The Water Cycle And Water Balance Of The Sand-lake System In The Badain Jaran Desert

Badain Jaran Desert is the second largest desert in China and its unique feature is the highest relative altitude of megadunes and largest number of lakes in its southeast,which is incomparable by desert in other areas.There has been a great debate over the source of lake and sand layer water in megadune-lake area of Badain Jaran Desert.The infiltration-excess surface runoff erosion landform,sand layer water seepage,runoff-produced physical deposits and the fan-shaped landform formed by it,precipitated salt of runoff-produced chemical deposits of megadunes,descending spring at the base of leeward slope of megadunes,groundwater spilling belt around lakes,vegetation bands and vegetation patches of megadunes and other natural phenomena in this area recently found by us in field survey conducted in 2010,2011,2015 and 2016 respectively indicate to different extent that this area has the characteristic of positive balance of water and there is conversion relation among atmospheric precipitation,vadose zone water of megadunes,underground water and lake water.Thus,with the identification of those natural phenomena,this paper makes determination of water content and grain size in sand layer of vadose zone of megadunes,aeolian sandy soil saturation water infiltration experiment,determination of field capacity of aeolian sandy soil,aeolian sandy soil preferential flow dye tracer experiment,runoff-produced physical deposits sand layer pore water and descending spring water hydrogen and oxygen isotopes determination in a further way,conduct research of the conversion mechanism between precipitation,vadose zone water of megadunes,underground water and lake water.Based on it,this paper establishes water circulation mode of megadune-lake system in this area and estimates water balance of megadunes and lakes respectively.Main conclusions got in this paper are as follows:1)The newly found infiltration-excess surface runoff erosion landform at depression of megadunes and the edge of dry lake basin indicate there is precipitation process of higher intensity favorable for recharging megadunes in this area.The newly found water seepage phenomenon at sand layer in the slope of megadunes,runoff-produced physical deposits and fan-shaped landform formed by it as well as precipitated salts of runoff-produced chemical deposits show the water of vadose zone of megadunes recharged by precipitation in this area could crop out the surface as sideward flow and form surface runoff.The runoff-produced physical deposits and fan-shaped landform formed by it and the emergence of precipitated salts of runoff-produced physical deposits composed of carbonate and sulfate indicate surface runoff formed by outcropping of sideward flow in vadose zone of this area is a long-term phenomenon instead of the one being caused by accidental precipitation event.2)Groundwater spilling belt distributed in a non-continuous manner around lakes shows the characteristic of concentrated runoff generation of underground water in some sections.The outcropping of descending spring at the base of megadune leeward slope provides an effective evidence for concentrated runoff generation of underground water in some sections.The difference in infiltration recharge from precipitation to underground water in different area of megadunes is the reason of concentrated runoff of underground water in some sections.The development of vegetation bands and vegetation patches of megadunes indicates in the process that vadose zone water of megadunes recharged by precipitation moves to the deeper part of megadunes,certain extent of concentration and sideward flow is produced to recharge sand layer water at vegetation bands and vegetation patches development part and then maintain the long-term development of vegetation bands and vegetation patches.3)The highest water content of send layer at the place with precipitated salts of runoff-produced physical deposits at slope of megadunes is about 3.5?8.0%and average value is 6.5%.3.3%of water content of sand layer in megadune vadose zone can still crop out and form surface runoff to maintain the continuous wetting of some sandy surface.The size composition of sand layer in vadose zone of megadune has strong spatial variation,the alterative variation between coarse grain and fine grain layer,which is relatively thicker,is obvious.The fine sand or finer component in fine grain layer reduces the infiltration rate of water content in sand layer,which make fine grain layer has relative impermeability and result in concentrated runoff of water content in vadose zone in movement process.4)With respect to water-holding capacity of water in sand layer in vadose zone accepting recharge of precipitation in field,only about 3.2%in aeolian sandy soil exists as gravity water.Water content in aeolian sandy soil can make preferential flow movement due to the variation of grain size in sand layer.The appearance of more than 3.2%of gravity water in aeolian sandy soil and preferential flow does not only beneficial to rapid infiltration and recharge of precipitation to water of megadunes,but is also favorable for water content of megadunes to move quickly to the base to recharge underground water and lake water.5)Relatively intense precipitation process with more than 15mm single precipitation,restraint evaporation of dried sand layer,low water-holding capacity and high infiltration of aeolian sandy soil,few vegetation and less water consumption,movement of preferential flow of gravity water in sand layer in vadose zone and water content in vadose zone provide an essential condition for precipitation in this area to recharge water to megadunes and then underground water and lake water.6)The composition of hydrogen and oxygen isotopes of pore water in sand layer at runoff-produced physical deposits and water of descending spring at the base of leeward slope of megadunes have homology and come from vadose zone of megadunes,which effectively prove the vadose zone water accepting recharge from precipitation in this area has formed a recharge to underground area and lake water in this area.The-28.9‰deuterium excess(d-excess)of pore water in sand layer of runoff-produced physical deposits in this area is lower than underground water and the reason is high evaporative fractionation.7)There is rest of water to feed the vadose zone water,underground water,and lakes after the loss of atmospheric precipitation by evaporation,transpiration,and surface runoff.The water cycle process is atmospheric precipitation-vadose zone water-underground water-surface water.It showed that 2.4×108m3 waters which come from atmospheric precipitation and took account of 12.2%can feed the megadunes water,underground water and lakes.The lakes had only the evaporation capacity of 4.8×107m3 Hence,the supplementary amount from atmospheric precipitation can provide enough water to maintain the lake evaporation and the existence of the lakes.