Wen-peng Li,Long-feng Wng,*,Yi-long Zhng,Li-jie Wu,Long-mei Zeng,Zhong-sheng Tuo

a China Institute of Geo-Environmental Monitoring, China Geological Survey, Beijing 100081, China

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

c Wuhan Zhi Bo Chuang Xiang Science and Technology Co., Ltd, Wuhan 430000, China

Keywords:Groundwater basin Surface watershed Ecological environment Hydrogeology condition Stable environmental isotopes Geologic stucture Dalinuoer Lake Mongolian Plateau

ABSTRACT The surface watershed and groundwater basin have fixed recharge scale,which are not only the basic unit for hydrologic cycle research but also control the water resources formation and evolution and its corresponding eco-geological environment pattern.To accurately identify the boundary of the surface watershed and groundwater basin is the basis for properly understanding hydrologic cycle and conducting the water balance analysis at watershed scale in complicated geologic structure area,especially when the boundary are inconsistent.In this study,the Dalinuoer Lake located in the middle of the Inner Mongolian Plateau which has complicated geologic structure was selected as the representative case.Based on the multidisciplinary comprehensive analysis of topography,tectonics,hydrogeology,groundwater dynamics and stable isotopes,the results suggest the following: (1) The surface watershed ridge and groundwater basin divide of Dalinuoer Lake are inconsistent.The surface watershed was divided into two separate groundwater systems almost having no groundwater exchange by the SW-NE Haoluku Anticlinorium Fault which has obvious water-blocking effect.The surface drainage area of Dalinuoer Lake is 6139 km2.The northern regional A is the Dalinuoer Lake groundwater system with an area of 4838 km2,and the southern regional B is the Xilamulun Riverhead groundwater system with an area of 1301 km2.(2) The groundwater in the southern of regional A and the spring-feeding river are the important recharge sources for the Dalinuoer Lake,and it has greater recharge effects than the northern Gonggeer River system.(3) It is speculated that the trend of Haoluku Anticlinorium Fault is the boundary of the westerlies and the East Asian summer Monsoon (EASM) climate systems,which further pinpoints the predecessor ’s understanding of this boundary line.At present,the Dalinuoer Lake watershed is proved to have gone through a prominent warming-drying trend periods,which leads to the precipitation reduction,temperature rise,human activities water usage increasement.So the hydrological cycle and lake eco-environment at watershed scale will still bound to be change,which may pose the potential deterioration risk on the suitability of fish habitat.The results can provide basic support for better understanding water balance evolution and lake area shrinkage cause as well as the ecological protection and restoration implementation of Dalinuoer Lake watershed.

1.Introduction

The concept of China’s water resources management in the new era is guided by the earth system science and hydrologic cycle theory at watershed scale.In order to realize the harmonious health of mountains,rivers,forests,fields,lakes,grass and sand,it is emphasized that carry out research on the cause of water imbalance and the accompanied ecogeological environment problems at watershed scale,as well as implement water resources regulation and ecological restoration.The key to properly understand the law of hydrologic cycle and water balance at watershed scale is to accurately identify the surface watershed and groundwater basin boundary.The surface watershed is referred to as the area encircled by the surface watershed ridge,which is comparatively in high research degree and easily identified.The groundwater basin divide need to be identified with the combination analysis of geology and hydrogeology condition,aquifer structure,groundwater dynamic field and isotope tracer due to its high born concealment (Li WP et al.,1995,1999,2006; Wang WX et al.,2016).In the complicated geologic structure area,the inconsistent of surface watershed ridge and groundwater basin divide is always ignored,which leads to much uncertainty in hydrologic cycle recognition and water resources evaluation.

The Dalinuoer Lake watershed that locates in the middle of the Inner Mongolia plateau has complicated geologic structure.In the 1970s and 1980s,the 1∶200000 hydrogeological survey were carried out in the Dalinuoer Lake watershed,and the corresponding reports were compiled.Although the detailed geological and hydrogeological data were obtained,the above research was based at the map-sheet scale and it lacks the groundwater system research at the watershed scale.In recent years,the Dalinuoer Lake area has changed from 225 km2in 1983 to 191 km2in 2018 (Muxi YL et al.,2020),which has caused widespread concern.A lot of research work have been done on the lake area evolution,recharge source,lake shrinkage cause,climate response,etc..Muxi YL et al.analyzed the trend and drivers of water area change of Dalinuoer Lake from 1983 to 2018 based on remote sensing interpretation method (Muxi YL et al.,2020).Ma C et al.illuminated the temporal and spatial evolution of ecological elements and revealed the mechanism and relationship of Dalinuoer Lake wetland change driving factors,as well as evaluated the impact of lake wetland change on the environment (Ma C et al.,2017).Zhen ZL et al.explored the recharge relationship between the groundwater and the Dalinuoer Lake based on the hydrogen and oxygen isotopes (Zhen ZL et al.,2014).Li WB et al.studied the variations of stable isotopes sampled in summer precipitation in the central Inner Mongolia (Li WB et al.,2019).Zhu BQ et al.explored the origin of fresh groundwater in the Otindag Desert from the aspects of climate,geomorphological,geological,hydrogen and oxygen stable isotopes and hydrochemical analysis (Zhu BQ et al.,2019).Yang XP et al.mentioned that the Otindag Desert was once teemed with lakes and a relict channel was directed 30 km north to megalake Dalinuoer before ca.4.2 ka (Yang XP et al.,2015).Xing ZQ et al.proposed the sustainable ecological water levels of Dalinuoer Lake based on ecohydrological process (Xing ZQ et al.,2021).

All the above research provide strong supports for the ecological protection and restoration of Dalinuoer Lake,but the drainage area of Dalinuoer Lake watershed given varies and differs considerably.Until now,the surface watershed and groundwater basin boundary of Dalinuoer Lake have not been thoroughly analyzes.Therefore,the multidisciplinary integrated reanalysis method is employed to further understand the surface watershed and groundwater basin boundary of Dalinuoer Lake based on the topography,tectonics,hydrogeology,groundwater dynamics and stable isotopes data.

2.Study area

2.1.General setting

Dalinuoer Lake which is the second largest endorheic salt lake in Inner Mongolia lies in the west of Hexigten.It is located at the junction of the major landform of the southern end of the Greater Khingan Mountains,the central part of the Inner Mongolia Plateau,and the northern part of the Otindag Desert.Due to the block of the Greater Khingan Mountains,Yanshan Mountains,Qinghai-Tibet Plateau,Himalayan Mountains and other mountains,water vapor from the Pacific Ocean and the Indian Ocean are difficult to reach and form precipitation.Dalinuoer Lake watershed belongs to the temperate continental monsoon climate zone,with the characteristic of dry climate,strong evaporation and low rainfall.The annual mean temperature ranges from 1°C to 2°C(Xing ZQ et al.,2021),and annual pan evaporation varied from 1521 mm to 2270 mm with an average of 1828 mm(Jiang MY et al.,2020).The annual precipitation ranged from 121 mm to 561 mm with an average of 280 mm,around 70.5% of which occurs in summer from June to August (Zhen ZL et al.,2021).Four inflow rivers,namely the Gonggeer River,the Liangzi River,the Shali River and the Haolai River flow into the lake with a large proportion coming from the Gonggeer River (Fig.1).The annual surface inflow of the Dalinuoer Lake watershed was 40 ×106m3in the last decade(Xing ZQ et al.,2021).

2.2.Geological and hydrogeological setting

Dalinuoer Lake watershed is situated at the intersection of the Yinshan complex fault belt stretching in the EW direction,the Xingmeng tectonic belt stretching in the S-NE direction and the Bayinheshuo fault depression basin,which shows complicated geological structure.It mainly develops three groups faults close to EW,NE and NW directions,which show relatively strong control on river.One of the most important active fault is the Xilamulun River Fault closed to EW direction,which controls the southern boundary of Dalinuoer and Ganggengnuoer Lake,and indirectly forms the boundary between Xilingol Grassland and Otindag Desert(Geng K et al.,1988).It also leads to the elevation differences between the northern and the southern.The Cenozoic fault basin of Dalinuoer Lake stretching in the NE direction is situated at the southeastern margin of Bayinheshuo fault depression basin group developed in late Mesozoic.The Haoluku Anticlinorium Fault developed in the south-central of Dalinuoer Lake watershed,and it controls the distribution of acidic extrusive rocks in Jurassic and intrusive rocks in late Yanshanian.The Haoluku Anticlinorium Fault divides the Dalinuoer Lake watershed into two small basins,Hankela and Yuanshuitou,in which the Quaternary strata with a thickness of more than 200 m accumulated (Fig.2).

The impermeable basement rocks of the Dalinuoer Lake watershed mainly consist of Mesozoic volcanic rocks.The Neogene sporadically and Quaternary strata widely distributed overlying the basement rock.The Quaternary sedimentary thickness is large and its genetic types are complex,on which successively deposited glacial facies,glacial water-lake facies,alluvial facies,slope diluvium,lake facies,lacustrine facies,eruptive basalt and aeolian facies.The glacial waterlake facies and aeolian sands are mainly distribute in the southern of Dalinuoer Lake,and the lithology is mainly silt,fine sand and gravel.There developed the tertiary basalt platforms in the northwestern,and the east lakeshore is covered by the Quaternary sediments (Fig.2,Fig.3).The permian sedimentary rock,shale,quartz-schist,intermediateacid volcanic rock and a small amount of Jurassic granite are cropped out in the middle and upper reaches of the Gonggeer River,and the Quaternary alluvial-diluvial layer developed in the gullies (Inner Mongolia Autonomous Region 101 Geological Team,1979).

Fig.1.Groundwater basin and surface watershed boundary as well as the collected water samplings sites from the scientific literature.

Fig.2.Hydrogeological sketch map of the Dalinuoer Lake and Xilamulun Riverhead.

Fig.3.I-I' hydrogeological profile map of the Dalinuoer Lake watershed.

Dalinuoer Lake is a tectonic dammed lake which was formed by basalt weir in northwestern on the basis of the tectonic dammed lake formed during the subsidence of a faulted basin.The groundwater and spring flow into the lake from the south,with a relatively high discharge.The groundwater types of Dalinuoer Lake watershed can be divided into five types: Pore water in unconsolidated rocks,basalt cave fissure-pore water in unconsolidated rocks,basalt cave fissure water-basalt volcanic cone group,magmatic fissure water and pore-fissure water in clastic rocks.The main aquifers of pore water in unconsolidated rocks are glacial deposits and diluvia layer,which distribute in the southern high plain and the eastern valley terraces with an area of 3309 km2.The main aquifers of basalt cave fissure-pore water in unconsolidated rocks are the first basalt platform or Quaternary cover,which mainly distribute in the northwestern of Dalinuoer Lake with an area of 739 km2.The magmatic fissure water and pore-fissure water in clastic rocks are poor in water-abundance,with an area of 498 km2and 63 km2respectively (Fig.2).

3.Materials and methods

The multidisciplinary integrated reanalysis method is employed to recognize the surface watershed and groundwater basin boundary based on the topography,structure,hydrogeology,groundwater dynamics and stable isotopes data.In which,(1) the surface water system,water conservancy projects,water resources development and utilization of the Dalinuoer Lake watershed were investigated,and a total of 82 local well were observed with the water level gauge to obtain groundwater level during the September 2019.(2) The isoline of groundwater level elevation was mapped by the Kriging interpolation method with the help of ArcGIS 10.1 version.(3) SRTM-DEM data was downloaded from the Chinese Academy of Sciences website (http://www.gscloud.cn/),and the surface watershed was identified with the help of hydrology module embedded in ArcGIS 10.1 version.(4)The relevant hydrogeology map-sheets of Dalinuoer Lake watershed named K-50-3,K-50-4,K-50-9,K-50-10,K-50-15 and K-50-20 were downloaded from the GeoCloud platform which is developed and operated by the China Geological Survey (https://www.cgs.gov.cn/ddztt/jqthd/dzy/).(5)The hydrogeology map-sheets were compiled with the unified symbol,graph parameter,attribute structure based on the real time human-computer interaction method with the help of MAPGIS 6.7 version.(6) A total of 46 water samplings including groundwater,river water,lake water,spring water collected from the Dalinuoer Lake watershed and its adjacent area during the summer and autumn seasons of 2011,the spring season of 2012 and September of 2013 were obtained from the scientific literature (Zhen ZL et al.,2014; Zhu BQ et al.,2019).All the samplings were analyzed for stable hydrogen and oxygen isotope,and the analysis results meet the accuracy requirements.

4.Results and discussion

4.1.Identification of the surface watershed

The point of the surface watershed is to identify the surface watershed ridge,water catchment line and range.In Fig.1,connecting the highest watershed ridge of the northern,southern,western and eastern of the Dalinuoer Lake based on the DEM model,a encirclement Dalinuoer Lake watershed including regional A and B with an area of 6139 km² was delineated clearly.The eastern watershed ridge is the remnant of the Greater Khingan Mountains lies in Hexigten,which forms the natural boundary of the Gonggeer River system flowing into Dalinuoer Lake.The western and northern of Dalinuoer Lake is the basalt platform formed by multiple Cenozoic volcanic eruptions or covered by Quaternary sand,which is widely distributed with volcanic cones.In the northwestern of the watershed,the river system is nondeveloped.The western watershed ridge is the natural boundary of the Huri Chaganor River system.The northern watershed ridge is the natural boundary of the Xilingol River system.Southeastern of the watershed ridge is the natural boundary of the Xilamulun Riverhead watershed located in Hexitenqi.The developmental rivers in the watershed eventually flow into the Dalinuoer Lake.

In this study,the drainage area of the Dalinuoer Lake watershed is 1943 km2larger than the area proposed by the water resources department (Xing ZQ et al.,2021).It is mainly differentiated by the regional B which lies in the southern of the Haoluku Anticlinorium Fault.Yang XP et al.(2015) mentioned that a relict channel flowing northward from the Otindag Desert to Dalinuoer Lake existed in regional B before ca.4.2 ka.Owing to the northern hemisphere climate shifts to extraordinary drought as a whole,the important geomorphic and hydrological events appeared in the Otindag Desert.In addition,the westward retrogressive erosion into the interior of the Otindag Desert which is caused by the Xilamulun Riverhead is being exacerbated.As a result,the groundwater is sapped,and the already scarce surface water flows into the Xilamulun River through the groundwater.It leads to the rapid decline of groundwater level and the lake vanished and degradation,as well as the river cut off.The authors of this study believe that once the high intensity rainfall events occur under climate change condition,the relict channel would likely to reflow.So the surface watershed of Dalinuoer Lake should contain regional A and regional B.

4.2.Identification of the groundwater basin divide

Based on the comprehensive analysis of terrain landform,geological structure and hydrogeological condition,the water resistance ability of the folds,faults and stratigraphic boundaries are usually identified to determine the groundwater basin divide.The relative water-resisting stratigraphic boundary is usually the metamorphic rock,igneous rock and fine grain clastic rock with a certain thickness.According to the occurrence,fracture development and filling condition,folds and fracture which show various degree of water-resisting ability become the impervious boundary.

4.2.1.Evidence from the geological structure,hydrogeology and groundwater flow system

Fig.2 showed that the northern,eastern and northwestern of the groundwater basin divide of Dalinuoer Lake are consistent with the surface watershed ridge.It is speculated that the southwestern boundary is the movable groundwater divide.The Haoluku Anticlinorium Fault in SW-NE direction which developed in the southern of the surface watershed of Dalinuoer Lake controlled the distribution of acidic extrusive rocks in the Jurassic and intrusive rocks in the late Yanshanian.Because of its obvious water-resisting character,it becomes the intermittent outcropped groundwater basin divide.The Haoluku Anticlinorium Fault divides the surface watershed of Dalinuoer Lake into the northern regional A named the groundwater system of Dalinuoer Lake and southern regional B named the groundwater system of the Xilamulun Riverhead (Fig.2,Fig.3).The area of regional A and B is 4838 km2and 1301 km2respectively.Fig.4 showed that the groundwater level elevation was not consistent in both sides of the water-blocking anticlinorium fault.The groundwater level elevation of the upstream of surface watershed named regional B is lower than the downstream named regional A,and the isoline of groundwater level elevation is perpendicular to the basin divide.It is well confirms the Haoluku Anticlinorium Fault acts as the groundwater basin divide with water-resisting character.The regional A and B are two independent groundwater system,and the groundwater discharges in the direction of Dalinuoer Lake and Xilamulun River separately.

It is noted that an EW fault extending westward through Jingfeng,Ganggenore,Dalainore,Duolunnore and Baiyinkulunnore existed in the southern of Dalinuoer Lake.The fault which formed by the compression in the NS direction is called the EW orientation major concealed fracture of the southern of Dalinuoer Lake in our research.It is speculated that it could be extended to the east and connected with the Xilamulun River Fault,and has the insidious step water blocking effect.This fault could further divide the groundwater system of Dalinuoer Lake (regional A) into the northern and southern as two secondary groundwater systems.The aquifer thickness of the northern secondary groundwater system is small,the groundwater discharges into the river directly.While the aquifer thickness of the southern secondary groundwater system is large,so the northward flowing groundwater is blocked on the concealed fault boundary and overflows upward,eventually forms springs or discharges into the lake.The characteristics of groundwater flow field on both sides of this fault are also inconsistent (Fig.4).

Fig.4.Isoline map of groundwater level elevation.

In addition,the eastern,western and southern divide of the Xilamulun Riverhead groudwater basin are consistent with the surface watershed ridge.But because the groundwater of regional B recharges the groundwater of regional C,the southwestern boundary of the groundwater system of Xilamulun Riverhead crosses the surface watershed boundary,and it extends westward to the Haoluku Anticlinorium Fault.

4.2.2.Evidence from the hydrogen and oxygen stable isotopes

The hydrogen and oxygen stable isotopes analytical data of the water samplings collected from the scientific literature are listed in Table 1.As shown in Table 1,the stable isotope compositions of the river samplings collected in the southern of regional A ranging from −84.04‰ to −81.94‰ forδD and−12.64‰ to −13.01‰ forδ18O,respectively.The arithmetic mean values ofδD andδ18O are −83.32‰ and −12.64‰,respectively.The stable isotope compositions of the spring samplings collected in the southern of regional A ranging from −84.3‰ to −82.21‰ forδD and −13.33‰ to −12.81‰forδ18O,respectively.The arithmetic mean values ofδD and δ18O are −83.26‰ and −13.07‰,respectively.The stable isotope compositions of the groundwater samplings collected in the southern of regional A ranging from −82.64‰ to−85.61 ‰ forδD and −13.11 ‰ to −13.53 ‰ forδ18O,respectively.The arithmetic mean values ofδD andδ18O are−84.25 ‰ and −13.32 ‰,respectively.The rivers in the southern of regional A are all centrally recharged by springs,in which the hydrogen and oxygen isotopes are basically the same with its surrounding groundwater samplings.In Fig.5,the river and groundwater samplings in the southern of regional A centrally located in the domain a2,which further demonstrated that the surface water and groundwater in the southern of regional A compose the integrated fluvial aquifer system.

Table 1.The stable isotope compositions of all the water samplings collected from the scientific literature.

The stable isotope compositions of the groundwater samplings collected in the regional B ranging from −75.9‰ to−63.4 ‰ forδD and −11.3 ‰ to −8.64 ‰ forδ18O,respectively.The arithmetic mean values ofδD andδ18O are−69.55‰ and −10.17‰,respectively.They are centrally located in the domain b,which are differentiate from the groundwater in the southern of regional A.This further illustrates that the regional A and B belongs to different groundwater systems.At the same time,the hydrogen and oxygen stable isotope compositions of spring samplings collected in the regional C are −72.6 ‰ and −10.5 ‰respectively,which are close to the groundwater samplings collected in the regional B.This declares that the regional B and C belong to the same groundwater system,and together constitute the groundwater system of the Xilamulun Riverhead.It is found that the stable isotope compositions of groundwater samplings are different between the northern and southern of the regional B with small area,which shows northern-southern zone features obviously.The value of the samplings collected in the southern are significantly poorer than in the northern of regional B,which is probably due to the elevation effects.At the same time,the groundwater in the southern does not flow into the northern,but flows towards the Xilamulun River.The TDS of groundwater samplingscollected in the southern is higher than that of the northern(Zhu BQ et al.,2019),which further verified this flow pattern.

In Fig.5,the river sampling collected in the northern Gonggeer River system is independent appeared in the domain a1.The Gonggeer Riverhead originates from the southern remnant end of the Greater Khingan Mountains,so the precipitation-runoff generation area has high elevation,and the rivers flowing into the lake are relatively poor in isotopes.

The southern of regional A,and regional B,C and D all located in the northern of the Otindag Desert,where the precipitation is the only recharge source for the groundwater and the recharge can account for more than 70% of precipitation normally.Therefore,the hydrogen and oxygen stable isotope compositions of the whole samplings collected in that regions all appeared near the GMWL,and had obvious zone features.So the samplings collected from the southern of regional A,and regional B,C and D appeared in the domain a2 and domain b+c+d separately (Fig.5).The groundwater and spring samplings collected in the regional D appeared in the domain d,the corresponding lake samplings formed by evaporation appeared in the domain d’.For this purpose,the evaporation line in regional D is calculated,and its slope value was 3.62.With the consideration that the groundwater and springs have similarity in geographical and hydrogeological conditions in the regional B,C and D,it is speculated that they have similar evaporation character.The combined evaporation line slope of regional B,C and D were calculated using the groundwater,spring,river and lake samplings collected in the regional B,C and D.Results showed that the combined evaporation line slope value is 4.10.Therefore,the evaporation line slope in this area was roughly 3.62-4.10 (Fig.5,Fig.6).

Fig.5.Relations of δD and δ18 O in all the water samplings.

Fig.6.Relations of δD and δ18 O in the water samplings collected from domain b+c+d.

In Fig.5,the samplings collected in the Dalinuoer Lake appeared in the domain a’.In order to explore the Dalinuoer Lake recharge source ration from the northern domain a1 and the southern domain a2,it is assumed that the recharge sources come from the Gonggeer River representing domain a1 and the southern part representing domain a2.The evaporation line of domain a1-domain a’ and domain a2-domain a’ named were plotted with its slope value of 5.11 and 4.78 respectively.The latter slope is closer to the aforementioned slope of the combined evaporation line of regional B,C and D.It is speculated that the original water recharging the Dalinuoer Lake should be the mixed water from the northern part and southern part,and the mixed water domain should appear in the middle of the northern domain a1 and southern domain a2.But the mixed water domain should be appeared more close to the domain a2.It is illustrated that in the current precipitation condition and water resources development and utilization situation,the effect of the surface water and groundwater in the southern of regional A play an even greater role than the northeastern Gonggeer River system in the Dalinuoer Lake recharge.Consequently,it is of great significance to protect water resources conservation area and reduce the development and utilization of groundwater resources in the southern sand of regional A for maintaining the ecology safety of the Dalinuoer Lake.

4.3.The impact of surface watershed and groundwater basin on the ecosystem of Dalinuoer Lake

The westerly winds circulation and the EASM circulation are two extremely important wind systems in the earth (An CB and Chen FH,2009; Gan TY,2000).Although the two circulations belong to different subsystems of atmospheric circulation,they are closely related.The two joint influence the regional precipitation and climate environment change in the middle and high latitudes in the northern hemisphere.The previous study showed that the boundary of the EASM and westerly winds lies through the Inner Mongolia Plateau (Chen FH et al.,2010; Zhu BQ et al.,2019,Li WB et al.,2017).The author of this article found that the arithmetic mean values ofδD andδ18O of the groundwater samplings collected in the regional B,C and D which located in the south of the Haoluku Anticlinorium Fault are −69.88‰ and −10.21‰,respectively,and the groundwater samplings centered in the domain b+c+d.The arithmetic mean values ofδD andδ18O of the groundwater samplings collected in the southern of the regional A are −84.00‰ and −13.25‰,respectively,and the groundwater samplings centered in the domain a2.There is no intersection between the domains,and the value of domain a2 is poorer than the domain b+c+d (Fig.5).This not only denies the possibility of water vapor originating from the Pacific Ocean,but also indicates that the southern of regional A and regional B,C and D are affected by different climatic water vapor sources.As a result,the Haoluku Anticlinorium Fault orientation is presumed to be the boundary of the EASM and westerly winds.

The ecology of the Dalinuoer Lake watershed is extremely sensitive to global climate change.During 52 years,the average precipitation decline rate was −10.5 mm/10a,and the trend rate of temperature increase was +0.35°C/10a,which performs a prominent warming-drying trend (Ma C et al.2017).The observation of Baiyinaobao weather station showed that the evaporation is 3.6 times greater than rainfall(Zhen ZL et al.,2014).The recharge and discharge relationship between groundwater and surface water of the Dalinuoer Lake watershed is that the groundwater in the southern recharges the river,and groundwater and the springfeeding river jointly recharge the Dalinuoer Lake.Warm and dry climate causes temperature rising and precipitation declining,which leads to the evaporation increases and runoff decreases.The Dalinuoer Lake water recharge is less than evaporation.In the 20thcentury,the human influence was weak.The relationship between man and land relationship showed less tension and the climate controls the ecological evolution of Dalinuoer Lake watershed.There are certain relations between the lake area and precipitation,but the evaporation does not have significant effect on the lake area.Rivers and groundwater have been exploited and utilized by the agriculture,industry and animal husbandry activities since the 21stcentury,which lead to soil erosion,land desertification and lake area shrinkage.The human activities have become the main contributor to the regional ecological environment degradation (Ma C et al.,2017).

The TDS,alkalinity,pH and hardness are important indicators for fish survival in the water body.The Dalinuoer Lake area shrinkage leads to the dissolved solids condensed during the evaporation process.As a result,the Dalinuoer Lake evolves into a saltwater lake.Yang FY et al.(2020)found that the average alkalinity and hardness of the water in Dalinuoer Lake were 69.34 mmol/L and 2.61 mmol/L respectively,with an average pH 8.80 in July 2019.From 1975 to 2019,the average alkalinity and hardness increased by 1.01% and 0.38% annually,respectively,while the average annual pH decreased by 0.16%.The high saline-alkaline water of Dalinuoer Lake had no serious negative impact on the original saline-alkali tolerant fish,such as Varsijaro and Dali Lake Plateau Loach now,but it might limit the migration of other ordinary economic freshwater fish species.So far,it was only found that naked carp from Qinghai Lake could better adapt to the high saline and alkaline water environment of Dalinuoer Lake.As the lake area shrinkage and water salinization of Dalinuoer Lake,the suitability of fish breeding habitat is getting worse,and the potential risk of fish species decline is increasing,which will have an important impact on the ecological diversity of Dalinuoer Lake (Yang FY et al.,2020).

5.Conclusion

(i) The correlations between DEM and drainage system suggest that the surface drainage area of Dalinuoer Lake is 6139 km2.The surface watershed of Dalinuoer Lake consists of the northern regional A and the southern regional B with an area of 4838 km2and 1301 km2,respectively.The northern regional A is the surface watershed in modern climate environment,and the surface water in the southern regional B could flow into regional A once the high intensity rainfall events occur.

(ii) The Haoluku Anticlinorium Fault developed in the southern of the Dalinuoer Lake determine the basement distribution of Jurassic acidic extrusive rocks controlled by Cenozoic and intrusive rocks of late Yanshanian.The NE-SE anticlinal axis intermittently outcropped and acts as the groundwater basin divide with obvious water-blocking effect,which divides the surface watershed of Dalinuoer Lake into two independent northern regional A and southern regional B groundwater systems.The surface watershed of Dalinuoer Lake is a paradigm which develops two groundwater systems in the same surface watershed.This conclusion is further confirmed by the comparison of groundwater level and stable environmental isotopes in the northern regional A and southern regional B.

(iii) The hydrogen and oxygen stable isotopes comprehensive analysis shows that the northern regional A and southern regional B bounded by the Haoluku Anticlinorium Fault had different isotopic characteristic.The value of theδD andδ18O sampled in the regional A groundwater system flowing into the southern of the Dalinuoer Lake are obvious poorer than that of the regional B groundwater system flowing into the Xilamulun Riverhead.But the value ofδD andδ18O in the samplings collected in the regional B,C and D are consistent.It can be inferred that the trend of Haoluku Anticlinorium Fault is the boundary of the westerlies wind and EASM climate systems,which improves the previous understanding of this boundary location.

(iv) The Dalinuoer Lake is an endorheic faulted depression lake recharged together by the surface water and groundwater.The river in the southern regional A rising in the Otindag Desert are all spring-feeding river,which is mainly recharged by the groundwater.The surface water and groundwater in southern regional A compose the integrated fluvial aquifer system.The value ofδD andδ18O in the river samplings collected in the southern regional A is consistent with its surrounding groundwater samplings.With the analysis of isotopes evaporation line of the Dalinuoer Lake watershed,it suggests that the effect of the surface water and groundwater in the southern part play an even greater role than the northeastern Gonggeer River system in the Dalinuoer Lake recharge source.So it is of great significance to protect water resources conservation area and reduce the development and utilization of groundwater resources in the southern of regional A for maintaining the ecology safety of Dalinuoer Lake.

(v) The high saline-alkaline water of Dalinuoer Lake had no serious negative impact on the original saline-alkali tolerant fish,such as Varsijaro and Dali Lake Plateau Loach now,but it might limit the migration of other ordinary economic freshwater fish species along with the lake area shrinkage and water salinization of Dalinuoer Lake.The suitability of fish breeding habitat is getting worse,and the potential risk of fish species decline is increasing,which will have an important impact on the ecological diversity of Dalinuoer Lake.

CRediT authorship contribution statement

Wen-peng Li and Long-feng Wang developed the concept and methodology,performed the data analysis and results interpretation,as well as took the lead in writing the manuscript.Wen-peng Li carried out the field investigation.Yi-long Zhang and Li-jie Wu observed the groundwater level.Wen-peng Li and Long-feng Wang complied all the maps with the help of the others authors.All authors discussed the results and improved the writing of this manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowlegement

The authors are grateful for the Chief Editor Zi-guo Hao and anonymous reviewers for insightful comments on the manuscript.This work was financially supported by the Hydrogeology and Water Resources Survey Program of China Geological Survey (20230006-06,DD20190322) and the National Natural Science Foundation of China(42130613).