Zhe Wang ,Li-juan Wang ,*,Jian-mei Shen,Zhen-long Nie,Ling-qun Meng,Le Cao,Shi-o Wei,Xiang-feng Zeng

a China University of Geosciences (Wuhan), Wuhan 430074, China

b Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China

c Center for Hydrogeology and Environmental Geology, China Geological Survey, Baoding 071051, China

d Alxa Right Banner administration of Alxa Desert World Geopark, Alxa Right Banner 750306, China

Keywords:Desert groundwater Water resource assessment Ecological environment Hydrogeological survey engineering Badain Jaran Desert Alxa Right Banner Southwest Mongolian Plateau China

ABSTRACT The Badain Jaran Desert is the third largest desert in China,covering an area of 50000 km2.It lies in Northwest China,where the arid and rainless natural environment has a great impact on the climate,environment,and human living conditions.Based on the results of 1∶250000 regional hydrogeological surveys and previous researches,this study systematically investigates the circulation characteristics and resource properties of the groundwater as well as the evolution of the climate and ecological environment since the Quaternary in the Badain Jaran Desert by means of geophysical exploration,hydrogeological drilling,hydrogeochemistry,and isotopic tracing.The results are as follows.(1) The groundwater in the Badain Jaran Desert is mainly recharged through the infiltration of local precipitation and has poor renewability.The groundwater recharge in the desert was calculated to be 1.8684×108 m3/a using the water balance method.(2) The Badain Jaran Desert has experienced four humid stages since the Quaternary,namely MIS 13-15,MIS 5,MIS 3,and the Early-Middle Holocene,but the climate in the desert has shown a trend towards aridity overall.The average annual temperature in the Badain Jaran Desert has significantly increased in the past 50 years.In detail,it has increased by about 2.5°C,with a higher rate in the south than in the north.Meanwhile,the precipitation amount has shown high spatial variability and the climate has shown a warming-drying trend in the past 50 years.(3) The lakes in the hinterland of the Badain Jaran Desert continuously shrank during 1973-2015.However,the vegetation communities maintained a highly natural distribution during 2000-2016,with the vegetation cover has increased overall.Accordingly,the Badain Jaran Desert did not show any notable expansion in that period.This study deepens the understanding of groundwater circulation and the climate and ecological evolution in the Badain Jaran Desert.It will provide a scientific basis for the rational exploitation of the groundwater resources and the ecological protection and restoration in the Badain Jaran Desert.

1.Introduction

Groundwater is a valuable natural resource,especially in arid deserts.The Badain Jaran Desert is located in Alxa Right Banner,Inner Mongolia of China.It is the third-largest desert in China,covering an area of about 50000 km2.It has an average annual precipitation of 30-120 mm,which is in sharp contrast to the potential evaporation of 3000 mm.Under such an extremely arid condition,the water shortage has seriously restricted the economic development in Alxa Right Banner.Recent studies mostly focus on the sources and formation of the groundwater in the Badain Jaran Desert (Cao L,et al.,2021; Chen JS et al.,2006; Ding HW et al.,2007; Huang TM et al.,2007; Ma JZh et al.,2007; Yang XP,2000a; Ma NN et al.,2008; Wang XS et al.,2014),while few studies have been conducted on the quantity and resource properties of the groundwater in the desert as well as the impacts of climate change on the groundwater and ecological environment in the desert in recent years.Therefore,it is greatly important for the rational exploitation of water resources and the protection and management of the fragile ecological environment in the Badain Jaran Desert to identify the circulation characteristics,quantity,and resource properties of the groundwater in the desert and to analyze the regional climate change and ecological evolution in the desert.

2.Overview of the study area

The Badain Jaran Desert lies in the southwestern part of the Mongolian Plateau and the western part of the Alxa Plateau.It is surrounded by the Beida Mountain in the south,the Yabulai Mountain in the southeast,the Zongnai Mountain in the east,and the border between China and Mongolia in the north.According to the data of physical exploration and ground surveys,the Badain Jaran Desert can be divided into the northern and southern parts with the Zongnai Mountain-Gurinai area as the boundary and thus possesses two relatively independent groundwater systems (Fig.1).

In terms of the terrain,the Badain Jaran Desert is high in the east and south and low in the west and north as a whole,with the elevation gradually decreasing from the southeastern lake area to the northwestern Gurinai Lake (Wang T,1990).Various longitudinal dunes formed from the continuous development of chains of sand dunes are distributed in the western and northern parts of the desert.They are transformed into mega dunes and complex mega dunes (relative height:300-500 m) inside the desert.Pyramid-shaped dunes are visible on the southeast margin of the desert,while low and gently undulating sandy land and grass-shrub sandpiles are mainly distributed on the outer edge of the desert (Qu JJ et al.,2003).More than 100 lakes are distributed among the mega dunes in the southern part of the desert.They are arranged in a moniliform shape and are mostly saline lakes,with an area of fewer than 1 km2each in general (Wang NA et al.,2016).The desert suffers scarce precipitation,with average annual precipitation of 30-120 mm,and the precipitation amount gradually decreases from southeast to northwest (Wang NA et al.,2013; Zhang KC et al.,2012).NEE- and NW-trending faults have mainly developed in the desert and they control the basement structure in the area.The tectonic units in the area mainly include the Yabulaishan uplift,Suhaitu depression,Zongnaishan uplift,Taolete depression,and low Teluoxitan uplift from south to north (Liu JL et al.,2011;Zhang KC et al.,2011).

3.Overview of hydrogeology

The southern and southeastern margins of the Badain Jaran Desert are adjacent to the Beida Mountain and the Yabulai Mountain and their hydrogeological conditions are as follows.The Quaternary basement is composed of rugged Cretaceous clastic rocks and local granites and the Quaternary is thin and mainly consists of piedmont alluvial-diluvial deposits.For instance,the alluvial-diluvial deposits discovered in borehole ZK02 in the Hongliugou area in front of the Yabulai Mountain are 4-5 m thick.They are mainly composed of sandy gravels and gravel-bearing sands and are overlain by thin-laminated eolian sands.The alluvial-diluvial deposits distributed in front of the Beida Mountain in the southern part of the desert are several meters to more than 20 m thick.They are mainly composed of interbeds consisting of grayish-yellow medium-fine-grained sands,mucky silty sands,and clay.They were deposited during the Late Pleistocene-Holocene.

Fig.1.Location and tectonic units in the Badain Jaran Desert.

In the mega dune-large lake area,the Quaternary thickens and the mega dunes are generally about 200-300 m and up to a maximum of 500 m in height.In the lake basins among the mega dunes,the Quaternary is more than 150-600 m thick,the aquifers consist of medium-fine-grained sands and fine sands,and the groundwater enjoys high water yield property,with a single well water yield of 1000-1400 m3/d.Meanwhile,the burial depth of the groundwater in interdune depressions in the basins is generally about 1-10 m,with feeble confined water locally occurring.To the west of the Cherigele area,the underlying bedrocks are uplifted,the Quaternary is less than 150 m thick,the burial depth of groundwater in the interdune depressions is generally more than 10 m,and the water yield property grows poor,with a single well water yield of less than 500 m3/d.The Quaternary began to be deposited in the middle-late stage of the Early Pleistocene and the Quaternary basement is mainly composed of Cretaceous clastic rocks (Fig.2).

The hydrogeological conditions in the Gurinai Lake in the west and its surrounding areas are as follows.Holocene lacustrine phreatic water is distributed,and the aquifers consist of fine sands and clayey sands,with a thickness of 3-10 m and a specific yield of 0.1-0.5 L/s·m.The burial depth of phreatic water is less than 1 m in the central part and generally 1-3 m in the peripheral areas,with overflow occurring in the central part.The salinity is 1-3 g/L generally and less than 3 g/L locally.

The Quaternary in the northern part of the desert is thin and mainly consists of eolian sands and medium-fine-grained sands.It overlies Cretaceous pebbled sandstones and sandstones.The water yield is less than 50 m3/h,the aquifers mainly include underlying bedrock fissure aquifers,and the salinity is greater than 1 g/L in this part.

4.Circulation characteristics and resource properties of groundwater

4.1.Division of hydrogeological units

The Badain Jaran Desert can be divided into two orders of areas according to the characteristics of the terrain,landforms,and geological environment.

The first-order areas.According to the watershed strike of terrain,landform characteristics,and the bedrock uplift distribution controlled by geological structures,the Badain Jaran Desert can be divided into two first-order areas,namely the southern and northern groundwater balance areas with the Zongnai Mountain in the eastern Badain Jaran Desert,which is concealed in the desert area and extends to the north of the Gurinai Lake as the boundary.The strike of the boundary is similar to that of the Beida Mountain and Yabulai Mountain.

Second-order areas.Each of the two first-order areas in the north and south can be further divided into three secondorder areas according to the major influencing factors and geographical distribution of the recharge,runoff,and discharge of the groundwater,springs,and lakes in the desert as well as the vegetation distribution pattern and differences in vegetation cover degree obtained from remote sensing interpretation (Fig.3; Table 1).

Table 1.Groundwater balance areas in the Badain Jaran Desert.

4.2.Characteristics of groundwater circulation

4.2.1.Characteristics of groundwater circulation in southern Badain Jaran Desert

Fig.2.Badain Lake in aerial photography.

(i) In terms of the stratigraphic structure,the Quaternary in the southern hinterland of the Badain Jaran Desert lacks weakly permeable beds such as clay or loam and is only interbedded with several layers of lacustrine mucky silty fine sands,which are discontinuous and do not constitute regional aquicludes.Therefore,the Quaternary groundwater system in the southern part of the desert is a unified single-layer waterbearing system.However,the groundwater at different depths shows significant differences in the hydrochemical and isotopic characteristics,indicating that the Quaternary groundwater system in the desert is a laminar flow system with slow runoff.

(ii) Shallow groundwater is mainly recharged through the infiltration of local precipitation and a small amount of upward runoff from deep runoff systems.According to tests,it is difficult for the condensed water to recharge the groundwater.However,condensed water plays an importantecological role in the desert vegetation in deep groundwater areas.The tritium content indicates that the infiltration depth of local precipitation is < 20 m.Shallow groundwater converges from surrounding mega dunes to lakes in the depression center.It discharges into the lakes finally and then is dissipated by the evaporation of lake water.In this process,some shallow groundwater is dissipated through evaporation and transpiration.Shallow groundwater forms local flow systems.Since different depressions have different scales and different burial depth and vegetation conditions,the shallow groundwater in different depressions has highly different hydrochemical and isotopic characteristics.

Fig.3.Groundwater balance areas and section positions in the Badain Jaran Desert.

(iii) Deep groundwater is recharged by rain-induced floods on the margins of the piedmont mega dunes in the south and east (Figs.4,5).It generally flows from east to west and from south to north and enters the desert hinterland.Under the control of the conditions such as terrain cutting,basement fluctuation,and the evaporation scale of lake water,some deep groundwater flows upward in the desert hinterland and discharges toward lakes in the form of springs,while the groundwater with deeper runoff slowly flows toward the Gurinai area in the northwest.

Fig.4.Groundwater circulation pattern in southern Badain Jaran Desert (EW-trending section).

Fig.5.NS-trending groundwater circulation pattern in the Badain Jaran Desert (NS-trending section).

(iv) As indicated by the hydrochemical and isotopic characteristics of groundwater in boreholes with different depths,the Quaternary groundwater system in the southern part of the desert is a unified 3D flow system with the basement composed of Cretaceous clastic rocks.It can be inferred from the hydrochemical characteristics that there is no direct hydraulic connection between the Cretaceous and Quaternary groundwater.The Quaternary basement forms a sag along boreholes ZK1-ZK7.The deepest part of the sag occurring near borehole ZK3,where the Quaternary is 500 m in depth.It is exactly the zone where large lakes and mega dunes are intensively distributed.The burial depth of the groundwater in this zone increases northwestward.As a result,the groundwater in this zone is difficult to be recharged by local precipitation and it mainly originates from slow regional runoff.The increase in the salinity of Quaternary groundwater in borehole ZK7 should act as evidence for the slow and even stagnant groundwater exchange.

4.2.2.Characteristics of groundwater circulation in northern Badain Jaran Desert

Field surveys and isotopic characteristics indicate that groundwater in the northern part of the desert is not closely related to that in the southern part.As revealed by the terrain and geophysical prospecting,there is a nearly EW-trending fault in the Gurinai-Zongnai Mountain area,and the Quaternary basement to the south and north of the fault sinks and rises,respectively.In addition,the survey results show that the burial depth of the groundwater to the north and south of the fault greatly differs.The manual trial pit near the fault zone reveals that the burial depth of the groundwater is < 2 m.Large diameter wells dug by many herdsmen are distributed in the zone.The burial level of groundwater in these wells increases toward borehole GZ3 in the north and is up to 50-60 m near borehole GZ3 (Fig.5).The presence of the hydraulic drop of groundwater indicates that the fault has a water blocking effect.In addition,the shallow groundwater near the fault zone contains no tritium,and the deep Quaternary groundwater discovered in borehole ZK10 has a14C content of 33.6 pMC and an age of 9 ka.These show that deep groundwater flows upward near the fault zone in the northern part of the desert and that the fault has a water blocking effect.

The groundwater in the northern desert hinterland features a large burial depth.Therefore,a few water samples were taken from it.According to the isotopic test results,the groundwater in the hinterland of the northern part may originate from the recharge of rain-induced floods from the Zongnai Mountain and discharges toward the western boundary of the desert (Ejin Basin) and the Guaizi Lake in the north.The isotopic characteristics show that the groundwater in the Guaizi Lake area greatly differs from that in the desert area,indicating that the Guaizi Lake is not the major discharge destination of the groundwater in the desert area.

Owing to scarce precipitation,the northern part of the desert (including the Zongnai Mountain) is lacking in groundwater.Especially in the area near borehole ZK11 in the north,the Quaternary groundwater aquifers are less than 10 m in thickness and thus groundwater is extremely short.

4.3.Renewability and resource properties of groundwater in the Badain Jaran Desert

Despite few groundwater samples collected from the northern part of the Badain Jaran Desert,the obtained test results reveal that the Quaternary groundwater in the deep part of borehole ZK10 has a14C content of 33.6 pMC and an age of 9 ka.Therefore,the groundwater in the northern part of the desert has limited renewability and is a non-renewable water resource.

The groundwater in the southern part of the desert also has limited renewability overall.Although the shallow groundwater in the hinterland of the southern part is recharged by local atmospheric precipitation,the recharge is low due to scarce precipitation and strong evaporation.This can be evidenced by the tritium content.Most of the shallow groundwater has a tritium content of < 5 TU (Fig.6),which is far below the tritium content in modern precipitation (26.7 TU on average for the precipitation monitored at the meteorological stations in the Badain Lake,Cherigele area,and Gurinai area in the desert).Furthermore,there is no tritium in the groundwater in some large diameter wells and trial pits,indicating little recharge from atmospheric precipitation.In addition,the influencing depth of local precipitation for shallow groundwater is not greater than 20 m and is less than 10 m in most areas,which also proves limited recharge from local precipitation.

According to the14C dating results,the14C content and age of groundwater without tritium in shallow wells are 40.5-77.5 pMC and 2.1-7.5 ka,respectively,those of water from Quaternary pumping wells are 18.4 -46.1 pMC and 6.4-14.0 ka,respectively,and those of ascending springs are 22.9-45.1 pMC and 6.6-12.2 ka,respectively.

Therefore,the groundwater in the desert hinterland is mostly “old water” with an age of thousands or even tens of thousands of years.The average annual renewal rate of the groundwater is 0.007%-0.047%.According to the definition of the United Nations Educational,Scientific and Cultural Organization (UNESCO),groundwater with a renewal rate of< 1% can be regarded as non-renewable groundwater resources.Therefore,the groundwater in the desert hinterland is the water stored in historic periods and should not be exploited on a large scale in a normal situation.

4.4.Groundwater resources in the Badain Jaran Desert

The groundwater recharge in the Badain Jaran Desert was calculated to be 1.8684×108m3/a using the water balance method and the groundwater discharge was calculated to be 1.8729×108m3/a.Therefore,the groundwater balance in the Badain Jaran Desert is −452×104m3/a and the groundwater is at a slightly negative balance state at present.These are completely consistent with the actual situation surveyed(Table 2).

Table 2.Summary of the balance condition of groundwater resources in the Badain Jaran Desert (108 m3/a).

5.Climate change and ecological evolution in the Badain Jaran Desert

5.1.Changes in precipitation

The recharge through precipitation infiltration is the most important recharge source of the shallow groundwater in the study area.Therefore,the quantity of the shallow groundwater depends on the precipitation amount.

Zhang KC et al.(2012) analyzed the law of the spatialtemporal changes in the regional precipitation in detail using the meteorological data of 18 meteorological stations in and around the Badain Jaran Desert of the past 40 years.Based on this,they considered that the precipitation amount monitored at these stations shares roughly the same interannual fluctuation trend.Specifically,the precipitation amount in the desert significantly decreased compared to that in the peripheral areas with the same latitude,and the average annual precipitation in the eastern part of the desert was significantly higher than that in the western part.The smallest annual precipitation mainly occurred in Ejin Banner and was less than 50 mm.In contrast,the average annual precipitation in the Norgong and Jilantai areas in the eastern part of the desert was 114.7 mm and 116 mm,respectively,which are greater than that in Ejin Banner and Guaizi Lake in the western part of the desert.The precipitation amount monitored at the meteorological stations in the Guaizi Lake and Ejin Banner in the western part of the desert has generally decreased since 1960,with the interannual fluctuation trends of the two stations highly correlated.In comparison,the precipitation amount in Alxa Right Banner and Zhongquanzi area on the southern margin of the desert has increased in recent years.

Ning WX et al.(2021) analyzed the temperature and the precipitation data of 1960-2018 from four meteorologicalstations around the Badain Jaran Desert and the precipitation data of 2016-2018 from one automatic meteorological station in the desert hinterland,obtaining the following knowledge.The annual precipitation around the Badain Jaran Desert showed a non-significant increasing trend,with a precipitation tendency rate of 4.57 mm/10a.Meanwhile,the increasing trend in the eastern part of the desert was more notable than that on the northern,western,and southern margins of the desert.The average annual temperature in surrounding areas of the desert showed a significant increasing trend,with an increasing rate of 0.34°C/10a.Meanwhile,the increasing rate of temperature in the northern part of the desert was higher than that on the eastern,southern,and western margins of the desert.The annual precipitation in the surrounding areas of the Badain Jaran Desert suffered multiple sharp changes during 1960-2018,especially during 1995-2000 and 2005-2015.However,the average annual temperature only experienced one sharp change during this period—roughly in 1990-1995,during which the increasing rate of temperature motored at these stations successively increased.

Fig.6.Distribution of tritium content in shallow groundwater in southern Badain Jaran Desert.

In this study,the interannual changes in primary meteorological elements in the study area were analyzed based on data of 1966-2017 of four national meteorological stations around the Badain Jaran Desert.Given that the desert has its microclimatic environments,the spatial changes in the primary meteorological elements were analyzed using the data of 2016-2018 of 10 small meteorological observation stations built in the desert area for this study,which cover different positions at an elevation of 900-1700 m.The model of the small meteorological stations is HOBO-U30 and they are mainly used for spatial change analysis of primary meteorological elements within a year.The results are as follows.The average annual temperature in the study area showed a significant increasing trend during the 50 years in 1966-2017.It increased by about 2.5°C overall,with a higher rate in the south than in the north.The precipitation amount in Ejin Banner in the northern part of the study area and to the south of the Guaizi Lake showed a non-significant decreasing trend,with a decrease rate of 0.21 mm/10a and 0.77 mm/10a,respectively.In contrast,the precipitation amount in the southern and western parts of the desert showed a nonsignificant increasing trend.Spatially,the precipitation in the desert area showed a gradually decreasing trend from south to north overall in the 50 years.The maximum and minimum precipitation amount occurred in Badain Jaran Town of Alxa Right Banner and the Tamusu area,respectively,where the annual precipitation was 153 mm and only 22.4 mm,respectively.The spatial variability of the precipitation was more notable than that of other meteorological elements.Furthermore,it was intensified by the complex and diverse microclimatic environments,which were formed under the influence of the mega dunes and lakes in the Badain Jaran Desert area.

5.2.Evaporation in the Badain Jaran Desert

The evaporation from water surfaces of lakes in the hinterland in the Badain Jaran Desert is a key control factor in the hydrological cycle in the whole desert.Chen JS et al.(2004) hold that annual evaporation from the lakes reached 4000 mm/a.Gates JB et al.(2008) inferred that the potential evaporation from the lakes was 2600 mm/a.Ma N (2012)observed the evaporation of the lakes using relevant eddy covariance technology and calculated that the annual evaporation from the lake was about 1400 mm/a according to the principle of energy balance.Wang XS et al.(2014)conducted a hydrogeological survey and comprehensive observation of typical lakes in the Badain Jaran Desert and determined that the evaporation intensity of the lake water was 1200-1500 mm/a,which is slightly higher than the annual evaporation of the lakes of 1040 mm/a calculated by Yang XP et al.(2010) using the Penman evaporation equation.Hu WF et al.(2015) calculated that the annual evaporation of the Yindeertu Lake in the hinterland of the Badain Jaran Desert was (1450 ± 10) mm/a according to the turbulence data observed from August,2012 to July,2013 using an eddy correlation system.Zhang J et al.(2017)established a groundwater-lake water dynamic simulation model in the Sumujinlin Lake area.Accordingly,they obtained the seasonal fluctuation of lake water level during 2012-2013 through fitting and inferred that the evaporation from the lake surface was 1261 mm/a.Han PF et al.(2018)determined that the evaporation from the lake surface from June,2013 to May,2014 was 1300-1500 mm/a (after the correction of the effect of salinity) according to the observation results of evaporation pans.In 2018,this study established a meteorological factor-evaporation model according to the observation results of evaporators and evaporating pans and calculated that the converted evaporation from freshwater lakes during 2016-2018 was 1295.94 mm/a.According to many years of research results above,the annual evaporation of lakes in the Badain Jaran Desert decreased overall during 2004-2018 despite fluctuations.The evaporation (observed using evaporating pans) monitored at two stations in the southern and northern parts of the desert showed a significant decreasing trend,with a decreasing rate of 166 mm/10a.In contrast,the evaporation monitored at the station in the Guaizi Lake area on the margin of the desert showed a significant increasing trend,with an increasing rate of 150 mm/10a.Meanwhile,the relative humidity generally decreased.The average wind speed monitored at the stations in Ejin Banner,Dingxing area,and Alxa Right Banner showed a significant decreasing trend,with a decreasing rate of 0.11-0.31 m/s.Wind speed is one of the major influencing factors of evaporation (Fig.7).Evaporation increases with an increase in wind speed,which is one of the reasons that the evaporation significantly increased in the Guaizi Lake area.Overall,the climate in the study area has shown a warming-drying trend over the recent 50 years.

5.3.Climate evolution in the Badain Jaran Desert

Aridification has been the most significant characteristic of the Badain Jaran Desert area since the Cenozoic.Since the Quaternary,the climate change in the desert area in western China has been closely related to the growth and shrinkage of the glaciers in the Qilian Mountains (Wang T,1990; Zhang WM et al.,2005; Li ZhL et al.,2013).From the Paleogene to the Neogene,this desert was in a lacustrine basin environment with an arid climate (Sun QF et al.,2008).The lakes in the desert were probably formed during the Late Pliocene-Early Pleistocene,that is,they were formed earlier than the desert(Ding HW et al.,2015).At 1.2 Ma,Pliocene red clays (the latest strata in the basement of mega dunes) were formed and the desert was in a warm and humid sedimentary environment(14-22°C) (Wang F,2015; Yan MC et al.,2001).At 1.2-1.1 Ma,the sedimentary environment was colder and drier (below 10°C) than before,the climate with frequently fluctuated,and near-surface winds were intensified (Ma NN et al.,2008).At 1.1-0.62 Ma,the climate was increasingly cold and dry.At 620-470 ka (MIS13-15),the sedimentary environment turned warm and humid,and the largest lakes since the Middle Pleistocene developed.During the Middle Pleistocene -Late Pleistocene (470-120 ka),the desert expanded again but gradually shrank late.Since the Late Pleistocene,rivers,and lakes shrank and gradually dried up (Yan MC et al.,2001).During the last interglacial period in the Late Pleistocene(120 -78 ka,MIS5),the East Asian monsoon further intensified,the climate turned warm and humid,and the lakes in the hinterland of the desert developed again,with the scale larger than today (Geng KH,1986; Yang XP,2001; Bai Y,2011; Bai Y et al.,2011; Fan XL et al.,2014).In the early glacial stage of the Last Glacial Period (LGP; 79.3-66.8 ka,MIS4),the climate was cold and dry,winter monsoon prevailed,there was little rain,and the salinity of the lakes increased (Fan XL,2015).In the interglacial stage of the LGP(MIS3),the climate in the desert was relatively warm and humid (Yang XP,2000a; Yang XP et al.,2003; Yang B et al.,2004; Yang XP and Scuderi LA,2010; Wang NA et al.,2011).At 66.8-41.0 ka,precipitation increased,the recharge of the lakes increased,and the lakes expanded,reflecting the characteristics of a warm and humid climate (Fan XL,2015).After 41.0 ka,the climate shifted toward drought and coldness.At 40-30 ka,calcareous cementation layers (Yang XP,2000a; Yang XP et al.,2003; Yang B et al.,2004) and calcareous root tubes developed in the desert.In this period,the average annual temperature in western Inner Mongolia was 2-3°C higher than today,and the annual precipitation in most areas in Northwest China was also higher than today and was 100-300 mm.During the Great Ice Age of the LGP(MIS2),the climate was relatively dry and cold,the lakes shrank,and the salinity of lake water increased.During the Late Pleistocene,the river flow in the desert gradually shifted from planar flooding to present flow in the riverbed.With the decrease in runoff quantity and the intensification of winter monsoons,large lakes in the Gurinai Lake - Guaizi Lake area gradually shrank and dried up.After the LGP,the climate has become increasingly dry (Tan JA,1964).Ancient sand dunes are preserved in the sediments of the first terrace in Subonaoer,indicating that the climate once deteriorated during the Late Pleistocene (Chi ZQ et al.,2006; Liu ZT et al.,2010).During the Early Holocene (11-8 ka),the climate shifted from the previous aridity toward wetness but was still arid.At 7.7-5.3 ka,the annual precipitation was 200 mm.Despite short-term dry-wet fluctuations,large lakes with water surface 20-30 m higher than the current lake surface occurred,which lasted until the Middle Holocene (4 ka)(Chen TY et al.,2019; Yang XP and Williams MAJ,2003;Yang XP et al.,2011; Wang NA et al.,2016).The first terrace in the Subonaoer area was formed during the Early-Middle Holocene,and it is a climatic terrace formed due to lake shrinkage in an arid climate.During the Middle Holocene,the effective moisture increased in the whole desert area in North China due to strong summer monsoons (Yang XP et al.,2011;Yang XP,2000b).The formation of the Swan Lake lacustrine terrace indicates that the climate was favorable for some time after 5 ka.Since the Middle Holocene (4 ka),the climate has shown a trend toward aridity overall (Fan XL et al.,2014;Chen TY et al.,2019; Wang NA et al.,2016; Yang XP,2000b; Xiao N et al.,2021).Accordingly,the river flow has decreased in a fluctuant manner,large numbers of dry riverbeds have occurred,human activities have gradually increased,and the desert has rapidly developed.

Fig.7.Evaporation and wind speed characteristics.

According to the above research results since the Quaternary,the Badain Jaran Desert has experienced four humid stages since the Quaternary,namely MIS13-15,MIS5,MIS3,and Early-Middle Holocene,but the climate in the desert has shown the trend toward aridity overall.

5.4.Ecological evolution of the Badain Jaran Desert

5.4.1.Dynamic trends in lakes in the Badain Jaran Desert

Lakes in the Badain Jaran Desert are the result of the longterm evolution of climate and environment,and their spatialtemporal variation characteristics are significant for understanding the evolution of the desert ecosystem.Zhang ZY et al.(2012) interpreted the changes in the lake area in the desert hinterland of nearly 40 years from 1973 to 2010 using the visual method and the band rationing technique.According to the interpretation results,the total area of the lakes decreased by 3.69 km2and the lakes shrank to different degrees in various periods.Jin XM et al.(2014) analyzed the interannual trends of the lake area in the Badain Jaran Desert from 1990 to 2010 using the water index MDLWI.They concluded that the lake area decreased by 0.59 km2within 20 years,indicating a slowly decreasing trend.This is consistent with the conclusion of Zhang ZY et al.(2012).In this study,monthly dynamic monitoring of the lakes for a complete hydrological year from August,2014 to August,2015 was conducted through the interpretation of remote sensing images and field verification photos.The results indicated that the water area of the lakes in August,2015 showed a decreasing trend compared to that in August,2014.Overall,the lakes in the hinterland of the Badain Jaran Desert showed a continuously shrinking trend during 1973-2015.

5.4.2.Trends in vegetation and plant communities in the Badain Jaran Desert

Plant communities in deserts serve as an important part of desert ecosystems and the change in their coverage plays a non-negligible role in measuring the stability of regional ecosystems.According to the study of Gong P et al.(2014),the overall vegetation coverage in the Badain Jaran Desert was low,but the normalized difference vegetation index(NDVI) in the desert showed an increasing trend overall from 2000 to 2012,with an increasing rate of 0.009/10a.This indicates that the vegetation coverage in the study area was gradually improved in that period.Liu C et al.(2019) studied the trend in the evolution of the vegetation cover in the desert in 2000-2016.As revealed by the results,the vegetation cover increased overall,and the increasing rate of the vegetation cover on the margin of the desert was higher than that inside the desert (1.53%/10a and 0.84%/10a,respectively).Meanwhile,the area with improved vegetation on the margin and inside of the desert accounted for 57.12% and about 21.26%,respectively.This indicates that the vegetation was improved overall during 2000-2016,implying that the Badain Jaran Desert did not suffer notable expansion in that period.Zhang YF et al.(2012) held that the plant communities in the Badain Jaran Desert were roughly at a natural state.Wang M et al.(2016) considered that natural Haloxylon ammodendron populations on the eastern margin of the Badain Jaran Desert maintain a relatively high natural distribution and growth.With more attention being paid to the ecological environment,Alxa Right Banner has implemented a series of key ecological protection and construction projects such as pastureland rehabilitation,compensation for ecological benefits of key public welfare forests,and grassland ecology conservation subsidy and reward mechanism.As a result,the deterioration trend in the ecological environment in the Badain Jaran Desert and its surrounding areas has generally been restrained overall,and the ecological environment in some areas has been notably improved.

Overall,the lakes in the Badain Jaran Desert show a shrinking trend.However,the vegetation communities maintained a highly natural distribution and the vegetation cover increased overall during 2000-2016,implying that the desert did not suffer any notable expansion in that period.

5.5.Discussion

The groundwater in the Badain Jaran Desert mainly originates from the direct infiltration of effective local precipitation in the desert,which infiltrates after forming dominant trickles and floods in low-lying areas under the influence of micro landforms.Meanwhile,it is indirectly recharged through the infiltration of precipitation-induced floods in the southern and southeastern catchment areas with comparatively rich precipitation.The groundwater resources in the desert are at a slightly negative balance state presently.In terms of resource properties,the groundwater in the northern part of the desert has limited renewability and is a non-renewable water resource.Meanwhile,the groundwater in the southern part of the desert also has limited renewability overall.Although the shallow groundwater in the hinterland of the southern part is recharged by local atmospheric precipitation,the recharge is low due to scarce precipitation and strong evaporation.The recharge and discharge of the groundwater in the Badain Jaran Desert occur in significantly different areas.In detail,various recharge occurs in wide areas (including the desert) bounded by watersheds on the ground surface,while the discharge only occurs in lake-spring areas and a few shallow groundwater areas with vegetation well growing.Despite a small scale,the discharge is relatively concentrated and in a large quantity,thus maintaining many years of the relative balance between groundwater and lakes and the ecosystems such as lacustrine wetlands.Given that the groundwater resources in the study area are limited and valuable,the exploitation and utilization quantity of the groundwater must be strictly restricted to avoid quantitative and qualitative changes that induce the deterioration of water quality.

6.Conclusions

(i) The recharge and discharge of the groundwater in the Badain Jaran Desert are 1.8684×108m3/a and 1.8729×108m3/a,respectively.Therefore,the groundwater balance in the desert is −452×104m3/a and the groundwater is at a slightly negative balance state at present.The average annual renewal rate of the groundwater is 0.007%-0.047%.Therefore,the groundwater is the water stored in historic periods and should not be exploited on a large scale in a normal situation.

(ii) The Badain Jaran Desert has experienced four humid phases since the Quaternary,namely MIS13-15,MIS5,MIS3,and Early-Middle Holocene,but the climate in the desert has shown a trend towards aridity overall.The average annual temperature in the Badain Jaran Desert and its surrounding areas showed a significant increasing trend over the past 50 years.It has increased by about 2.5°C throughout the past 50 years,with a higher rate in the south than in the north.The precipitation amount in Ejin Banner in the northern part of the study area and to the south of the Guaizi Lake shows a nonsignificant decreasing trend,while that in the southern and western parts of the desert shows a non-significant increasing trend.The evaporation (observed using evaporating pans)monitored at two stations in the southern and northern parts of the desert has shown a significant decreasing trend,with a decreasing rate of 166 mm/10a.In contrast,the evaporation monitored at the station in the Guaizi Lake area on the margin of the desert has shown a significant increasing trend,with an increasing rate of 150 mm/10a.Changes in evaporation are mainly related to changes in wind speed.The relative humidity has generally decreased.Overall,the climate of the working area has shown a warming-drying trend in the recent 50 years.

(iii) Since the 1970s,the lakes in the Badain Jaran Desert show a shrinking trend overall.However,the vegetation communities maintained a highly natural distribution and the vegetation cover increased overall during 2000-2016.Meanwhile,the desert has not notably expanded over the past ten years.

CRediT authorship contribution statement

Zhe Wang and Li-juan Wang conceived of the presented idea.Zhe Wang,Jian-mei Shen and Zhen-long Nie developed the theory and performed the computations.Ling-qun Meng,Le Cao and Shi-bo Wei verified the analytical methods.Xiang-feng Zeng gave experimental support.Jian-mei Shen and Zhen-long Nie encouraged Zhe Wang and Li-juan Wang to investigate a specific aspect and supervised the findings of this work.All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgment

This research was funded by projects of the National Natural Science Foundation of China (41702285),the National Geological Survey Project (121201106000150093),the National Natural Science Foundation of China(41807214),the Fundamental Scientific Research Funds from the Chinese Academy of Geological Sciences (No.SK202011).