Groundwater Dynamic Characteristics with the Ecological Threshold in the Northwest China Oasis

Abstract

Suitable groundwater level is an important foundation for the stability of the ecological environment, and the healthy development of the social economy, in the arid area of Northwest China. The Manas River Basin is a typical oasis in an arid area, where the problems of salinization and desertification are prominent. By analyzing the variation characteristics of groundwater in the study area from 2013 to 2019 combined with remote sensing technology—according to the theory of capillary water rise and phreatic evaporation—a mathematical calculation model of the ecological threshold is established to determine the ecological groundwater level. The results show that (1) the groundwater level in the study area fluctuates by 0.2–18 m throughout the year, and the variation of groundwater drawdown is 5–35 m from 2013 to 2019; (2) the upper threshold of the ecological groundwater level is 0.82–4.05 m and the lower threshold is 3.35–10.23 m; (3) the ecological water shortage area in the study area is 9755.36 km², and the groundwater ecological deficit is 105.741 × 10⁸ m³. This study can provide a theoretical basis for the determination of the ecological groundwater level, the optimal allocation of water resources, and ecological environment management in the arid area of Northwest China.

1. Introduction

The arid area of Northwest China has scarce precipitation, strong evaporation, and extremely scarce water resources. Groundwater is used as the main water supply source due to its extensiveness and stability, playing an important role in socio-economic development and ecological environment stability. Due to the impact of climate change and human activities, people’s demands for water resources are increasing, groundwater resources have been overexploited, and groundwater buried depths have been increasing. Changes in groundwater buried depth directly affect the growth and development of surface vegetation and the safety of human water supply, causing serious damage to fragile ecosystems and socio-economic development in arid and semi-arid areas. In irrigated oases in arid areas, artificial ecology and natural ecology coexist, and surface water groundwater frequently transforms. The ecological level of oasis groundwater has become the key factor in determining the health status of an oasis ecosystem. The Manas River Basin is located in the hinterland of the arid area in Northwest China, which is a core economic area at the northern foot of the Tianshan Mountains and the largest oasis farming area in Xinjiang. Affected by human activities, the balance of groundwater in the Manas River Basin changed to a negative balance in 1995. So far, the change in groundwater reserves has reached 2.81 × 10⁸ m³/a [1]. Agricultural water accounts for more than 90% of the total water resources in the basin, squeezing out ecological water use, leading to the withering of oasis shelter forests and downstream desert plants [2].

Changes in the groundwater level will lead to different ecological effects. Too great a burial depth can easily lead to withering vegetation and ecological degradation, while a burial depth that is too shallow can easily cause the secondary salinization of the land due to strong evaporation [3]. The ecological groundwater level is an important condition for coordinating the economic and social development of the region and the stability of the ecosystem, and it is an important indicator for measuring the health of the ecological environment in arid and semi-arid areas [4,5]. At present, there is no consensus regarding the definition of ecological groundwater level, and different scholars have given different explanations based on different research perspectives. The basic theories formed refer to a suitable underground ecological groundwater level [6,7,8], ecological groundwater warning level [9], a critical salinization water level [10], etc. For example, in the Shiyang River Basin, Hu et al. [11] established the relationship between surface vegetation and groundwater buried depth and provided a theoretical basis for determining the regional ecological water delivery volume by calculating the appropriate groundwater buried depth. The ecological groundwater level can also be used as a quantitative index to assess the risk of regional groundwater development and utilization. Li et al. [12] constructed the relationship between groundwater buried depth and groundwater exploitation risk level and evaluated the development and utilization potential of regional groundwater resources based on the groundwater level in order to reasonably plan and utilize regional water resources.

Salinization and desertification coexist in the Manas River Basin, and the oasis scale keeps expanding; the whole oasis is in a sub-stable state, and the government is actively implementing a policy of “land retreat and water reduction”, so it is necessary to carry out ecological groundwater level research in order to provide support for the sustainable utilization of groundwater resources in the basin. Combined with the environmental problems and diverse ecosystems in the study area, the definition of the ecological groundwater level adopted in this study is the groundwater level, which meets the needs of the ecological environment and does not cause its deterioration under the changing environment. It is an interval of a series of groundwater level values, a function varying with time and space, and is jointly affected by hydrogeological conditions, vegetation conditions, topography, climate conditions, and other relevant factors [13,14,15]. A reasonable ecological groundwater level determines the appropriate upper and lower thresholds according to its influencing factors, to guide the coordinated development of ecosystems and socioeconomic systems in a complex ecological environment [16,17]. Therefore, this study uses remote sensing and GIS to construct a database of land use types, soil texture, water depth, and crop distribution in the watershed. The upper limit of the ecological groundwater level is determined by the rising height of the capillary and the thickness of the vegetation root system. The limit depth of phreatic evaporation is calculated by the dynamic data method as the lower limit of the ecological groundwater level [18,19]. The threshold range of the ecological groundwater level under multi-dimensional conditions such as natural vegetation, farmland crops, and the urban landscape is established from the vertical category to provide quantitative indicators for the prevention and control of salinization and desertification in the basin. Moreover, it is used to analyze and study the characteristics of groundwater buried depth changes in various regions in the area to determine the ecological water shortage of groundwater. This study provides the theoretical basis and technical support for the restoration and management of groundwater resources in arid areas, rational development and utilization, sustainable development of agriculture, and the health and stability of the ecological environment.

2. Materials and Methods

2.1. Study Area

The Manas River Basin is located in the middle section of the northern foot of the Tianshan Mountains, from the northern slope of the Tianshan Mountains in the south to the southern margin of the Junggar Basin in the north (43°21′ N–45°20′ N, 84°43′ E–86°35′ E); the drainage area is approximately 34,050 km² (Figure 1). This area belongs to the mid-temperate continental arid climate zone of the northern hemisphere, with a dry climate, sparse precipitation, and strong evaporation. The annual evaporation is greater than 2000 mm, but the precipitation is less than 200 mm, and 68% of the annual precipitation is concentrated from June to August [2]. The terrain of the entire watershed slopes from south to north, forming a mountain basin system with typical mountain, oasis, and desert characteristics. The basin water systems all originate in mountainous areas, and from west to east there are five inland rivers: Jingou River, Bayingou River, Ningjia River, Manas River, and Tasi River. Alpine snowmelt water and mountain rainfall are the main sources of water resources in the basin. In the plain area, water resources are used in large quantities for irrigation, and the proportion of agricultural water consumption is as high as 94%. Groundwater and surface water are frequently converted in this area and, eventually, both are dissipated through evaporation and in desert areas, resulting in a rapid decline in the groundwater level of the watershed and the gradual degradation of natural vegetation.

From south to north, the Manas River Basin encompasses mountains, hills, piedmont inclined plains, alluvial plains, and deserts. Therefore, the hydrogeological structure of this area is zoning, namely: mountain vertical exchange zone → hilly runoff belt → vertical alternating belt of alluvial fans → horizontal runoff belt of fine soil plain → runoff consumption zone in a desert area [17]. The study area is mainly located in the oasis plain in the mountain basin system of the Manas River Basin (Figure 2), which is divided into piedmont inclined plain and fine soil plain. Both of them are areas of intense movement of the water cycle in the basin and are also important residential areas and industrial gathering areas on the northern slope of the Tianshan Mountains. The hydrogeological conditions of the study area are relatively complicated, and the lithology of the aquifer is dominated by thick loose sediments of the third and quaternary series. The structure of the regional aquifer varies greatly from south to north. In the piedmont inclined plain, gravel, sand, and coarse sand are the main contents; in the fine soil plain, sandy soil and clay soil are the main contents; in the northern desert zone, cohesive soil and fine sand are the main contents. The particles of the aquifer change from coarse to fine from south to north, the rock structure changes from single to multi-layer, and the aquifer crosses each other, lacking stable regional aquicludes [1].

2.2. Data Sources

We collected remotely sensed land use data and soil texture spatial distribution data of the Manas River Basin in 2020 from the Resource and Environmental Science Data Center of the Chinese Academy of Sciences (RESDC; http://www.resdc.cn, accessed on 17 March 2022). The land use data is obtained based on the interpretation of the US Landsat 8 remote sensing image with a resolution of 30 m (Figure 3); the soil texture data in the study area is based on the 1:1 million soil type map and the soil profile data obtained from the second soil survey in China and combined reclassification according to the International Soil Classification Standard (Figure 4).

The observation data of the groundwater level in the study area were mainly provided by the water conservancy administration department of Shihezi city, with a total of 84 observation wells, including the position, elevation, groundwater level, and groundwater buried depth of observation wells. A Zkgd-3000 type water level and water temperature observation instrument were used, and the data period was from 1 January 2013, to 30 September 2019. The hydrogeological map of the study area was mainly derived from the hydrogeological map of the southern margin of the Junggar Basin in the Xinjiang Uygur Autonomous Region, and the crop species were mainly obtained from the Xinjiang Statistical Yearbook in 2019.

2.3. Methods

2.3.1. Measurement of Groundwater Level and Groundwater Buried Depth

The groundwater level is mainly determined by the groundwater level monitoring system. The water level pipe is placed in the groundwater observation well based on the standard monitoring method, and the groundwater level is measured by the water level well. Groundwater buried depth refers to the distance from the phreatic surface to the ground surface. The groundwater burial depth is as follows [20]:

$$h_g=H_s-h_w,$$

(1)

where $h_g$ is the groundwater buried depth, $H_s$ is surface elevation, and $h_w$ is groundwater level.

2.3.2. Determination of the Lower Limit of the Ecological Groundwater Level

The lower limit threshold of the ecological groundwater level can be determined by calculating the limit evaporation depth (the depth of the phreatic level at which phreatic evaporation is zero). When the buried depth of groundwater is less than this depth, the soil moisture is mainly recharged by surface water infiltration. If the replenishment is not timely, the surface of the soil will be dry, and shallow-rooted herbaceous plants will decay or die due to insufficient water, so this should be used as the lower limit of the ecological groundwater level [9].

The limit evaporation depth is usually calculated by the dynamic data correlation method, which determines the relationship between the original depth of groundwater in the study area, the difference in groundwater level, and the evaporation intensity when the water is close to the surface and uses its correlation to establish a regression equation to determine the evaporation depth limit. Firstly, to determine the calculation period, each well should select no less than four groups of calculation periods, and each period should be greater than or equal to three days. Secondly, calculate the saturated evaporation of topsoil water accumulated during the selected period $\epsilon$. $\epsilon$ can be calculated according to the saturated evaporation intensity of surface soil water E. The saturated evaporation intensity of surface soil water generally refers to the phreatic evaporation intensity when the buried depth of groundwater is zero, which can generally be replaced by the water surface evaporation intensity measured by an E601 evaporation dish [21].

Then, according to the observation data of the groundwater level, the difference between the maximum and minimum groundwater buried depth $\left(\Delta h\right)$ in the selected period and the average groundwater buried depth $\left(\overline{h}\right)$ in the selected period are calculated:

$$\Delta h=h_{max}-h_{min},$$

(2)

$$\overline{h}=\frac{h_{max}+h_{min}}{2} ,$$

(3)

where $h_{max}$ is the maximum depth of groundwater in the selected time, $h_{min}$ is the minimum depth of groundwater in the selected time, $\Delta h$ is the difference in buried depth, and $\overline{h}$ is the average buried depth of the groundwater.

Taking $\overline{h}$ as the abscissa and $\Delta h/\epsilon$ as the ordinate, we establish the discrete relationship between the average groundwater level $\overline{h}$, the ratio of groundwater buried depth change to cumulative evaporation $\Delta h/\epsilon$, and determine its trend line. The intersection of the trend line and the abscissa represents the groundwater buried depth when evaporation is 0, so the groundwater burned depth responding to this point is the limit evaluation depth; then the limit evaluation depth is taken as the lower limit of the ecological groundwater level at this point.

2.3.3. Determination of the Upper Limit of the Ecological Groundwater Level

The upper limit of the ecological groundwater level refers to the highest groundwater level that can prevent soil salinization and maintain normal plant growth. Therefore, the upper limit of the ecological groundwater level can be determined by the sum of capillary water rise and the thickness of the root layer of vegetation [19]:

$$h_k=h_p+\Delta z$$

(4)

where $h_k$ is the upper limit of the ecological groundwater level, $h_p$ is the highest rising height of soil capillary water, and $\Delta z$ is the safe ultra-high (the thickness of the main active layer of crop roots).

When the groundwater buried depth is greater than this depth, it will help the plant root layer to obtain good aeration, and generally will not cause the salinization of the soil root layer [22]. The height of capillary water rise can be measured by collecting soil samples in the field and using direct observation in the laboratory [23,24], and the thickness of the root layer of vegetation can be obtained by referring to relevant literature and field measurements according to the type of crop in the study area.

2.3.4. Estimation of Ecologically Regulated Water Quantity

The groundwater ecologically regulated water quantity refers to the total amount of groundwater resources needed to maintain the stability of the groundwater ecosystem and restore it to the range of the ecological groundwater level [25]. According to the actual groundwater buried depth in the study area, and the determined ecological groundwater level, the spatial analysis, filling, and excavation functions of ArcGIS are used to calculate the ecological regulation amount of the oasis plain area in the Manas River Basin. The formula is:

$$\mathrm{W}=\left({\mathrm{FBH}}_{\mathrm{tif}}-{\mathrm{EBH}}_{\mathrm{tif}}\right)\cdot {\mu }_{\mathrm{tif}}$$

(5)

where $\mathrm{W}$ is the groundwater ecologically regulated water quantity in the study area, ${\mathrm{FBH}}_{\mathrm{tif}}$ is the buried depth distribution of the groundwater level calculated by interpolation in the study area, ${\mathrm{EBH}}_{\mathrm{tif}}$ is the calculated depth interval of the ecological groundwater level in the study area, and ${\mu }_{\mathrm{tif}}$ is the specific yield in each subregion of the study area.

Based on the calculated vectorized data of the upper and lower limit of the ecological groundwater level in the study area (${\mathrm{EBH}}_{\mathrm{tif}}$) and the current annual buried depth distribution of groundwater (${\mathrm{FBH}}_{\mathrm{tif}}$), ArcGIS was used to analyze the spatial overlap and fill and cut analysis, and the deficit volume and surplus volume of groundwater aquifer was determined. We used the Kovda–Averyanov empirical formula and the results of previous studies to determine the distribution of specific yield in the study area, and superimpose the division of specific yield with the administrative division of the study area [26]. The basic form of the Kovda–Averyanov empirical formula is:

$$\mathrm{E}={\mathrm{E}}_0{\left(1-\frac{\mathrm{H}}{{\mathrm{H}}_{max}}\right)}^n,$$

(6)

where E₀ is the water surface evaporation intensity (or the evaporation intensity when the phreatic depth is zero): H is the phreatic depth; Hmax is the ultimate evaporation depth of groundwater, and n is the empirical constant related to soil texture and vegetation, generally 1–3.

For the decline of the groundwater level caused by phreatic evaporation, Equation (6) can be rewritten as:

$$\mu \cdot \Delta h={\mathrm{E}}_0{\left(1-\frac{\mathrm{H}}{{\mathrm{H}}_{max}}\right)}^n$$

(7)

where $\mu$ is the specific yield and $\Delta h$ is the change value of the phreatic groundwater level in the $\Delta t$ period. The formula can be transformed into the following formula:

$$\lg \Delta h=n\lg {\mathrm{E}}_0\left(1-\frac{\mathrm{H}}{{\mathrm{H}}_{max}}\right)-\lg \mu .$$

(8)

The limit evaporation depth of groundwater is calculated based on the dynamic data method, and the evaporation depth of the corresponding area can be calculated by using the data of each observation well $\mu$ and n. Then, according to the weights of different specific yield divisions, we determine the average specific yield of the corresponding administrative area, and vectorize it to obtain the regional specific yield (${\mu }_{\mathrm{tif}}$).

3. Results and Analysis

3.1. Dynamic Changes of the Groundwater Buried Depth

3.1.1. Spatial Variation Characteristics of the Groundwater Buried Depth

According to the observation data of groundwater buried depth in the plain area of the Manas River Basin from 2013 to 2019, vectorization is carried out for them, and spatial interpolation of the groundwater buried depth in the study area is carried out by using an inverse distance weight method with ArcGIS software to generate the groundwater buried depth distribution map (Figure 5). Comparing and analyzing Figure 5, it can be found that there is a “head fall” between the piedmont inclined plain area and the fine soil plain area, and the groundwater buried depth in this area exceeds 100 m. This is due to the influence of folds and faults; four rows of anticlines parallel to the Tianshan Mountains are formed at the southern end of the piedmont inclined plain area, blocking the hydraulic connection between mountain groundwater and plain groundwater, and the existence of faults leads to the formation of “head fall”. The south of the anticline is a low mountain and hilly area, which is mainly composed of impermeable mudstone, sandy mudstone, semi cemented sandstone, and conglomerate, so water permeability is poor. The anticline is an impermeable dam, which blocks the hydraulic connection between the groundwater in mountainous areas and the groundwater in plain areas, so that a large amount of water from the south is blocked. Due to the reduction in the groundwater buried depth, the water returns to the surface in the form of springs from the riverbed and both sides of the river in order to recharge the river. After the river flows through the mountain pass, due to the influence of the terrain, the flow velocity slows down, and the loose sediments that are carried gradually accumulate. In front of the mountain, gentle alluvial proluvial fans of different sizes are formed, which are connected into an alluvial proluvial fan belt with an altitude of about 450–1000 m, forming a huge thick loose sedimentary layer of the quaternary system. The stratum in this area is mainly composed of coarse pebbles and gravel. The valley is deeply incised, resulting in the mutual supply of groundwater and surface water. A large amount of leakage is transformed into groundwater. The groundwater buried depth in this area is large, and the leakage surface water is consumed in the aeration zone. In addition, a large amount of river water for irrigation has led to a decrease in the amount of groundwater replenishment, and a distribution zone with a groundwater buried depth of 100–200 m is formed in the southeast of the study area. Due to the excessive exploitation of groundwater, the boundary of this distribution zone tended to move northward during 2013–2019. Shihezi city and Manas County, which are located on the alluvial fan formed at the river’s estuary, exploited a large amount of groundwater for urban construction and agricultural irrigation, resulting in a sharp increase in groundwater buried depth, which aggravated the expansion of this area. However, in the north of Shihezi city, the altitude changes rapidly, the diving level is close to the altitude, and the spring water overflow zone is formed in some areas. In the north of the spring water overflow zone, as the aquifer particles become finer, the groundwater buried depth begins to increase.

The groundwater buried depth of the fine soil plain area is significantly different in space, and the groundwater buried depth of the central and northeastern regions of the fine soil plain has dropped significantly. In the middle of the fine soil plain, except for the decrease in groundwater buried depth in the area around the reservoir, due to its infiltration, the groundwater recharge items in most areas are reduced due to a large amount of water diversion irrigation, drip irrigation, canal system seepage prevention, and other reasons, and the increase in groundwater buried depth is 10–35 m, which has formed an obvious underground funnel in the middle of the water basin. The area with lower groundwater buried depth in the basin generally coincides with the direction of the river, and the groundwater buried depth gradually increases from the vicinity of the river channel to the areas on both sides. Due to the continuous expansion of artificial oases in the transition zone between plains and deserts, a large amount of groundwater has been exploited. In the northeastern part of the basin, deep into the desert and far from rivers and reservoirs, groundwater replenishment is small, and consumption is large, so the groundwater buried depth varies greatly, increasing by 5–10 m.

3.1.2. Temporal Variation Characteristics of the Groundwater Buried Depth

According to the difference between the average groundwater buried depth in 2019 and 2013, the groundwater buried depth change map in the study area has been obtained (Figure 6). Based on the dynamic groundwater buried depth data from 2013 to 2019 (Figure 6 and Figure 7), the interannual variation characteristics of groundwater buried depth in this region were analyzed. The results show that from 2013 to 2019, the buried depth of groundwater in the plain area of the Manas River Basin showed an increasing trend except for the southeast, with an increase of 5–35 m. Although the groundwater buried depth in the southeastern region has recovered slightly and the groundwater buried depth has decreased by 0–10 m, the overall groundwater buried depth is still greater than 80 m. In Figure 7, a and b are the changes of the groundwater buried depth of the typical observation points in the western line (W-W’) and the eastern line (E-E’) of the watershed in Figure 1, from the piedmont inclined plain to the desert transition zone. The groundwater buried depth from the south to the north has experienced a large–small–large change, which is in line with the spatial distribution characteristics of the groundwater buried depth in the basin. As shown in Figure 7, the groundwater buried depth in the study area changes sharply during the year. The fluctuation range of groundwater buried depth in the year is 0.2–18 m, and the groundwater buried depth changes in the form of “small–large–small” with time. In the fine soil plain and desert transition zone, the change of groundwater buried depth is basically synchronous within the year. The groundwater buried depth decreases gradually from January to April, increases gradually after May, reaches the maximum around September, and gradually recovers in April of the next year, with a fluctuation of 5–18 m. The large fluctuation of groundwater buried depth is mainly caused by large-scale water diversion and irrigation in the region. Agriculture is the main pillar industry in the region, and its water use structure is shown in

Table 1. Utilization of water resources of industries in the Manas River Basin (%).

Time	1990	1995	2000	2005	2010	2015
Agriculture water	96.53	96.04	95.65	94.57	94.27	94.03
Industrial water	1.53	1.83	2.28	3.24	3.20	3.53
Domestic water	1.10	1.11	0.82	1.18	1.36	1.42
Ecological water	0.84	1.01	1.25	1.01	1.18	1.02

. The proportion of agricultural water accounts for more than 94% of the regional water resources, and the artificial exploitation of groundwater accounts for 57.1% of the regional groundwater discharge [27]. The period of high water consumption is from May to August. Water resources are mainly used for farmland irrigation. The groundwater buried depth continues to increase due to large-scale exploitation of groundwater and diversion and irrigation of river water. The groundwater buried depth reaches the maximum in September. After that, due to the end of irrigation, irrigation infiltration, and groundwater recharge by lateral runoff, the groundwater buried depth gradually rises. However, in the piedmont inclined plain area, the fluctuation range of groundwater buried depth is smaller than that in the fine soil plain area and the desert transition zone, ranging from 0.2 to 5 m, and it reaches the maximum groundwater buried depth earlier than the fine soil plain area and the desert transition zone. The specific performance is that the buried depth of groundwater continues to increase from January to June, reaches the maximum from July to August, and then gradually decreases. Around November, the buried depth reaches the minimum and fluctuates by 1–5 m within the year. This is because the groundwater in the piedmont inclined plain area is gradually replenished by a large amount of leakage after the flood season from July to August so that the groundwater buried depth begins to recover. After 2015, the interannual variation of groundwater in the eastern part of the piedmont inclined plain changed significantly, and the groundwater buried depth began to decrease as a whole. This is due to the implementation of “land withdrawal and water reduction” under the guidance of China’s “three red lines” policy, the recalculation of farmland irrigation quota, and the strict control of the development and utilization of water resources. However, due to the serious overexploitation of regional water resources, the continuous improvement of water-saving irrigation technology, the reduction in regional groundwater recharge items, and the continuous increase in cultivated land area caused by drip irrigation, the groundwater buried depth in the rest of the basin continues to increase. Although the policy of “land withdrawal and water reduction” has been implemented in this region in recent years, the recovery of groundwater is relatively slow, lagging, and other control measures are needed.

3.2. Determination of the Ecological Groundwater Level Threshold

3.2.1. Determination of the Lower Limit of the Ecological Groundwater Level

The southern part of the study area is a piedmont inclined plain, which is 500–600 m above sea level and is composed of alluvial–diluvial fan belts, distributed with huge thick loose sediments, and with a large infiltration coefficient. The downward erosion of the river leads to the violent movement of groundwater, causing the groundwater buried depth to exceed 100 m, and there is a “head fall” with the adjacent fine soil plains. Therefore, this study mainly examines the ecological groundwater level of the fine soil plain.

Excluding observation wells with huge water level fluctuations and excessive groundwater buried depth caused by mining, 25 groundwater buried depth observation wells in the fine soil plain area were selected as objects to analyze and calculate the lower limit of the ecological groundwater level in the study area. The groundwater fluctuates slightly in April, May, and September in the study area, so April, May, and September are selected as the calculation period. The saturated evapotranspiration intensity of the surface soil in April, May, and September is 3.5 mm/d, 5.2 mm/d, and 4 mm/d, respectively. Taking wells 1, 11, 19, and 75 as examples, we calculate the maximum groundwater buried depth, minimum groundwater buried depth, groundwater buried depth difference, and average depth of groundwater during the selected time (

Table 2. Statistical results of wells 1, 11, 19, and 75 groundwater monitoring logging.

Number	Time	${\mathit{h}}_{\mathit{m}\mathit{a}\mathit{x}}\left(\mathbf{m}\right)$	${\mathit{h}}_{\mathit{m}\mathit{i}\mathit{n}}\left(\mathbf{m}\right)$	$\Delta \mathit{h}\left(\mathbf{m}\right)$	$E\left(mm/d\right)$	$\epsilon \left(mm\right)$	$\overline{\mathit{h}}\left(m\right)$	$\Delta \mathit{h}/\epsilon$
1	5.13–5.17	4.86	4.81	0.05	5.2	26.0	4.84	1.92
	5.26–5.30	4.84	4.79	0.05	5.2	26.0	4.82	1.92
	9.01–9.05	4.97	4.95	0.02	4.0	20.0	4.96	1.00
	9.10–9.14	5.02	5.01	0.01	4.0	20.0	5.02	0.50
11	4.03–4.07	4.14	4.09	0.05	3.5	17.5	4.12	2.86
	4.22–4.26	4.23	4.19	0.04	3.5	17.5	4.21	2.29
	9.03–9.07	4.47	4.44	0.03	4.0	20.0	4.46	1.50
	9.18–9.22	4.60	4.58	0.02	4.0	20.0	4.59	1.00
19	4.22–4.26	6.89	6.81	0.08	3.5	17.5	6.85	4.57
	5.12–5.16	7.38	7.32	0.06	5.2	26.0	7.35	2.31
	9.01–9.05	8.13	8.11	0.02	4.0	20.0	8.12	1.00
	9.14–9.18	8.25	8.24	0.01	4.0	20.0	8.24	0.50
75	4.28–4.30	2.16	2.06	0.10	3.5	10.5	2.10	9.52
	5.09–5.11	2.52	2.39	0.13	5.2	15.6	2.44	8.33
	9.03–9.05	2.99	2.93	0.06	4.0	12.0	2.97	5.00
	9.27–9.29	3.31	3.27	0.04	4.0	12.0	3.29	3.33

).

With $\overline{h}$ as the abscissa and $\Delta h/\epsilon$ as the ordinate, we establish the discrete relationship between the average groundwater buried depth $\overline{h}$, the ratio of groundwater buried depth change to cumulative evaporation $\Delta h/\epsilon$, and determine the trend line of wells 1, 11, 19, and 75 (Figure 8).

Using this method to calculate the limit evaporation depth corresponding to the 25 observation wells in the study area, the interval distribution is 3.35–10.23 m; that is, the lower limit of the ecological groundwater level is 3.35–10.23 m. The determined limit depths of the observation wells are vectorized, and the interpolation analysis in the ArcGIS spatial analysis is used to generate the lower limit burial depth distribution map of the ecological groundwater level in the study area (Figure 9). The results show that the lower limit of the ecological groundwater level in the study area is 3.35–6 m in the northeast, and 7.5–10.23 m in the northwest and southwest.

3.2.2. Determination of the Upper Limit of Ecological Groundwater Level

From indoor experiments and by referring to previous results [28,29,30], the capillary rise heights of different soil textures in the study area can be obtained. The thickness of the plant root layer is mainly determined by combining land use types and maincrop varieties in the study area with reference to the “Root System of Herbal Plants in Northern China” [31] and “Monographs on Crop Cultivation” [32]. Figure 3 and Figure 4 show the distribution of soil texture and land use types in the study area. The soil texture in the study area is mainly clay loam, loam, sandy loam, and sandy soil. The land use types are mainly farmland, grassland, and woodland. Because the area of the remaining land use types is small, ArcGIS is used to reclassify the land use types, and the remaining land use types are integrated into the nearby cultivated land, grassland, and woodland. The study area is located in a typical mountain basin system, belonging to the desert oasis zone, which is also an important agricultural economic belt and cotton producing area on the northern slope of the Tianshan Mountains. Therefore, the arable land is mainly composed of cotton crops, and the grassland is dominated by cold-resistant plants represented by alpine pastures. Woodlands are mostly distributed in the vicinity of mountainous areas and in artificial forests in some urban areas. The upper limit of the ecological groundwater level in the study area is obtained by adding the capillary water rise height and the plant root layer thickness in the study area (

Table 3. Calculation results of the upper limit buried depth of the ecological groundwater level in the study area.

Soil Texture	Land Use Types	Capillary Rise Height/m	Plant Root Thickness/m	Ecological Threshold of Groundwater/m
Silty clay	Cultivated land	1.1	0.6	1.7
	Grassland		0.35	1.45
	Forest		2	3.1
Clay loam	Cultivated land	1.71	0.6	2.31
	Grassland		0.35	2.06
	Forest		2	3.71
Loamy clay	Cultivated land	1.67	0.6	2.27
	Grassland		0.35	2.02
	Forest		2	3.67
Loam	Cultivated land	2.05	0.6	2.65
	Grassland		0.35	2.4
	Forest		2	4.05
Sandy loam	Cultivated land	1.46	0.6	2.06
	Grassland		0.35	1.81
	Forest		2	3.46
Sandy clay loam	Cultivated land	1.48	0.6	2.08
	Grassland		0.35	1.83
	Forest		2	3.48
Sandy soil and loamy sand	Cultivated land	0.47	0.6	1.07
	Grassland		0.35	0.82
	Forest		2	2.47

).

Using the overlay analysis function of ArcGIS, the processed soil texture map and land use map are superimposed, and the spatial distribution of the upper limit of the ecological groundwater level in the corresponding area is determined according to the soil texture attributes and land use attributes (Figure 10). The results show that the upper limit of the ecological groundwater level in the study area is 0.82–4.05 m, of which the upper limit is 2–2.5 m in most areas, 0.82–1.5 m in some northern areas, and 2.5–4.05 m in a few areas.

3.3. Estimation of Ecologically Regulated Water Quantity

The study area is an important agricultural gathering area. Taking the average buried depth distribution of groundwater in 2019 as the current level, the distribution of groundwater buried depth in 2019, and the ecological groundwater level threshold of groundwater are superimposed and analyzed using ArcGIS to draw a distribution map of the suitable interval of groundwater level (Figure 11). As shown in Figure 10, the areas where the groundwater buried depth in the study area is lower than the lower threshold of ecological groundwater level are classified as a deficit area. The groundwater buried depth near the spring overflow zone is small, which is higher than the upper limit threshold of ecological groundwater water level, and is divided into the excess area. The rest of the regional groundwater buried depth between the ecological groundwater level threshold is divided into the suitable area.

Using the empirical formula method, and based on the existing research results of the various research stages of the Manas River Basin, as well as comprehensively referring to the results of the previous numerical model identification, and the simplified results of the water-bearing rock formation determined by the cumulative hydraulic conductivity method, the specific yield distribution map of the study area are determined (Figure 12).

ArcGIS was used to divide the Manas River Basin’s average groundwater buried depth distribution map and suitable groundwater buried depth distribution map in 2019 according to administrative regions, and determine the aquifer volume of the corresponding regions by filling and digging instructions. Multiplying the aquifer volume in the deficit area by the specific yield of the corresponding area equates to the ecologically regulated water quantity for the area needed to restore the groundwater depth to the appropriate range of the ecological groundwater level (

Table 4. Analysis of groundwater volume in each city and county in the study area.

Administrative Area	Area			Volume		Specific Yield	Deficit (108 m3)	Excess (108 m3)
	Deficit Area (km2)	Excess Area (km2)	Suitable Area (km2)	Deficit Area (108 m3)	Excess Area (108 m3)
Shehezi	235.57	4.02	101.86	49.28	0.027	0.15	7.392	0.004
Manasi	3076.47	0	4.88	501.92	0	0.05	25.096	0
Shawan	4019.61	20.37	634.12	488.07	0.181	0.09	43.926	0.016
Kelamayi	2022.00	0	0	240.08	0	0.1	24.008	0
Kuitun	401.70	0	0	53.19	0	0.1	5.319	0
Total	9755.36	24.39	740.86	1332.54	0.208		105.741	0.020

). The ecologically regulated water quantity for the study area to be restored to the suitable range of the ecological groundwater level is 105.741 × 10⁸ m³.

4. Discussion

4.1. Differences in Groundwater Dynamic Characteristics

From 2013 to 2019, the groundwater buried depth in the Manas River Basin changed, except that the piedmont inclined plain in the southwestern part of the basin became less fluctuating and the groundwater buried depth recovered, while the groundwater buried depth of the rest of the area continued to increase. Changes in groundwater buried depth during the year are still affected by agricultural irrigation, showing a “small–large–small” situation, and the changes in time tend to be synchronized in various regions. The groundwater dynamic types of the basin include hydrological type, meteorological type, artificial exploitation type and irrigation infiltration type, which are presented according to the temporal and spatial variation. Some studies [20] believe that the groundwater level in the Manas River Basin continued to show a downward trend from 1995 to 2015, and the decline rate gradually increased from south to north, which is different from the results of this study. The main reason is that in recent years, the Manas River Basin has strictly implemented the “most stringent water resources management system”, promoted land reclamation and water reduction, shut down a large number of groundwater pumping wells, and reduced the total amount of groundwater extraction in the study area. In addition, irrigation water consumption was strictly controlled to reduce the water diversion from river runoff and increase the recharge item of groundwater, so that the groundwater buried depth in the piedmont inclined plain gradually decreased, and the water resource management system in the study area achieved the initial results. There are also some studies [33] that indicate that the buried depth of regional groundwater is generally large, and the buried depth boundary moves northward, which is consistent with the results of this study. The main reason is that the groundwater resources in the basin have turned into a negative equilibrium state since 1995. After years of development, the static reserves of groundwater in the region have gradually decreased, resulting in the continuous northward shift of the buried depth boundary and the spring water overflow zone.

4.2. Groundwater Ecological Threshold Rationality

The groundwater buried depth is closely related to the ecological health of the arid zone oasis, while the plain oasis of the Manas River Basin is already in a sub-healthy state [34]. Artificial oases have been expanding to replace natural oases, and cultivated land has become the main type of land expansion in this region. The high degree of artificial disturbance has greatly changed the water cycle of the basin. Among the previous studies on ecological groundwater level in arid areas, some studies [9,24] determined that the appropriate ecological groundwater level was 2.0–4.5 m based on phreatic evaporation theory, and through experimental methods, some studies [35,36] confirmed that salinization is likely to occur when the groundwater level is less than 2 m, and desertification is likely to occur when the underground depth is greater than 6 m. In the Qaidam Basin, some scholars [37] determined the suitable burial depth interval for different plant types according to the dependence relationship between ecological vegetation and groundwater, and believed that the groundwater buried depth of 5.3 m as the lower threshold value is beneficial to maintain the ecological security of most regions. Cheng et al. [38] analyzed the relationship between vegetation characteristics and groundwater buried depth in the water source area of the Manas River Valley based on the Gaussian model. It is considered that the suitable range of ecological groundwater levels of overall vegetation in the study area is 1.0–5.5 m, that the ecological groundwater warning level is 5.5 m, and the lower limit of the ecological groundwater level is 9 m. Based on the control of salinization and desertification, the upper and lower limits of the ecological groundwater level determined in this study are 0.82–4.05 m and 3.35–10.23 m, respectively, and the upper and lower limits of the ecological groundwater level threshold have been expanded. The results of this study are partially different from previous studies, but the overall law is similar. The main reason for this difference is that there are differences in research scales and external environmental factors affecting vegetation. Cheng et al. take the water source area of the Manas River Valley as the study area. This area is the key water conservation area of the Manas River Basin. There are rich types of vegetation, mostly natural vegetation, which is convenient to establish the relationship between vegetation frequency and groundwater buried depth, and can formulate the ecological protection strategy of this area more efficiently. In the oasis plain area of the Manas River Basin, cultivated land is the main land use type. Affected by human activities, the relationship between crop growth and groundwater buried depth is weakened. It is necessary to comprehensively consider other influencing factors to determine the ecological groundwater level. Based on the actual situation of different soil textures and land use types, this study established the threshold interval of ecological groundwater levels under the multi-dimensional natural vegetation, farmland crops, urban landscape, and other categories, and determined the upper limit of the ecological groundwater level under the consideration of rivers and lakes, wetlands, vegetation types, and other conditions. The lower limit of the ecological groundwater level ranges from 3.35 m to 10.23 m, in which the lower limit of ecological groundwater level ranges from 3.35 m to 8 m in most regions, which is basically consistent with previous studies. The remaining ranges are mostly distributed in the desert edge and groundwater funnel areas, and the originally buried depth of regional groundwater is relatively large, resulting in dry surface soil and large limit evaporation depth.

4.3. Suggestions for the Ecological Regulation of Groundwater

The current level of groundwater in the Manas River Basin is quite different from the ecological threshold of groundwater in the region. Extensive exploitation of groundwater has led to a continuous decrease in static reserves of groundwater. After years of depletion, the ecological deficit has reached 105.741 × 10⁸ m³. Due to the high intensity of human regulation in the basin, the regional ecological environment has not been seriously deteriorated, but its development mode is not conducive to the sustainable and coordinated development of the region. A large amount of water resources is consumed by evapotranspiration, resulting in serious soil secondary salinization in the middle of the basin and vegetation apoptosis in the downstream desert area. The regulation of regional groundwater resources based on ecological thresholds can start from three aspects: groundwater replenishment, drainage, and development and utilization [39]. Groundwater exploitation is a major way of discharging groundwater in the study area and should be based on “the strictest water resources management system” for approving the regional groundwater recoverable amount, for the regional economic development and ecological stability constraints, and optimization of the regional groundwater exploitation plan; it makes full use of the rich area of groundwater resources in the study area and restores the deficiency area of groundwater resources. In terms of groundwater replenishment, due to global climate change, the climate in the study area has shifted to warm and humid, the amount of upstream water resources has increased, and the lateral recharge of groundwater has increased. On the other hand, it is also possible to carry out ecological water transfer from neighboring areas, based on water right transactions, to alleviate the ecological deficit of groundwater in the study area. In terms of the groundwater development and utilization, as the main agricultural irrigation area, the mechanism of agricultural water irrigation groundwater recharge should be studied, and groundwater recharge measures should be improved to promote the benign operation of regional water cycles and avoid ineffective dissipation of water resources. At the same time, in conjunction with the regional “land retreat and water reduction” policy, the regional water use structure will be optimized and ecological agriculture will be developed [11].

The rational exploitation and protection of groundwater are of great significance to the arid area of Northwest China. The determination of an ecological threshold value can provide theoretical guidance for regional groundwater regulation and restoration, and optimize the allocation of regional water resources based on the perspective of ecosystem stability. This study can provide theoretical guidance for the utilization of groundwater resources and ecological environment protection in the arid oasis area of Northwest China, but it still has certain limitations. As an artificial oasis under the strong influence of human activities, the study area has greatly changed the regional water cycle pattern, water resource consumption, and recharge mechanism, and its eco-hydrological response mechanism and evolution law need to be further studied, so as to regulate groundwater resources in the arid area of Northwest China from various respects.

5. Conclusions

Based on the analysis of dynamic characteristics of groundwater changes, the authors of this paper carried out research on the regional ecological groundwater level threshold and ecological regulation quantity and put forward strategies to maintain the stability of the regional ecosystem and optimize the allocation of groundwater resources. The conclusions are as follows:

(1) From 2013 to 2019, the groundwater buried depth in the study area continued to increase, except for in the southeast. There were obvious underground funnels, groundwater resources were over exploited, the groundwater balance was in a negative equilibrium state, and the stability of the groundwater ecosystem was destroyed.

(2) A suitable ecological groundwater level is of great significance to the stability of the ecological environment of the Manas River Basin. Considering hydrogeological differences and the impact of human activities, the upper limit of the ecological groundwater level in the study area was determined to be 0.82–4.05 m, and the lower limit was 3.35–10.23 m.

(3) Using ArcGIS software, based on the calculated suitable interval of the ecological groundwater level, we calculated the difference between the groundwater buried depth in the study area and the suitable interval of the ecological groundwater level. The calculation showed that the groundwater ecological deficit in the study area was 105.741 × 10⁸ m³ and the water shortage area was 9755.36 km².

(4) The goal of ecological groundwater regulation research is to provide the basis for regional water resource allocation and ecological environment protection, combined with determining the general situation of the regional groundwater resources of the Manas River Basin and improving the regional groundwater phenomenon. In addition to adhering to the “land retreat and water reduction”, we still need to: 1) carry out water rights trading, calculate water resource deficits and shortages, carry out ecological water diversion, and replenish the deficit and shortage of water resources in the region. 2) The mechanism of agricultural water irrigation recharge groundwater should be studied, and groundwater recharge measures should be improved to promote the benign operation of the regional water cycle and avoid the ineffective dissipation of water resources. 3) Carry out research on blue water and green water resources, optimize the water resource utilization mode, and improve the regional water resources’ effective utilization efficiency. 4) Optimize regional water use structure, adjust the industrial layout, and develop ecological industries.