The Ecological Relationship of Groundwater–Soil–Vegetation in the Oasis–Desert Transition Zone of the Shiyang River Basin

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Article The Ecological Relationship of Groundwater–Soil–Vegetation in the Oasis–Desert Transition Zone of the Shiyang River Basin

Le Cao 1,2,3,* , Zhenlong Nie 1, Min Liu 1 , Lifang Wang 1, Jinzhe Wang 1 and Qian Wang 1

1 Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China; [email protected] (Z.N.); [email protected] (M.L.); wlfﬁ[email protected] (L.W.); [email protected] (J.W.); [email protected] (Q.W.) 2 Chinese Academy of Geological Sciences, China University of Geosciences (Beijing), Beijing 100083, China 3 Key Laboratory of Groundwater Sciences and Engineering, Ministry of Natural Resources, Shijiazhuang 050061, China * Correspondence: [email protected]

Abstract: Groundwater is an important ecological water source in arid areas. Groundwater depth (GWD) is an important indicator that affects vegetation growth and soil salinization. Clarifying the coupling relationship between vegetation, groundwater, and soil in arid areas is beneficial to the prevention of environmental problems such as desertification and salinization. Existing studies lack research on the water–soil–vegetation relationship in typical areas, especially in shallow groundwater areas. In this study, the shallow groundwater area in Minqin, northwest China, was taken as study area, and vegetation surveys and soil samples collection were conducted. The relationships between vegetation fractional coverage (VFC) and GWD, soil salinity, soil moisture, and precipitation were comprehensively analyzed. The results showed low soil salinity in the riparian zone and high soil salinity in other shallow-buried areas with salinization problems. Soil salinity was negatively Citation: Cao, L.; Nie, Z.; Liu, M.; correlated with VFC (R = −0.4). When soil salinity >3 g/kg, VFC was less than 20%. Meanwhile, Wang, L.; Wang, J.; Wang, Q. The Ecological Relationship of when GWD >10 m, VFC was usually less than 15%. In the areas with soil salinity <3 g/kg, when Groundwater–Soil–Vegetation in the GWD was in the range of 4–10 m, VFC was positively correlated with soil moisture content (R = 0.99), Oasis–Desert Transition Zone of the and vegetation growth mainly depended on surface soil water, which was significantly affected by Shiyang River Basin. Water 2021, 13, precipitation. When GWD was less than 4 m, VFC was negatively correlated with GWD (R = −0.78), 1642. https://doi.org/10.3390/ and vegetation growth mainly relied on groundwater and soil water. There are obvious ecological w13121642 differences in the shallow-buried areas in Minqin. Hence, it is reasonable to consider zoning and grading policies for ecological protection. Academic Editor: Frédéric Frappart Keywords: groundwater depth (GWD); soil salinity; soil moisture; vegetation fractional coverage Received: 13 May 2021 (VFC); NDVI; ecological relationship; Shiyang River Basin Accepted: 8 June 2021 Published: 11 June 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- Desertification is the main ecological and environmental problem in arid iations. areas [1–3], and vegetation is the main body of ecology in arid areas. Maintaining high vegetation coverage is the key to preventing desertification [4,5]. As vegetation growth relies on water, groundwater is an important ecological water source in addition to limited precipitation [6–8]. Groundwater depth (GWD) is an important factor affecting the growth of ecological vegetation [9–12]. Greater GWD leads to lower vegetation fractional Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. coverage (VFC) [9,11], whereas lesser GWD causes salinization [13], which can restrict This article is an open access article vegetation growth and affect VFC. Therefore, studying the ecological relationship between distributed under the terms and GWD, soil, and vegetation has important scientific significance. conditions of the Creative Commons The Shiyang River Basin in China faces serious salinization [14] and desertification Attribution (CC BY) license (https:// problems [15], both of which are closely related to the GWD, especially in the Minqin creativecommons.org/licenses/by/ Oasis at the lower reaches of the basin. After nearly 60 years of excessive groundwater 4.0/). exploitation, the GWD in the basin has increased significantly, affecting the growth of

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natural vegetation around the oasis. The sandstorm intrusion from the Badain Jaran Desert to the northwest of the oasis has gradually expanded [16,17], and the Badain Jaran Desert is now close to become merging with the Tengger Desert in the east to the oasis. Sandstorms in this area are also one of the main sources of sandstorms in China [18]. In addition to the increase of the desertification area, the proportion of salinized soil accounts for 80% of the Minqin area [14], and the salinization problem in this area is extremely serious. Therefore, studying the ecological value of groundwater in the basin to natural vegetation, the GWD, and the ecological relationship between soil and vegetation is of important significance to the ecological protection, management, and restoration of this area. Many scholars have conducted research on the ecological environmental problems (desertification and salinization) related to groundwater and vegetation in the Minqin area [14–16,19–21]. Huang [19,22,23] and Chunyu [24] studied the dynamic simulation of the oasis driven by ecological water transfer and its significance for the restoration of natural oases in arid inland basins, and obtained an optimal water diversion amount of 45 million m3. Cao [25] concluded that the most suitable groundwater depth for natural vegetation growth in the Minqin Oasis was 2.5–3.9 m, which is not completely consistent with other basins in Northwest China [26–28]. Wang [29,30] improved the theory and method of groundwater function evaluation and regionalization, and delineated the nature reserves of groundwater maintaining vegetation ecology, which are mainly distributed in the shallow groundwater buried areas. Ding [16] and Ren [17] investigated the source and spatiotemporal pattern of sand migration in the Minqin Oasis. Yang [14] reported that the area of soil salinization in the Minqin Oasis has gradually increased in recent years. The degree of salinization is high in spring and low in autumn, and a large proportion of the salinized land is related to climate and hydrological factors. Liu [31] investigated the spatial distribution characteristics of soil water and salt under different land use types in Minqin Oasis. The average salt content is 47.02 g/kg and the degree of salinization is severe. Existing studies have mostly focused on the analysis of regional and basin- scale remote sensing of vegetation coverage as well as salinization degree and evolution. However, the analysis of the coupling effect of groundwater, salinization, and ecological vegetation is still lacking. Detailed analysis of the groundwater ecological effects based on the spatial heterogeneity of the Minqin area also required, especially for the study of shallow groundwater areas. In this study, we conducted a vegetation ecological survey, measured GWD, and collected groundwater and soil samples in the shallow GWD area of the Shiyang River Basin. By analyzing the ecological relationship of groundwater–soil–vegetation, the main factors that control and affect the VFC in the region were examined. By combining the characteristic differences of basic ecological conditions with zoning, targeted ecological protection and management suggestions were proposed. The objective of this study is to clarify the VFC under the control of GWD and soil salt in the oasis–desert transition zone of the Shiyang River Basin and obtain important ecological GWD and salinization indicators, and provide a reference and scientific basis for the formulation of ecological policies in the basin.

2. Materials and Methods 2.1. Study Area The Shiyang River Basin is located in the eastern part of Gansu Province, China. It is one of the three inland river basins in the Hexi Corridor, with a total area of 4.16 × 104 km2 [32]. The Minqin Basin is located in the lower reaches of the Shiyang River Basin with an area of about 1.58 × 104 km2. The northwestern part is the Badain Jaran Desert, the southeastern part is the Tengger Desert (Figure1), and the central part of the basin is the Minqin Oasis. The study area is mainly located in the oasis and desert transition zone from 38◦000 N to 39◦200 N and 102◦370 E to 103◦550 E. These areas are subject to serious ecological problems and risks. The Minqin Oasis area is characterized by a typical arid desertiﬁcation climate with an average annual temperature of 7.8 ◦C, an average annual Water 2021, 13, x FOR PEER REVIEW 3 of 17

from 38°00’ N to 39°20’ N and 102°37’ E to 103°55’ E. These areas are subject to serious ecological problems and risks. The Minqin Oasis area is characterized by a typical arid desertification climate with an average annual temperature of 7.8 °C, an average annual precipitation of about 116.5 mm, and an average annual evaporation of 2308 mm [33]. Water 2021, 13, 1642 The landform types include mobile dunes, semi‐fixed dunes,3 of 17 fixed dunes, interlaced deserts, and Gobi. Shrubs are the dominant component of natural vegetation in the area,

precipitationand include of about Nitraria 116.5 mm, spp., and an averageTamarix annual spp., evaporation Haloxylon of 2308 mmammodendron [33]. The , Kalidium foliatum, landformReaumuria types soongorica include mobile, and dunes, Artemisia semi-ﬁxed arenaria dunes, ﬁxed. The dunes, trees interlaced include deserts, Populus euphratica, Populus andgansuensis Gobi. Shrubs, Elaeagnus are the dominant angustifolia component, ofand natural Salix vegetation matsudana in the area,. The and include herbs are mainly Phragmites Nitraria spp., Tamarix spp., Haloxylon ammodendron, Kalidium foliatum, Reaumuria soongorica, andaustralisArtemisia, arenariaAchnatherum. The trees includesplendensPopulus, euphraticaSophora, Populus alopecuroides gansuensis, Elaeagnus L, Bassia dasyphylla, and angustifoliaAgriophyllum, and Salix squarrosum matsudana. The [34–36]. herbs are mainly Phragmites australis, Achnatherum splendens, Sophora alopecuroides L., Bassia dasyphylla, and Agriophyllum squarrosum [34–36].

Figure 1. Remote-sensing1. Remote‐sensing image and image sampling and plots sampling of the lower plots reaches of of Shiyangthe lower River reaches Basin. of Shiyang River Basin. In recent years, the GWD of agricultural irrigation area in the Minqin area has been aroundIn 15–35 recent m [25 years,,37,38]. the The GWD shallow of groundwater agricultural area (GWDirrigation < 15 m) area is mainly in the Minqin area has been distributed in the oasis–desert transition zone around the irrigation area [25], the riparian zonearound [35], and15–35 the Qingtu m [25,37,38]. Lake Wetland The in the shallow terminal lakegroundwater of the Shiyang Riverarea [19(GWD]. < 15 m) is mainly Accordingdistributed to the in geographical the oasis–desert location, landform, transition groundwater zone around depth, and the vegetation irrigation area [25], the riparian community,zone [35], the and shallow the groundwater Qingtu areaLake was Wetland divided into in six the zones; terminal detailed information lake of the Shiyang River [19]. is shown in Figure1 and Table1. These zones represent the riparian zone in the middle reachesAccording (WW), theto shallowthe geographical groundwater area location, in Nanhu Town landform, (NH), Qingtu groundwater Lake wetland depth, and vegetation (QTH),community, the west of the Minqin shallow oasis (MQW), groundwater Minqin Oasis (MQEW) area andwas the divided east of Minqin into six zones; detailed oasisinformation (MQEE). is shown in Figure 1 and Table 1. These zones represent the riparian zone in the middle reaches (WW), the shallow groundwater area in Nanhu Town (NH), Qingtu Lake wetland (QTH), the west of Minqin oasis (MQW), Minqin Oasis (MQEW) and the east of Minqin oasis (MQEE).

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Table 1. Sampling sites information and the vegetation communities.

Site Number of Groundwater Landform Vegetation Types Typical Communities Abbreviation Quadrats Depth (m) Oasis–Desert NH Transition 17 1.4–5.9 Shrubs/Herbs Haloxylon ammodendron/Kalidium foliatum Zone/Wetland Elaeagnus angustifolia/Tamarix WW Riparian zone 26 1.3–6.0 Trees/Shrubs/Herbs spp./Achnatherum splendens QTH Desert/Wetland 24 1.1–4.0 Shrubs/Herbs Kalidium foliatum/Phragmites australis Oasis–Desert MQW 8 9.0–17.8 Shrubs/Herbs Haloxylon ammodendron/Tamarix spp. Transition Zone Oasis–Desert MQEW 15 5.2–20.8 Shrubs/Herbs Haloxylon ammodendron/Nitraria spp. Transition Zone Oasis–Desert MQEE 14 1.97–5.0 Trees/Shrubs/Herbs Kalidium foliatum/Achnatherum splendens Transition Zone

2.2. Data Sources and Methods The ecological survey was conducted in August 2018. According to different landform types and GWD intervals [25,37], representative plots in each district were selected to perform tree surveys with 30 × 30 m2 vegetation quadrats. Within the large quadrat, ﬁve 5 × 5 m2 quadrats were arranged in the four corners and the center according to the diagonal method to investigate shrubs. Within each shrub quadrat, a 1 × 1 m2 quadrat was set up to conduct herb surveys. The ecological survey recorded the community types (trees, shrubs, and herbs), number of vegetation species, number of single species, coverage, growth, and other ecological indicators [39]. The importance of vegetation in the quadrat was ranked according to the coverage of each species. In the center of the quadrat, soil samples were collected at 20 cm intervals for a maximum depth of 1 m [31]. Five samples were thus taken from each sampling point, stored in aluminum boxes and Ziplock bags, and tested for the soil moisture content and soluble salt. The moisture content was determined using the drying method [40]. The averaged value of the ﬁve samples was calculated and used as the soil characteristic data of the sampling point. The GWD was measured using a Luoyang shovel and steel tape. The normalized difference vegetation index (NDVI) is a commonly used indicator for evaluating vegetation development. Landsat NDVI data have been widely used for studying changes in regional ecological environments [8,19,20,41–43]. In general, NDVI values range between −1 and 1. A high NDVI value indicates that the vegetation is well- developed, whereas a low NDVI value indicates that the vegetation is in poor condition. This study used the Landsat satellite data (from 1980 to 2018) with a temporal resolution of 16 d and a spatial resolution of 30 m. Firstly, the satellite image was preprocessed using atmospheric and geometric correction. Then, image stitching, cropping, and band calculation were performed. Finally, the maximum value extraction and annual average calculation were performed. The vegetation fractional coverage is estimated using NDVI and the following formula [44]: NDVI − NDVI VFC = min × 100% , (1) NDVImax − NDVImin

where VFC is the vegetation fractional coverage and NDVImin and NDVImax are NDVI values with cumulative probabilities of 5% and 95%, respectively. The long-term dynamic observation data of groundwater were obtained from the Shiyang River Administration. They include the data of typical groundwater depth monitoring wells in the Minqin area (1980–2016) and were used to calculate the annual average GWD. Water 2021, 13, x FOR PEER REVIEW 5 of 17

monitoring wells in the Minqin area (1980–2016) and were used to calculate the annual average GWD. The meteorological data of the Minqin area were downloaded from the China Meteorological Science Data Sharing Service Network (http://data.cma.cn accessed on July 1, 2019) of the China Meteorological Administration. These data mainly include air Water 2021, 13, 1642 temperature (T), relative humidity (RH), and precipitation (P) from 2004 to 2016. The5 of 17 annual average temperature, annual average relative humidity, and annual rainfall were calculated here based on the above data.

3. ResultsThe meteorological data of the Minqin area were downloaded from the China Meteoro- logical Science Data Sharing Service Network (http://data.cma.cn; accessed on 1 July 2019) 3.1. Surface Soil Moisture and Salt Content of the China Meteorological Administration. These data mainly include air temperature (T), relativeThe average humidity surface (RH), soil and moisture precipitation contents (P) (Figure from 2004 2a) toof 2016.the areas The annualin descending average temperature,order were QTH annual > MQEE average > NH relative > MQEW humidity, > WW > and MQW. annual The rainfallsoil moisture were calculatedcontents of hereall basedareas ranged on the abovefrom 0% data. to 30%. MQW had the lowest average soil moisture content (only 2.5%), whereas QTH had the highest average soil moisture content (about 16%). 3.However, Results the soil moisture content of QTH varied widely (5–30%). 3.1. SurfaceThe average Soil Moisture surface and soil Salt salt Content contents (Figure 2b) of the areas in descending order wereThe NH average > QTH surface> MQEE soil > MQEW moisture > MQW contents > WW. (Figure The2a) salt of thecontents areas inof descendingall areas ranged order werefrom QTH0 to 40 > MQEEg/kg. The > NH average > MQEW salt content > WW >of MQW. WW was The the soil lowest moisture (at 1.5 contents g/kg), of whereas all areas rangedthat of NH from was 0% the to 30%.highest MQW (approximately had the lowest 17.5 average g/kg). The soil soil moisture salt content content of NH (only varied 2.5%), whereasfrom 2 to QTH 38 g/kg. had theWith highest the exception average of soil the moisture WW area, content the average (about 16%).salt content However, of the the soilother moisture five areas content exceeded of QTH 3 g/kg, varied indicating widely salinized (5–30%). land.

Min–Max Mean

Sample

25–75% and Median

1–99%

Sampling site Sampling site (a) (b)

FigureFigure 2. 2. SurfaceSurface soilsoil moisture and salinity in different sampling sampling sites: sites: ( (aa)) surface surface soil soil moisture; moisture; (b (b) )surface surface soil soil salinity. salinity.

3.2. EcologicalThe average Vegetation surface Coverage soil salt and contents Groundwater (Figure Depth2b) of the areas in descending order wereThe NH remote > QTH sensing > MQEE data > MQEW showed > the MQW average > WW. VFC The of salt each contents area (Figure of all areas3 or Figure ranged from4a). The 0 to VFCs 40 g/kg. of the The areas average in descending salt content order of WW were was as thefollows: lowest WW (at > 1.5 MQEE g/kg), > whereasQTH > thatMQEW of NH > NH was > theMQW. highest The (approximately VFCs of all areas 17.5 were g/kg). less The than soil 60%. salt The content average of NH VFC varied of fromWW was 2 to 38the g/kg. highest With at the32%, exception which indicated of the WW medium area, the vegetation average saltcoverage. content The of theVFC other of fiveMQW areas was exceeded the lowest 3 g/kg, at 8%, indicating which indicated salinized mostly land. bare land. In contrast, the VFCs of other areas ranged from 10 to 20% and were classified as medium and low vegetation 3.2.coverage Ecological areas. Vegetation The VFCs Coverage of the quadrats and Groundwater in the WW Depth area varied widely (with a maximum of 60%The and remote a minimum sensing of data 2%), showed with a high the averagecoefficient VFC of ofvariation. each area (Figure3 or Figure4a). The VFCs of the areas in descending order were as follows: WW > MQEE > QTH > MQEW > NH > MQW. The VFCs of all areas were less than 60%. The average VFC of WW was the highest at 32%, which indicated medium vegetation coverage. The VFC of MQW was the lowest at 8%, which indicated mostly bare land. In contrast, the VFCs of other areas ranged from 10 to 20% and were classified as medium and low vegetation coverage areas. The VFCs of the quadrats in the WW area varied widely (with a maximum of 60% and a minimum of 2%), with a high coefficient of variation. Water 2021, 13, x FOR PEER REVIEW 6 of 17 Water 2021, 13, 1642 6 of 17

FigureFigure 3. 3.Distribution Distribution of groundwater of groundwater depth (GWD) depth and(GWD) vegetation and vegetation fractional coverage fractional (VFC). coverage (VFC).

Min–Max Mean

Sample

25–75% and Median

1–99%

Figure 4. Vegetation fractional coverage and groundwater depth in different sampling sites: (a) vegetation fractional coverage; (b) groundwater depth. Sampling site Sampling site (a) (b) Figure 4. Vegetation fractional coverage and groundwater depth in different sampling sites: (a) vegetation fractional coverage; (b) groundwater depth.

The GWDs of the areas in descending order were MQW > MQEW > NH > MQEE > QQ > QTH (Figure 4b). The average GWD in MQW was 13.94 m; that in MQEW was 11.31 m; those in WW and NH were similar (3.42 m and 3.43 m, respectively); those in QTH and MQEE were less than 3 m; and that in QTH, at 2.16 m, was the lowest.

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Water 2021, 13, x FOR PEER REVIEW 7 of 17 The GWDs of the areas in descending order were MQW > MQEW > NH > MQEE > QQ > QTH (Figure4b). The average GWD in MQW was 13.94 m; that in MQEW was 11.31 m; those in WW and NH were similar (3.42 m and 3.43 m, respectively); those in QTH andThe MQEE vegetation were less quadrats than 3 m; demonstrate and that in QTH, that ata combination 2.16 m, was the of lowest. shrubs and herbs is the The vegetation quadrats demonstrate that a combination of shrubs and herbs is the main community type in the study area (Figure 5). There were 66 quadrats of this type, main community type in the study area (Figure5). There were 66 quadrats of this type, accounting for 71% of all investigated quadrats. The average VFC was 15%. The number accounting for 71% of all investigated quadrats. The average VFC was 15%. The number of ofcommunities communities with with trees trees was was small, small, and and they they were were only only distributed distributed in the in WW the area. WW In area. the In thecommunities communities with with trees, trees, the VFCthe VFC was signiﬁcantlywas significantly higher higher (25–45%). (25–45%).

70 60 VFCmin VFCmax VFCmean Number of quadrats 60 50

50 40

40 30 30

20 20 of quadrats Number

10 Vegetation fractional coverage(%) 10

0 0

Vegetation community structure and importance ranking

Figure 5. Statistics of vegetation community structure types and vegetation fractional coverage. Figure 5. Statistics of vegetation community structure types and vegetation fractional coverage. 4. Discussion 4.4.1. Discussion Ecological Relationship among Groundwater, Soil and Vegetation 4.1. EcologicalFigure6 Relationshipshows that the among quadrats Groundwater, with higher Soil vegetationand Vegetation fractional coverage were mainly concentrated in areas with shallow groundwater depth (<6 m) and relatively lowFigure surface 6 soil shows salinity that (<5 the g/kg). quadrats These with samples higher were vegetation distributed fractional in WW, whereascoverage the were mainlycoverage concentrated rates of other in areas areas were with around shallow 10–20%. groundwater When the depth GWD was(<6 morem) and than relatively 10 m, the low surfaceVFC was soil less salinity than 10% (<5 and g/kg). the surface These was samples bare soil, were reﬂecting distributed the poor in overallWW, ecologicalwhereas the coverageconditions rates of shallow-buried of other areas areaswere inaround the Shiyang 10–20%. River When Basin. the GWD was more than 10 m, the VFC was less than 10% and the surface was bare soil, reflecting the poor overall ecological conditions of shallow‐buried areas in the Shiyang River Basin.

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FigureFigure 6. 6.The The vegetation vegetation fractional fractional coverage coverage (VFC) with (VFC) different with soil different salinity soil and groundwatersalinity and groundwater depthdepth of of the the Shiyang Shiyang River River Basin. Basin. The correlation analysis results of the water–soil–vegetation relationship showed that the VFCThe had correlation the closest relationshipanalysis results with surface of the soil water–soil–vegetation salinity, with a correlation coefficientrelationship showed that thebetween VFC them had ofthe− 0.43,closest whereas relationship the VFC correlationwith surface with soil GWD salinity, was only with−0.21. a correlation The coefficient betweenrelationship them between of − GWD0.43, and whereas vegetation the growth VFC wascorrelation less obvious, with meaning GWD that was the only −0.21. The GWD does not significantly affect the VFC. Higher soil salinity may limit the types of relationshipvegetation and between vegetation GWD growth, and resulting vegetation in a lower growth GWD that was cannot less produceobvious, high meaning VFC. that the GWD doesTo not better significantly understand theaffect ecological the VFC. effect Higher of GWD soil and confirmsalinity the may weak limit relationship the types of vegetation andbetween vegetation GWD depth growth, and VFC, resulting we analyzed in a the lower changes GWD of GWD that and cannot vegetation produce conditions high VFC. in a long time series. Figure7 presents the change trend of groundwater from the 1980s to 2017.To The better GWDs understand of MQEE, NH, the MQEW, ecological WW, effect and MQW of GWD have trended and confirm upward. the After weak relationship between2012, the GWDs GWD of thedepth NH, MQEE,and VFC, and MQW we monitoringanalyzed wells the decreased changes and of the GWD water and vegetation conditionslevels increased. in a Afterlong 2010,time theseries. GWD Figure of the QTH7 presents area slowly the change decreased trend due toof the groundwater from theimplementation 1980s to 2017. of ecological The waterGWDs transfer of MQEE, measures. NH, As a wetland,MQEW, the WW, surface and water MQW area have trended of QTH has also expanded [19,24,45]. The vegetation data in the same period show that, upward.with the increase After in 2012, GWD, the the GWDs vegetation of coverage the NH, conditions MQEE, did and not MQW decrease monitoring significantly wells decreased andin the the NH, water WW, levels MQEW, increased. and MQEE After areas (Figure 2010, 8thea,b,e,f). GWD From of the 1980sQTH to area 1996 slowly and decreased due tothen the from implementation 2000 to 2017, the of vegetation ecological NDVI water did not transfer decrease measures. significantly, As but a therewetland, the surface waterwere fluctuations. area of QTH The has average also value expanded of NDVI [19,24,45]. in MQW changed The vegetation from 0.22 in data 1980 in to the same period 0.15 in 1996, and subsequently remained stable at around 0.15 (Figure8d). The NDVI of showvegetation that, in with QTH the fluctuated increase around in GWD, 0.13 from the 1980 vegetation to 2010, and coverage increased steadilyconditions after did not decrease significantly2010 (corresponding in the to NH, the decrease WW, MQEW, in GWD) (Figureand MQEE8c). areas (Figure 8a,b,e,f). From the 1980s to 1996 and then from 2000 to 2017, the vegetation NDVI did not decrease significantly, but there were fluctuations. The average value of NDVI in MQW changed from 0.22 in 1980 to 0.15 in 1996, and subsequently remained stable at around 0.15 (Figure 8d). The NDVI of vegetation in QTH fluctuated around 0.13 from 1980 to 2010, and increased steadily after 2010 (corresponding to the decrease in GWD) (Figure 8c).

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FigureFigureFigure 7. 7.7.Variation Variation of of of groundwatergroundwater groundwater depth depth from from from 1980 1980 1980 to to 2016. 2016. to 2016.

(b) (a)(a) (b)

(d) (c) Min–Max Mean Ecological water diversion(c) (d) SampleMin –Max Mean Ecological water diversion 25–75% and Median Sample 1–99% 25–75% and Median

1–99%

Figure 8. Cont.

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(e) (f)

WaterWater2021 202113 , 13, x FOR PEER REVIEW 10 of 17 , , 1642 10 of 17

(e) (f)

Figure 8. Normalized difference vegetation index (NDVI) changes in different zones from 1980 to 2016. (a) NH; (b) WW; (c) QTH; (d) MQW; (e) MQEW; (f) MQEE.

In QTH, the buried depth was usually less than 3 m, and the surface vegetation was Figure 8. Normalized difference vegetation index (NDVI) changes in different zones from 1980 to 2016. (a) NH; (b) WW; Figure 8. Normalized differenceconsistent vegetation with index the (NDVI) downward changes in differenttrend zonesof ecological from 1980 to water2016. (a) NH;transfer (b) WW; and buried depth, (c) QTH; (d) MQW; (e) MQEW; (f) MQEE. (c) QTH; (d) MQW; (e) MQEW;reflecting (f) MQEE. the ecological contribution of groundwater to the vegetation in the area. In otherIn Inareas, QTH, QTH, thethethe buried buried vegetation depth depth was waschanges usually usually less and less than buried than 3 m, 3and m,depths the and surface the were surface vegetation inconsistent vegetation was with the evolutionwasconsistent consistent ofwith GWD, with the thedownward indicating downward trend trendthe of weakecological of ecological influence water water transferof transfer GWD and and andburied buried the depth, close depth, correlation betweenreflectingreflecting the vegetationthe ecological ecological contributionand contribution surface of of groundwatersoil. groundwater For areas to to the thewith vegetation vegetation deeper in in theGWD the area. area. (>10 In In other m), the soil moistureareas,other the areas, vegetationcontent the vegetation is changes mainly changes and affected buried and depths buriedby meteorological were depths inconsistent were inconsistent factors, with the such evolutionwith asthe precipitation, of GWD, indicating the weak influence of GWD and the close correlation between vegeta- temperature,evolution of GWD,and indicatingrelative thehumidity weak influence [34], of especiallyGWD and thesummer close correlation precipitation. The tionbetween and surface vegetation soil. and For surface areas with soil. deeper For areas GWD with (>10 deeper m), the GWD soil (>10 moisture m), the content soil is meteorologicalmainlymoisture affected content by data meteorologicalis mainly of the affected Minqin factors, by meteorologicalarea such asfrom precipitation, 2004 factors, to temperature,2017such as(Figure precipitation, and 9) relative showed certain fluctuationshumiditytemperature, [34], especiallyinand annualrelative summer precipitationhumidity precipitation. [34], duringespecially The meteorological thissummer period. dataprecipitation. ofThere the Minqin wereThe area very high precipitationfrommeteorological 2004 to 2017 values data (Figure of in the 92007,) Minqin showed 2011, area certain and from fluctuations2014 2004 and to 2017 the in annual(Figurefluctuations precipitation9) showed in annual certain during precipitation andthisfluctuations period.annual There relativein annual were humidity veryprecipitation high were precipitation during consistent this values period. with in 2007, theThere 2011,change were and ofvery 2014 vegetation andhigh the NDVI in precipitation values in 2007, 2011, and 2014 and the fluctuations in annual precipitation WW,fluctuations NH, MQEW, in annual and precipitation MQEE (Figure and annual 10). relative This indicates humidity werethat the consistent growth with of thevegetation in changeand annual of vegetation relative humidity NDVI in WW,were consistent NH, MQEW, with and the MQEE change (Figure of vegetation 10). This NDVI indicates in thesethatWW, the areasNH, growth MQEW, mainly of vegetationand depends MQEE (Figure in on these precipitation 10). areas This mainly indicates and depends that soil the growthwater on precipitation of[21,46], vegetation and and in soil is not closely relatedwaterthese [ 21areas to,46 groundwater.], mainly and is notdepends closely on related precipitation to groundwater. and soil water [21,46], and is not closely related to groundwater.

Figure 9. Meteorological changes in Minqin from 2004 to 2016.

FigureFigure 9.9.Meteorological Meteorological changes changes in Minqin in Minqin from 2004from to 2004 2016. to 2016.

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0.5 180 PRE/mm RH% NH WW QTH MQW MQEW MQEE 150 0.4

120 0.3

90 NDVI 0.2 60 Precipitation (mm) Relative humidity (%) 0.1 30

0 0

Year

Figure 10. NDVI and meteorological factors changes in different zones from 2004 to 2016. Figure 10. NDVI and meteorological factors changes in different zones from 2004 to 2016. Among these areas, the VFC in the WW area had the closest relationship with precipi- tationAmong with these a correlation areas, the coefficient VFC in the R ofWW 0.63 area (Table had2 ).the This closest means relationship that WW with has a close precipitationrelationship with with a correlation the surface coefficient soil moisture R of content.0.63 (Table Figure 2). This 11 ameans shows that all WW the quadrats has a and closesoil relationship survey points with in the WWsurface area. soil According moisture content. to the differences Figure 11a in shows GWD, all these the survey quadratspoints wereand soil divided survey into points three in groups.the WW When area. According the local GWD to the was differences less than in 2 GWD, m (Group 1), thesethe survey soil moisture points contentwere divided and VFC into were three higher, groups. and When VFC the reached local GWD 40–50%. was When less than GWD was 2 m (Group 1), the soil moisture content and VFC were higher, and VFC reached 40– about 2–4 m (Group 2), the relationship between VFC and soil moisture content was not 50%. When GWD was about 2–4 m (Group 2), the relationship between VFC and soil significant and VFC ranged from 20–50%. When the GWD was above 4 m (Group 3), VFC moisture content was not significant and VFC ranged from 20–50%. When the GWD was was less than 35% and there was a significant linear relationship between soil moisture above 4 m (Group 3), VFC was less than 35% and there was a significant linear content and VFC (R = 0.99). The higher the soil moisture content, the higher the VFC. relationship between soil moisture content and VFC (R = 0.99). The higher the soil moistureFrom the content, perspective the higher of community the VFC. From structure, the perspective Group 1 wasof community composed structure, of trees/shrubs Groupand herbs,1 was wherecomposed shrubs of weretrees/shrubs the dominant and herbs, community, where shrubs and marsh were meadowsthe dominant were more community,developed and [35 marsh]. Group meadows 2 was were dominated more developed by a combination [35]. Group of 2 trees,was dominated shrubs, and by herbs, a wherecombination trees were of trees, usually shrubs, the dominant and herbs, species. where Group trees 3were included usually shrubs the anddominant herbs, where species.herbs Group were the 3 included dominant shrubs species and in someherbs, of where the quadrats. herbs were Desert the dominant vegetation species communities in in somethe Shiyangof the quadrats. River Basin Desert are vegetation usually composed communities of shrubs in the and Shiyang herbs (FigureRiver Basin5). The are roots of usuallyshrubs composed and herbs of in shrubs arid areas and herbs are mainly (Figure distributed 5). The roots at a of depth shrubs of and 1.2 m herbs below in arid the soil sur- areasface are(Figure mainly 12 distributeda) [47–50]. Theat a depth water of in this1.2 m layer below is thethe mainsoil surface water source(Figure that 12a) supports [47– the 50].growth The water of vegetation, in this layer which is indicates the main the water ecological source significance that supports of soil the moisture growth inof the sur- vegetation,face unsaturated which indicates zone. For the example, ecological the significance root system of Haloxylonsoil moisture ammodendron in the surfaceis generally unsaturatedwithin 1 m zone. of the For surface, example, and the the root horizontal system rootof Haloxylon system isammodendron concentrated is 50–100generally cm below withinthe soil 1 m surface of the [surface,47,48,50 ].and The the root horizontal system ofrootTamarix systemspp. is concentrated is distributed 50–100 40 cm cm below the belowsoil surface.the soil surface The roots [47,48,50]. of Nitraria Thespp. root are system distributed of Tamarix approximately spp. is distributed 120 cm below40 cm the soil belowsurface the [soil47]. surface. The main The roots roots of of the Nitraria thin-branched spp. are distributedKalidium foliatum approximatelypenetrate 120 to 80–145cm cm belowbelow the the soil soil surface surface, [47]. where The the main main roots root systemof the isthin distributed‐branched inKalidium the 10–30 foliatum cm soil layer, penetrateand the to root 80–145 width cm ranges below fromthe soil 100–115 surface, cm where [51]. the For main vegetation root system in the is WW distributed area, 4 m is an in ecologicalthe 10–30 water cm soil level layer, boundary and the for theroot structure width ranges of the vegetationfrom 100–115 community cm [51]. ( FigureFor 12a). vegetationWhen the in GWD the WW is greaterarea, 4 m than is an 4 m,ecological the vegetation water level of the boundary community for the dominated structure of by small theshrubs vegetation and herbs community mainly (Figure depends 12a). on summerWhen the precipitation GWD is greater and surface than soil4 m, moisture the for vegetationgrowth. of The the vegetation community roots dominated extend by laterally small shrubs to access and water, herbs especially mainly depends in annual on plants summer precipitation and surface soil moisture for growth. The vegetation roots extend (Figure 12b). The degree of lushness is greatly affected by the total amount of precipitation and its annual and seasonal distribution. When the precipitation is appropriate, it can form Nitraria spp. + Calligonum shrubs and “Halogeton arachnoideus + Reaumuria soonorica shrubs with a coverage of more than 40%. This type of vegetation has become an important Water 2021, 13, 1642 12 of 17

part of the vegetation in this area [46,52]. The trees and some shrubs with a root depth of Water 2021, 13, x FOR PEER REVIEW 12 of 17 Water 2021, 13, x FOR PEER REVIEW4–10 m mainly rely on deep groundwater to survive [49,53]. When the GWD is12 less of 17 than

4 m, the roots of the vegetation can grow vertically and penetrate the groundwater surface or reach the groundwater capillary water uplift zone (Figure 12c). Hence, the vegetation laterally to access water, especially in annual plants (Figure 12b). The degree of lushness no longer completely depends on the surface soil moisture to survive. When the GWD is islaterally greatly toaffected access water,by the especially total amount in annual of precipitation plants (Figure and 12b). its Theannual degree and of seasonal lushness further reduced to 2 m, vegetation growth can fully access soil moisture and groundwater distribution.is greatly affected When the by precipitation the total amount is appropriate, of precipitation it can form and Nitraria its annual spp. +and Calligonum seasonal to survivedistribution. [54] When (e.g., the some precipitation species of isTamarix appropriate,[49,55 it]), can and form thus Nitraria VFC spp. can + reach Calligonum a higher Water 2021, 13, x FOR PEER REVIEW shrubs and “Halogeton arachnoideus + Reaumuria soonorica shrubs with a coverage of more 13 of 17 valuethanshrubs [ 40%.56]. and FigureThis “Halogeton type 11b of displays vegetation arachnoideus the has relationship + becomeReaumuria an between soonoricaimportant GWDshrubs part of and with the VFC avegetation coverage when GWD ofin morethis <4 m. Theseareathan two[46,52]. 40%. were This The highly typetrees of and negatively vegetation some shrubs correlated has become with a (R root an = −important depth0.78). of The 4–10 part smaller mof mainlythe the vegetation buriedrely on depth,indeep this the highergroundwaterarea the[46,52]. VFC. Theto This survive trees was and [49,53]. consistent some shrubsWhen with withthe an GWD a analysis root isdepth less of theofthan 4–10 NDVI–GWD 4 mm, mainly the roots rely relationship onof deepthe in thegroundwater Minqin Oasis to conducted survive [49,53]. by Cao When [25]. the GWD is less than 4 m, the roots of the Table 2.vegetation Correlation can analysisgrow vertically between and NDVI penetrate and meteorological the groundwater factors surface in different or reach zones. the groundwatervegetation can capillary grow watervertically uplift and zone penetrate (Figure the12c). groundwater Hence, the vegetationsurface or no reach longer the Tablegroundwater 2. Correlation capillary analysis water between uplift NDVI zone and (Figure meteorological 12c). Hence, factors the in vegetation different zones. no longer completelyR dependsWW on the surfaceNH soil moistureMQW to survive.MQEW When the GWDMQEE is further QTH reducedcompletely to 2 dependsm, vegetation on the growth surface cansoil fullymoisture access to soilsurvive. moisture When and the groundwater GWD is further to n R WW12 12 NH MQW3 MQEW11 MQEE5 QTH 4 survivereduced [54] to 2(e.g., m, vegetationsome species growth of Tamarix can fully [49,55]), access and soil thus moisture VFC andcan reachgroundwater a higher to PRE/mmvaluesurviven [56]. [54] Figure (e.g.,0.63 12 11b some displays species0.30 12the of relationship Tamarix−0.43 3[49,55]), between and GWD 11 0.17thus and VFC VFC can 5 when reach−0.05 GWD a higher <4 4 −0.13 TEM/°Cm.PRE/mmvalue These [56]. two Figure were−0.630.29 11b highly displays −negatively0.150.30 the relationship correlated−−0.050.43 between (R = −0.17 0.78).GWD−0.06 The and smallerVFC−0.05 when−0.12 the GWD buried−0.13 <4 0.49 ◦ − − − − − depth,TEM/m. These theC highertwo were0.29 the highlyVFC. Thisnegatively0.15 was consistent correlated0.05 with (R =an − 0.060.78).analysis The of smaller the0.12 NDVI–GWD the buried0.49 RH/%RH/% 0.23 0.030.03 −−0.250.25 0.040.04 0.09 0.09 −0.37−0.37 relationshipdepth, the inhigher the Minqin the VFC. Oasis This conducted was consistent by Cao [25].with an analysis of the NDVI–GWD relationship in the Minqin Oasis conducted by Cao [25]. R: 0 11

4.2. Ecological Degradation of Transition Zone Dominated by Groundwater Changes in the GWD can cause salinization and desertification [13]. A large GWD can result in reduced surface vegetation (as in MQW), which can in turn cause desertification (Figure 13), whereas a small GWD affected by surface evaporation can result in soil salinization. The growth of a small amount of salt‐tolerant vegetation can limit the VFC [31], whereas lower vegetation cover can increase the risk of desertification. This process can make soil salinization a major environmental problem leading to land desertification [57,58].

FigureFigure 11. Relationship11. Relationship between between vegetation vegetation fractional fractional coverage (VFC) (VFC) and and soil soil moisture moisture and and groundwater groundwater depth depth (GWD): (GWD): Figure 11. Relationship between vegetation fractional coverage (VFC) and soil moisture and groundwater depth (GWD): (a) VFC–soil(a) VFC–soil moisture; moisture; (b) ( VFC–GWD.b) VFC–GWD. (a) VFC–soil moisture; (b) VFC–GWD.

Figure 13. Ecological evolution process of desertification and salinization in shallow groundwater FigureFigure 12. 12.(a )(a Sketch) Sketch of of rootroot distribution in in the the profiles proﬁles of different of different groundwater groundwater depth depthin Shiyang in Shiyang River Basin; River (b Basin;) Figure 12. (a) Sketcharea of of root Shiyang distribution River in Basin. the profiles The circle of different size of groundwater each area representsdepth in Shiyang the mean River valueBasin; of(b )the number (b) ReaumuriaReaumuria soongorica; ;((cc) )HaloxylonHaloxylon ammodendron ammodendron. . Reaumuria soongoricaof; vegetation(c) Haloxylon speciesammodendron in the. area, with the minimum of 2.6 and the maximum of 5.2; green represents the level of VFC, blue represents the level of salinization, and orange represents the level of desertification.

In addition to the WW area, the VFCs of other areas were not high, but their soil salinity values were above 3 g/kg (Figure 2b). This suggested that the soil salinity may have constrained the VFC. For the NH area, with the highest salinity, the VFC was not high with scattered distribution of Kalidium foliatum and Sophora alopecuroides L. In the NH area, the correlation between NH and precipitation (R = 0.3) was also smaller than that of WW (R = 0.63), reflecting the obvious limitation of soil salinity on VFC. The expansion of the saline area in the Minqin Oasis and parts of the Qingtu Lake and the increase in salinity were mainly driven by ecological water transfer [14,46]. Figure 13 shows the relationship between desertification and salinization in the shallow buried area of the Shiyang River. When GWD was < 4 m and soil salinity was < 3 g/kg, the VFC was the highest and the number of species was the largest. There were 8–9 species of plants in the community of trees near the river bank (Figure 14). When the soil salinity

Water 2021, 13, x FOR PEER REVIEW 13 of 17

Table 2. Correlation analysis between NDVI and meteorological factors in different zones.

R WW NH MQW MQEW MQEE QTH n 12 12 3 11 5 4 PRE/mm 0.63 0.30 −0.43 0.17 −0.05 −0.13 TEM/°C −0.29 −0.15 −0.05 −0.06 −0.12 0.49 RH/% 0.23 0.03 −0.25 0.04 0.09 −0.37

R: 0 1

Water 2021, 13, 1642 13 of 17 4.2. Ecological Degradation of Transition Zone Dominated by Groundwater Changes in the GWD can cause salinization and desertification [13]. A large GWD 4.2. Ecological Degradation of Transition Zone Dominated by Groundwater can result in reduced surface vegetation (as in MQW), which can in turn cause Changes in the GWD can cause salinization and desertification [13]. A large GWD candesertification result in reduced (Figure surface 13), vegetation whereas (as a in small MQW), GWD which affected can in turn by cause surface deser- evaporation can tificationresult in (Figure soil salinization. 13), whereas aThe small growth GWD affectedof a small by surfaceamount evaporation of salt‐tolerant can revegetation can sultlimit in the soil VFC salinization. [31], whereas The growth lower of a vegetation small amount cover of salt-tolerant can increase vegetation the risk can of desertification. limitThis the process VFC [31 can], whereas make lowersoil salinization vegetation cover a major can increase environmental the risk of desertifica-problem leading to land tion. This process can make soil salinization a major environmental problem leading to landdesertification desertification [57,58]. [57,58].

FigureFigure 13. 13.Ecological Ecological evolution evolution process process of desertification of desertification and salinization and in salinization shallow groundwater in shallow groundwater areaarea of of Shiyang Shiyang River River Basin. Basin. The circle The size circle of each size area of represents each area the represents mean value the of the mean number value of of the number vegetationof vegetation species species in the area, in withthe area, the minimum with the of 2.6minimum and the maximum of 2.6 and of 5.2; the green maximum represents of the 5.2; green levelrepresents of VFC, blue the represents level of theVFC, level blue of salinization, represents and the orange level represents of salinization, the level of and desertification. orange represents the level of desertification. In addition to the WW area, the VFCs of other areas were not high, but their soil salinity values were above 3 g/kg (Figure2b). This suggested that the soil salinity may have constrainedIn addition the VFC.to the For WW the NH area, area, the with VFCs the highest of other salinity, areas the VFC were was not not highhigh, but their soil withsalinity scattered values distribution were above of Kalidium 3 g/kg foliatum (Figureand Sophora 2b). This alopecuroides suggestedL. In the that NH the area, soil salinity may thehave correlation constrained between the NH VFC. and precipitation For the NH (R =area, 0.3) was with also the smaller highest than salinity, that of WW the VFC was not (R = 0.63), reflecting the obvious limitation of soil salinity on VFC. The expansion of the salinehigh areawith in thescattered Minqin Oasisdistribution and parts of of Kalidium the Qingtu foliatum Lake and and the increase Sophora in salinityalopecuroides L. In the wereNH mainly area, driventhe correlation by ecological between water transfer NH and [14,46 precipitation]. Figure 13 shows (R the= 0.3) relationship was also smaller than betweenthat of desertification WW (R = 0.63), and salinization reflecting in the shallow obvious buried limitation area of the of Shiyang soil salinity River. on VFC. The Whenexpansion GWD was of the <4 m saline and soil area salinity in the was Minqin <3 g/kg, Oasis the VFC and was parts the highest of the and Qingtu the Lake and the number of species was the largest. There were 8–9 species of plants in the community of increase in salinity were mainly driven by ecological water transfer [14,46]. Figure 13 trees near the river bank (Figure 14). When the soil salinity was greater than 5 g/kg, VFC beganshows to decrease.the relationship When the soilbetween salinization desertification reached 20 g/kg, and it begansalinization to transform in the into shallow buried barearea land of the (Figure Shiyang6) and desertificationRiver. When occurred. GWD was When < 4 GWD m and was soil >6 m, salinity the vegetation was < 3 g/kg, the VFC VFCwas began the tohighest drop. When and GWD the number was >10 m, of VFC species was less was than the 10% andlargest. began There to transform were 8–9 species of intoplants bare in land the (Figure community6). of trees near the river bank (Figure 14). When the soil salinity

Water 2021, 13, x FOR PEER REVIEW 14 of 17

was greater than 5 g/kg, VFC began to decrease. When the soil salinization reached 20 g/kg, it began to transform into bare land (Figure 6) and desertification occurred. When GWD was >6 m, the vegetation VFC began to drop. When GWD was >10 m, VFC was less Water 2021, 13, 1642 14 of 17 than 10% and began to transform into bare land (Figure 6).

30 Number of quadrats 30 VFCmean 25 25

20 20

15 15

10 10 Number of quadratsNumber of 5 5

Vegetation fractional coverage (%) 0 0 123456789 Plant species

Figure 14. Statistics of species number, VFC and number of quadrats.

Figure4.3. Suggestions 14. Statistics and Measures of species for Ecological number, Protection VFC and in Different number Levels of quadrats. and Districts Over-exploitation of groundwater in the Shiyang River Basin has been detrimen- 4.3.tal. Suggestions Since the 1960s, and the Measures groundwater for level Ecological has declined Protection significantly in Different [25,38,59], andLevels the and Districts ecological environment has deteriorated considerably. Since the implementation of eco- logicalOver protection‐exploitation measures of (such groundwater as ecological water in transferthe Shiyang and a limitation River onBasin ground- has been detrimental. Sincewater the extraction), 1960s, the the initial groundwater achievements oflevel ecological has restoration declined in somesignificantly areas have [25,38,59], and the appeared [25,37]. The shallow groundwater area (<10 m) in the basin is the key area for ecologicalprotection andenvironment the easiest area forhas governance. deteriorated Therefore, considerably. due to differences Since in GWD, the soil implementation of ecologicalquality, and protection salinity, it is necessary measures to implement (such as targeted ecological ecological water protection transfer measures and a limitation on groundwaterat different levels. extraction), For the existing the shallow initial groundwater achievements areas, theof ecological WW area has arestoration small in some areas GWD and low soil salinity. Its vegetation should be mainly protected by forest farms and havegrasslands. appeared Groundwater [25,37]. exploitation The shallow in the groundwater riparian zone should area be (<10 restricted. m) in At the the basin is the key area forsame protection time, attention and shouldthe easiest be paid area to the for salinization governance. of areas withTherefore, a GWD of due less thanto differences in GWD, soil2 m.quality, The current and ecological salinity, environment it is necessary of QTH is significantlyto implement affected targeted by ecological ecological protection water transfer [24]. It is recommended to maintain the existing water transfer measures measuresand gradually at different expand the impactlevels. on theFor ecological the existing vegetation shallow around the groundwater wetland. The soil areas, the WW area hassalinity a small in the GWD MQW and and MQEW low areas soil are salinity. not very high, Its andvegetation the main factor should limiting be VFC mainly protected by is a large GWD. By restricting groundwater exploitation in the irrigation area, planting and forestmaintaining farms ecological and grasslands. forest construction, Groundwater and expanding exploitation the ecological forests in the with riparian small zone should be restricted.trees, we can At expect the same ecological time, improvement, attention which should may gradually be paid serve to the as an salinization effective of areas with a GWDbarrier of against less thethan Badain 2 Jaranm. The Desert. current The NH andecological MQEE areas environment had GWDs of less of than QTH is significantly 5 m, but the high soil salinity limited the VFCs of these areas. Salinization control measures affectedshould beby adopted ecological in these areas.water transfer [24]. It is recommended to maintain the existing water transfer measures and gradually expand the impact on the ecological vegetation 5. Conclusions around the wetland. The soil salinity in the MQW and MQEW areas are not very high, In this work, the soil moisture, soil salinity, GWD, and VFC were integrated to study andthe ecologicalthe main relationship factor of groundwater–soil–vegetationlimiting VFC is a large in Shiyang GWD. River Basin. By Therestricting role groundwater exploitationof GWD and soilin salinity the inirrigation desertification area, and salinization planting were and innovatively maintaining analyzed, ecological forest construction,and the key GWD and and expanding soil salinity standard the ecological for ecological forests vegetation with maintenance small andtrees, we can expect restoration were proposed. ecological improvement, which may gradually serve as an effective barrier against the Badain Jaran Desert. The NH and MQEE areas had GWDs of less than 5 m, but the high soil salinity limited the VFCs of these areas. Salinization control measures should be adopted in these areas.

5. Conclusions In this work, the soil moisture, soil salinity, GWD, and VFC were integrated to study the ecological relationship of groundwater–soil–vegetation in Shiyang River Basin. The role of GWD and soil salinity in desertification and salinization were innovatively analyzed, and the key GWD and soil salinity standard for ecological vegetation maintenance and restoration were proposed.

Water 2021, 13, 1642 15 of 17

In the Shiyang River Basin, soil salinity is the factor most closely related to VFC and is inversely correlated with VFC (R = −0.4). High salinity can significantly limit VFC. When soil salinity is greater than 3 g/kg, VFC is generally less than 20%. The surface vegetation community is dominated by shrubs and herbs. In the areas with soil salinity less than 3 g/kg, when GWD >10 m, the vegetation fractional coverage is usually less than 15%. For areas with GWD <10 m, when GWD >4 m, VFC and soil moisture content are positively correlated (R = 0.99), or significantly affected by annual rainfall, and vegetation growth mainly depends on surface soil moisture. When GWD <4 m, VFC and GWD are negatively correlated (R = −0.78), and vegetation growth mostly relies on groundwater and soil moisture. The water–soil–ecological conditions of the shallow-buried areas of the Shiyang River Basin have obvious differences, and ecological management measures should be formu- lated in specific areas. Further work is required to better understand the water use strategies of typical vegetation under different GWD and salinization levels. In addition, it is beneficial to ecological protection to study the causes of soil salinity difference in the transition area.

Author Contributions: Conceptualization, L.C. and Z.N.; methodology, L.C. and M.L.; formal analysis, L.C.; investigation, L.C., M.L., L.W., J.W. and Q.W.; data curation, M.L., J.W. and Q.W.; writing—original draft preparation, L.C.; writing—review and editing, L.C. and J.W.; supervision, Z.N.; funding acquisition, Z.N. and M.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key R&D Program of China, grant number 2017YFC0406103; by the National Natural Science Foundation of China, grant nos. 41807214 and 41902262; by the China Geological Survey Project, grant number DD20190349. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Acknowledgments: We thank Huixiong Lu and his team for their acquisition and collation of remote sensing data. Great thanks to the editors and anonymous reviewers for providing valuable comments that significantly improved this paper. Conflicts of Interest: The authors declare no conflict of interest.

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