit.Observations were made during 25-27 September every year at different spots in the hinterland of that sand sea.The results show that for all of the observed megadunes,the vadose water content appears to increase volumetrically with the depth with an amount of about 3% at the depth round 2 m below the megadune surface.It reaches to about 65% of the maximum wetness held by matrix potential of the deposit (HB) which was measured after rainfall events during the expeditions of 2000 and 2003.The positive δ18O values of the vadose water at the depths from 25 cm to 1 m beneath the megadune surface reveal a repeated recharge-evaporation process.The infiltration depth of a few centimeters after rainfall event was also observed along a slope of 120 m on a sand dune,it could be quickly exausted through the evaporation measured by energy budget.It is concluded that the local precipitation with annual mean of about 50 mm from its northwest to 80 mm southeastwards seems not sufficient for the output amount of annual evaporation from megadune surface even if a portion of it being condensed.If take into account the high percentage of HB of the vadose water within the surface layer of megadunes and the enrichment process of 18O,the current concept that the main recharge source of the megadune area is the local precipitation will be challenged.The existence of an important groundwater recharge source far away from this dune desert,perhaps,from the mountainuous area to its south,is not directly identified,but it seems to be implied.
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Key words:
- Badain-jaran dune desert /
- megadune /
- pore water /
- evaporation and condensation /
- saturation /
- environmental isotope
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