**1. Introduction**

In order to adapt to the new requirements of "clear waters and green mountains are as good as mountains of gold and silver" and "ecological construction and development of the Yellow River" proposed by President Xi Jinping, domestic researchers have carried out a lot of research work. Since the 1950s, relevant experts in China have conducted a lot of experimental and practical studies on the deterioration of lake ecological water environments and ecosystems caused by unreasonable human activities and resource utilization. One of the themes of the International Conference on Ecological Restoration, held in Beijing in 1996, was "Ecological Restoration of Degraded Ecosystems". In 2000, according to the new requirements of national economic and social development, the Ministry of Water Resources proposed the working idea of giving full play to the power of nature and relying on ecological self-healing ability to accelerate the pace of lake ecosystem degradation prevention and control, and adopted a series of countermeasures and measures around this idea. The calculation method of ecological water demand under changing environments was studied by Wang, Q et al., 2021 [1], who compared different calculation results based on the analysis of river runoff restoration and variability, and determined the ecological water demand of internal flow, which provided a new idea for the calculation method of ecological water demand in the future. According to remote sensing and GIS technology, the spatial distribution of ecological water demand in the

**Citation:** Wan, F.; Zhang, F.; Zheng, X.; Xiao, L. Study on Ecological Water Demand and Ecological Water Supplement in Wuliangsuhai Lake. *Water* **2022**, *14*, 1262. https:// doi.org/10.3390/w14081262

Academic Editors: Xiangyi Ding, Qiting Zuo, Guotao Cui and Wei Zhang

Received: 4 March 2022 Accepted: 11 April 2022 Published: 13 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

study area was simulated by Wu, J. Q. et al., 2017 using Penman-Monteith method and ArcGIS software [2], and analyzed the correlation between ecological water demand and landscape pattern. In the past 20 years, the target of ecological water demand, water demand category, and water demand calculation results of the Yellow River estuary have been obtained by Yu, S. B. et al., 2020 [3], who combined the evolution characteristics of the Yellow River estuary, and proposed the key points of, and directions for, ecological water demand research. Four broad categories and hundreds of assessment methods for hydrological methods, hydraulic methods, habitat simulation methods, and global analysis methods have been developed by international researchers focused on ecological water demand [4,5]. For example, according to biological preference for habitat environment, the habitat simulation methods were used to evaluate the ecological water demand [6], and established the direct connection between runoff and organisms, etc. Meanwhile, various types of numerical models have been developed to assess the status and succession of ecological vegetation and water environment [7]. Internationally, early studies on ecological water use were on the minimum flow of rivers for the purpose of shipping functions. As pollution problems intensified, leading to damage to ecological structures and functions in some countries, ecological studies were carried out one after another. A hierarchical modeling approach was used by Ocock, J. F. et al., 2018 [8], who identified the impact of habitat on water environment management. Surface water monitoring data were used by Wolfram, J et al., 2021 to comprehensively assess the past and present environmental risks of several aquatic species [9]. The ecosystem payment service was established by Salzman, J et al., 2018 aiming to provide an exchange-value scenario of soil and water management practices as a way to assess the trends and status of these policy tools [10]. The SWAT watershed simulation model was developed by Neupane, P. et al., 2020 to assess the fate and transport of soils, groundwater, and rivers at the watershed scale [11]. The InVEST tools and GIS spatial analysis were used by Tran, D. X. et al., 2022 to derive measures of forage productivity, soil erosion control and water supply [12].

For the current ecological water supplement, the benefits of economic and social development in domestic catchment area are comprehensively considered by ecological water supplement research, and improves the local water ecological environment, which is also the specific practice of ecological operation. The eutrophication of Taihu Lake has been solved by the ecological water supplement of Baiyangdian Wetland and the project of diversion from the Yangtze River to the Taihu Lake. The optimization model of the ecological water supplement of Boluo Lake Wetland was established by Huang et al., which improved the utilization rate of flood resources and reduced the contradiction between supply and demand of local economic and social water consumption, and the mutual advance and retreat of ecological environment and economic development were realized [13]. In recent years, under the dual influence of international climate change and human activities, the water quantity, water quality, and physical structure of global rivers have changed to varying degrees, which has changed the composition of biological communities, resulting in the gradual degradation of river ecosystem and the reduction of biodiversity [14]. Globally, population explosion and climate change have highlighted the need to enhance freshwater security and diversify water supplies, and groundwater storage in aquifers is increasingly used to mitigate the supply–demand gap usually caused by extreme climates [15]. In addition, in the context of water scarcity and pollution on an international scale, it is also crucial to identify and estimate the potential of these resources for ecological planning [16] Since the beginning of the new century, the theory and demonstrations of eco-hydrological research have been vigorously promoted by the implementation of the United Nations International Hydrological Program, which emphasized the ability to sustain ecosystem demand, and maximize the ecological and environmental benefits [17,18]. For example, the ecological quality of the oasis was monitored by constructing the Remote Sensing Ecological Index (RSEI) for Arid Regions, and the temporal and spatial changes of oasis ecological status and its internal and external factors were analyzed [19].

In this paper, the ecological water demand of Wuliangsuhai Lake was estimated according to the present water quality and the goal of ecological environment protection. The ecological water demand and timely ecological water supplement of Wuliangsuhai Lake were considered with different water quality targets and different times of reaching the standards, so as to restore the water ecological environment of Wuliangsuhai Lake in stages and steps. The requirements of ecological water demand were met in four ways: agricultural wastewater in irrigation area; strengthening water saving in the main canal; carrying out water-saving system reform; and transporting ecological water use for washing salt to Wuliangsuhai Lake during an irrigation gap period. At the same time, this paper takes ecological priority, compatibility, maximum value and hierarchy as its principles; namely, taking the protection of the ecological environment function of lakes as the premise, carefully distinguishing the types of ecological water demand, taking the maximum value of each water demand as the final water demand, and scientifically managing and rationally allocating water resources. The damage degree and dominant factors of water ecological environment in Wuliangsuhai Lake were analyzed and studied to provide a favorable basis for the reconstruction of biodiversity and ecological integrity.

#### **2. Overview of the Study Area**

Wuliangsuhai Lake was formed in the middle of the 19th century and is a furiotile lake formed by the diversion the Yellow River, which is located in Bayannur City, Inner Mongolia Autonomous Region of China, as shown in Figures 1 and 2. The current lake area of Wuliangsuhai Lake is 300 km<sup>2</sup> , and the reed area accounts for 41.2% of the lake area and dense regions of aquatic plants form 20.9% of the lake area. Its water depth is 0.5~2.5 m, and 90% of its water supply depends on the total drainage and irrigation ditches of the Yellow River [20,21]. It is one of eight freshwater lakes in China, and it is also the largest shore lake in the upper Yellow River. At the same time, it is an extremely rare large grassland lake with biodiversity and environmental protection functions in desert and semidesert regions of the world, and it is also the largest natural wetland in the same latitude of the Earth.

**Figure 1.** Location map of study area.

**Figure 2.** Map of Wuliangsuhai Lake.

Wuliangsuhai Lake has become an important ecological barrier and habitat for birds in northern China, but its ecological environment is extremely fragile and greatly affected by human activities, so it needs to be renovated urgently. At present, the water quality of Wuliangsuhai Lake wetland is seriously polluted, and annual or inter-annual pollution, agricultural non-point source pollution, point source pollution, and internal pollution make it a typical plant type lake with severe eutrophication [22]. Half of the water surface is occupied by emergent plants, and almost all of the open water surface is filled with submerged macrophytes. According to People's Political Consultative Conference Newspaper, the lake bottom is raised at a rate of 6~9 mm per year. If rescue and treatment are not accelerated, Wuliangsuhai Lake will disappear in 10 to 20 years, posing a new threat to the ecological security of the north [23].

Wuliangsuhai Lake is the only drainage and vented area in the Hetao irrigation area, which plays an important role in purifying the water withdrawal of Hetao irrigation area and ensuring the safety of the water environment of the Yellow River. Wuliangsuhai Lake, as a disaster reduction, division of ice-run and flood diversion reservoir of the Yellow River, plays an important role in the division of ice-run in flood season and the water replenishing in dry period. Wuliangsuhai Lake also plays a key role in the normal operation of irrigation and drainage engineering in the Hetao irrigation area, controlling salinization, the water-salt balance in the irrigation area and maintaining the balance of the water environment system in the irrigation area. At present, the main problems facing Wuliangsuhai Lake are: Firstly, the ecological water supply is obviously insufficient, the area of the lake area is shrinking and the ecological function is seriously degraded; Second, the long-term accumulation of pollutants, the internal source is seriously polluted and the trend of swampiness is obviously accelerated; Third, the overall level of environmental governance in the watershed is not high, and exogenous pollution and soil erosion are still serious.

According to the Water Resources Bulletin of Bayannur City in 2019, total nitrogen, ammonia nitrogen, Chemical Oxygen Demand (COD), mercury and chloride are ranked in the top five pollutant concentrations in Wuliangsuhai Lake. The pollution load ratio is basically more than 10%, and the total reaches 75%. Although total nitrogen is ranked first, more than 80% of its source is ammonia nitrogen, and its standard is the same as ammonia nitrogen. Controlling ammonia nitrogen can achieve the purpose of controlling

total nitrogen. At the same time, the mineralization degree is mainly affected by the high chloride content, so the control of total pollutants mainly considers two pollution parameters: COD and ammonia nitrogen. Now, the water quality concentrations of COD and ammonia nitrogen when entering the lake and in the lake are listed in Table 1.

**Table 1.** The water quality concentrations of COD and ammonia nitrogen when entering the lake and in the lake (mg/L).


The COD is Chemical Oxygen Demand.

According to the management requirements of water functional area, the incoming water quality of Bayannur agricultural water area in the upper section of sewage discharge control area of Urad Front Banner where the waste canal enters Huangkou should be class III. Therefore, the COD concentration values of Shagedu ferry in the upstream background section are taken as 20 mg/L, and the ammonia nitrogen concentration values are taken as 1.0 mg/L.

#### **3. Materials and Calculation Methods**

#### *3.1. Calculation of Ecological Water Demand in Wuliangsuhai Lake*

Different types of lakes have different ecological construction and protection objectives and maintain different ecological functions. In this paper, combined with the characteristics and current situation of Wuliangsuhai Lake, the ecological water demand of Wuliangsuhai Lake was estimated according to the goal of ecological environment protection. Water of evaporation and leakage in the lake and water demand of pollutant dilution were mainly considered in the calculation of ecological water demand.

#### 3.1.1. The Water of Evaporation Leakage and Other Loss

The water surface evaporation was calculated by using the series values of Urad Front Banner meteorological station from 1966 to 2018, and the annual average evaporation is 2382.1 mm (20 cm diameter evaporating dish). The evaporation reduction coefficient is 0.56 when 20 cm diameter evaporation pan is converted into 20 m<sup>2</sup> evaporation tank (which can represent water surface evaporation). Thus, the annual average water surface evaporation is 1334.0 mm. Land surface evaporation is the difference between multi-year average precipitation and annual average runoff depth. The multi-year average precipitation is calculated from 1966 to 2018, which is 221.1 mm, and the annual average runoff depth is 8.8 mm, so the land surface evaporation is 212.3 mm. Evaporation gain and loss is the difference between water surface evaporation and land surface evaporation, which is 1121.7 mm. The increase and loss of lake surface evaporation were calculated based on the gain and loss of evaporation and the water surface area over the years, and the annual average is 365 million m<sup>3</sup> . The seepage factor of Wuliangsuhai Lake is 0.67 mm/d, and

about 71 million m<sup>3</sup> is leaked into the surrounding groundwater every year. Therefore, the water of evaporation and leakage of Wuliangsuhai Lake is 436 million m<sup>3</sup> .

#### 3.1.2. Water Demand of Pollutant Dilution

For Wuliangsuhai Lake, to ensure a high ecosystem service function, it is necessary to make sure the already polluted water quality can be gradually improved to meet certain water quality standards. Among them, one method is to input a certain amount of clean water, so that the polluted water can be replaced constantly and reaches a water quality standard after a certain time. In this way, the input water becomes the water for pollutant dilution. For Wuliangsuhai Lake, ecological water demand is provided in two main ways: water inflow of the main drain system, and water diversion from the Yellow River. Under the condition of water balance, the water inflow of main drain system can be used for ecological water use. For the estimation of ecological water demand when pollutants are diluted, due to the fact that COD and ammonia nitrogen of the water body from the main drain system are greater than the corresponding pollutant concentration of Wuliangsuhai Lake water body, if the drained water of the irrigation area is directly introduced, it will inevitably lead to a decline in the water quality of Wuliangsuhai Lake. Therefore, water can only be diverted directly from the Yellow River as ecological water use, and certain discharge waters from Wuliangsuhai Lake must be guaranteed. When estimating the water demand of pollutant dilution, the water body of Wuliangsuhai Lake should reach the following standards: the class IV water quality targets of COD and ammonia nitrogen are 30 mg/L and 1.5 mg/L, respectively; The class V water standards are 40 mg/L and 2.0 mg/L, respectively. The water demand of pollutant dilution is considered to be directly diverted from the Yellow River, and the solution method is as follows:

The dynamic viewpoint is used to estimate the water demand of pollutant dilution, and considering the time (*r*) required for water quality to reach the standard (*Cstd*), the total volume of water in the lake (*Q*) and pollutant concentration (*C*0). After the Yellow River water is directly diverted, the mixed concentration of pollutants with Wuliangsuhai Lake is *Cout*. At this time, the concentration of lake water body should reach *Cstd* after *r* years, and it can be assumed that the amount of ecological water demand can be provided every year is certain, which is *Qeco*.

Assuming that ecological water use is provided in the first year is *Weco*, and pollutant concentrations in lake water at the beginning and end of the year are *C*<sup>0</sup> and *C*01, respectively. According to the material balance principle, the pollutant reduction of the lake is the output minus the input:

$$Q(\mathbb{C}\_0 - \mathbb{C}\_{01}) = (Q\_{\text{out}} + Q\_{\text{eco}})\mathbb{C}\_{\text{out}} - (Q\_{\text{eco}}\mathbb{C}\_{\text{eco}} + Q\_{\text{in}}\mathbb{C}\_{\text{in}}) \tag{1}$$

where *Q* is the total volume of water in the lake (hundred million m<sup>3</sup> ), *Cout* is the mixed concentration of pollutants (mg/L), *Qeco* is the amount of water directly diverted from the Yellow River each year (m<sup>3</sup> ), *Ceco* is background concentration value of water directly diverted from the Yellow River (mg/L), *Cin* is pollutant concentration of farmland drainage, domestic sewage and so on (mg/L), *Qin* is water of entering the lake (m3/s). Among them:

$$Q\_{\rm in} = \mathbb{R} + F + A \tag{2}$$

where *R* is the rainfall of lake surface (hundred million m<sup>3</sup> ), *F* is the water of flood entering the lake by rainfall (hundred million m<sup>3</sup> ), *A* is the water of farmland drainage from main drain system, production and domestic wastewater.

$$Q\_{out} = P + D \tag{3}$$

where *P* is the amount of water discharged from the lake into the Yellow River (hundred million m<sup>3</sup> ), *D* is the water of evaporation leakage and loss in the lake (hundred million m<sup>3</sup> ).

$$\mathbf{C}\_{\rm out} = (\mathbf{C}\_{\rm esc} \mathbf{Q}\_{\rm esc} + \mathbf{Q}\_{\rm in} \mathbf{C}\_{\rm in}) / (\mathbf{Q}\_{\rm esc} + \mathbf{Q}\_{\rm in}) \tag{4}$$

the parameter description in the formula is the same as that in formula (1).

$$Q\_{out} \mathcal{C}\_{out} = P \mathcal{C}\_p + D \mathcal{C}\_d \tag{5}$$

$$Q\_{in} \mathbf{C}\_{in} = R \mathbf{C}\_{r} + F \mathbf{C}\_{f} + A \mathbf{C}\_{a} \tag{6}$$

where *C<sup>p</sup>* is the concentration of pollutants discharged from lakes into the Yellow River (mg/L), *C<sup>d</sup>* is the pollutant concentration of the water body of evaporation and leakage in the lake (mg/L), *C<sup>r</sup>* , *C<sup>f</sup>* and *C<sup>a</sup>* are the pollutant concentrations of lake surface precipitation, the flood into the lake and farmland drainage from main drain system, production and domestic wastewater, respectively (mg/L).

Equations (5) and (6) are substituted into Equation (1) to obtain:

$$Q(\mathbb{C}\_0 - \mathbb{C}\_{01}) = PC\_{01} + DC\_{01} + Q\_{\text{eco}}\mathbb{C}\_{\text{out}} - \left(Q\_{\text{eco}}\mathbb{C}\_{\text{eco}} + \mathbb{R}\mathbb{C}\_r + FC\_f + A\mathbb{C}\_4\right) \tag{7}$$

from the above formula, it can be concluded that:

$$\mathbf{C}\_{01} = \left(Q\_{\rm eco}\mathbf{C}\_{\rm eco} + R\mathbf{C}\_{r} + F\mathbf{C}\_{f} + A\mathbf{C}\_{a} - Q\_{\rm eco}\mathbf{C}\_{\rm out} + Q\mathbf{C}\_{0}\right) / \left(Q + P + D\right) \tag{8}$$

this formula is simplified as:

$$\mathbb{C}\_{01} = \mathbb{K}\_1 + \mathbb{K}\_2\mathbb{C}\_0 \tag{9}$$

among them:

$$K\_1 = \left(Q\_{\rm eco}\mathbb{C}\_{\rm eco} + R\mathbb{C}\_r + FC\_f + A\mathbb{C}\_a - Q\_{\rm eco}\mathbb{C}\_{\rm out}\right) / \left(Q + P + D\right) \tag{10}$$

$$K\_2 = Q/(Q+P+D)\tag{11}$$

similarly, the pollutant concentration of lake water body (*C*02) at the end of the second year is:

$$\mathbf{C}\_{02} = \mathbf{K}\_1 + \mathbf{K}\_2 \mathbf{K}\_{01} = \mathbf{K}\_1 + \mathbf{K}\_1 \mathbf{K}\_2 + \mathbf{K}\_2^2 \mathbf{C}\_0 = \mathbf{K}\_1 (1 + \mathbf{K}\_2) + \mathbf{K}\_2^2 \mathbf{C}\_0 \tag{12}$$

therefore, the pollutant concentration of lake water body (*Cor*) at the end of the *r* year is obtained as:

$$\mathbf{C}\_{or} = \mathbf{K}\_1 \left( \mathbf{1} + \mathbf{K}\_2 + \dots + \mathbf{K}\_2^{(r-1)} \right) + \mathbf{K}\_2^r \mathbf{C}\_0 \tag{13}$$

if the water quality is to be standardized in *r* year, there are:

$$\mathbb{C}\_{or} = \mathbb{C}\_{std} \tag{14}$$

thus, if the following equation can be solved, the ecological water demand (*Qeco*) to be provided each year can be obtained:

$$\mathbf{C}\_{std} = \mathbf{K}\_1 \left( \mathbf{1} + \mathbf{K}\_2 + \dots + \mathbf{K}\_2^{r-1} \right) + \mathbf{K}\_2^r \mathbf{C} \tag{15}$$

#### *3.2. Ecological Water Demand of Wuliangsuhai Lake*

(1) The amount of water discharged from Wuliangsuhai Lake into the Yellow River: *P* =14 million m<sup>3</sup> .

(2) The water of evaporative leakage and loss in Wuliangsuhai Lake: *D* =436 million m<sup>3</sup> .

(3) It can be known from the above-mentioned theories and calculation formulas, the water demand of pollutant dilution depends not only on the ecological protection target, but also on the length of time required to meet the water quality standards of Wuliangsuhai

Lake. The shorter the time, the greater the ecological water demand. The size of ecological water demand is also determined by the amount of water (in fact, the amounts of pollutants) entering Wuliangsuhai Lake from main drainage channel each year.

After the water is diverted directly from the Yellow River, the mixed concentration of pollutants retreating from Wuliangsuhai Lake into the Yellow River. When the water quality reaches the class IV and V water standards, respectively, the ecological water demand of COD and ammonia nitrogen meeting the standards in 1, 10 and 15 years are shown in Table 2.

**Table 2.** The water demand of dilution of COD and ammonia nitrogen meeting to standard in 1, 10 and 15 years (hundred million m<sup>3</sup> ).


The larger values of COD and ammonia nitrogen can be taken as the water demand of pollutant dilution. It can be seen from Table 2 that when the water demand of pollutant dilution can make ammonia nitrogen reach the water quality standard, COD can also reach the standard.

Therefore, the ecological water demand of Wuliangsuhai Lake (*Q*0 ) is:

$$Q' = P + D + Q\_{\text{eco}} \tag{16}$$

the parameter descriptions in the formula are the same as above.

The amount of ecological water needs to be replenished from the Yellow River (*Q*s) is:

$$Q\_s = Q' - Q\_{\rm in} \tag{17}$$

the parameter descriptions in the formula are the same as above.

The ecological water demand and timely ecological water supplement of Wuliangsuhai Lake with different water quality targets and different time of reaching the standards are listed in Table 3.

**Table 3.** Ecological water demand and ecological water supplement in different water quality standards.


According to the calculation in Table 3, the water quality of Wuliangsuhai Lake can reach the standard of class IV water in one year under the condition of the current water discharge from the main drainage channel and exhaust contaminant, and the timely ecological water supplement is 6.12 hundred million m<sup>3</sup> . The water quality of Wuliangsuhai

Lake can be reached the standard of class IV water in 10 years, and the timely ecological water supplement is 3.80 hundred million m<sup>3</sup> . The water quality of Wuliangsuhai Lake can be reached the standard of class IV water in 15 years, and the timely ecological water supplement is 3.58 hundred million m<sup>3</sup> . The water quality of Wuliangsuhai Lake can be reached the standard of class V water in one year, and the timely ecological water supplement is 4.65 hundred million m<sup>3</sup> . The water quality of Wuliangsuhai Lake can be reached the standard of class V water in 10 years, and the timely ecological water supplement is 3.01 hundred million m<sup>3</sup> . The water quality of Wuliangsuhai Lake can be reached the standard of class V water in 15 years, and the timely ecological water supplement is 2.66 hundred million m<sup>3</sup> .

#### **4. Results and Discussion**

#### *4.1. Results*

At present, the shortage of ecological water supplement in Wuliangsuhai Lake has become the main restrictive factor to maintain the existing water surface and water ecological health of Wuliangsuhai Lake. Rescuing the Wuliangsuhai Lake must take the lead in scientific planning of water resources to ensure its ecological water demand requirements. The ecological water supplement of Wuliangsuhai Lake can be solved through the following four ways:

(1) In 2015, the water-saving capacity of water consumption diverted from the Yellow River in irrigation area increased by 6.83 hundred million m<sup>3</sup> , the amount of water diverted from the Yellow River was reduced to 43 hundred million m<sup>3</sup> . About 3.01 hundred million m<sup>3</sup> of farmland drainage in irrigation area, which was reduced to 2.22 hundred million m<sup>3</sup> after passing through estuary wetlands. In 2020, it was reduced to 40 hundred million m<sup>3</sup> , and about 2.55 hundred million m<sup>3</sup> of farmland drainage in irrigation area, which was reduced to 1.42 hundred million m<sup>3</sup> after passing through estuary wetlands. These return water will be replenished into Wuliangsuhai Lake.

(2) By means of lining the aboveground paragraph of the total main canal and strengthening the management of water-saving measures, water saving was achieved by 1 hundred million m<sup>3</sup> in 2015. After considering that the engineering was partially implemented, the utilization coefficient of canal system of the total main canal and the main canal was 0.94 and 0.96, respectively. In 2015, 0.9 hundred million m<sup>3</sup> of water was added to Wuliangsuhai Lake. In 2020, 2.02 hundred million m<sup>3</sup> of water consumption was saved. After considering that the engineering was fully implemented, the utilization coefficient of canal system of the total main canal and the main canal was increased to 0.95 and 0.97 respectively. In 2020, 1.86 hundred million m<sup>3</sup> of water was added to Wuliangsuhai Lake.

(3) By means of deepening the management of water-saving measures, carrying out the reform of group management system with the participation of users, strengthening water management, and comprehensively implementing the water-saving irrigation technology in the field, 0.86 hundred million m<sup>3</sup> of water was saved by 8.615 million mu of the whole irrigation area in 2015, and 0.78 hundred million m<sup>3</sup> of water was added to Wuliangsuhai Lake. In 2020, 2.58 hundred million m<sup>3</sup> of water was saved by the whole irrigation area. The utilization coefficient of canal system of the total main canal and the main canal was 0.95 and 0.97 respectively, and 2.38 hundred million m<sup>3</sup> of water was added to Wuliangsuhai Lake.

(4) During the irrigation gap period, 1.74 hundred million m<sup>3</sup> of ecological water use for salt washing was transported to Wuliangsuhai Lake through existing irrigation channels. It was transported to Wuliangsuhai Lake through the total main canal and the main canal, and 1.60 hundred million m<sup>3</sup> was discharged into the Yellow River after circulation.

The second water balance of Wuliangsuhai Lake in 2020 is listed in Table 4.


**Table 4.** The analysis table of second water balance of Wuliangsuhai Lake in 2020.

The results show that the requirements of ecological water demand in Wuliangsuhai Lake can be solved through the above four ways.

#### *4.2. Discussion*

#### 4.2.1. The Effect of Ecological Water Supplement in Wuliangsuhai Lake

The calculation results show that through the four ways of water replenishment, the ecological water supplement of Wuliangsuhai Lake can achieve water balance, and the equilibrium difference reaches 0.20 hundred million m<sup>3</sup> in 2020, which can effectively control the shrinkage of the lake and ensure the water quantity needed to make Wuliangsuhai Lake have basic environmental functions. However, the improvement of water quality requires a continuous and long process, and the estimation of ecological environment water demand is an urgent problem that needs to be solved in the protection and sustainable development of wetland biodiversity. This paper adopts the estimation method of dynamic point of view, when the water quality reaches the class IV and V water standards respectively, reasonably calculates the ecological water demand and the timely ecological water supplement of meeting the standards in 1, 10 and 15 years. In this paper, the problems of insufficient ecological water supply are effectively alleviated by tapping the potential of water saving and reasonable water diversion scheduling and taking the supplement of agricultural wastewater in the irrigation area, lining the aboveground area of the total main canal, deepening the management of water-saving measures, and transporting salt washing ecological water to Wuliangsuhai Lake during the irrigation gap period in the water supply.

Since Wuliangsuhai Lake is located in the transition zone of arid and semi-arid areas in northwest China, water resource carrying capacity and environmental carrying capacity are seriously overloaded due to the loss of a certain amount of water from reed transpiration and lake water evaporation. Thus, the restoration of water ecological environment in Wuliangsuhai Lake can only be realized in stages and steps.

#### 4.2.2. The Necessity of the Ecological Water Supplement in Wuliangsuhai Lake

The primary task of rescuing Wuliangsuhai Lake is to ensure that the existing water surface of Wuliangsuhai Lake is no longer reduced and the lake is no longer shrinking. According to the analysis of water balance state of Wuliangsuhai Lake, the existing water surface of Wuliangsuhai Lake should be maintained. In the current situation, the ecological water demand gap is 4.36 hundred million m<sup>3</sup> . Therefore, the ecological supplement of Wuliangsuhai Lake is a fundamental measure to ensure that the existing water surface of Wuliangsuhai Lake is no longer reduced and the lake water is no longer shrinking.

The most important task for rescuing Wuliangsuhai Lake is to curb the increase of water salinity year by year. To maintain the water-salt balance in Wuliangsuhai Lake, 1.60 hundred million m<sup>3</sup> of water should be discharged to the Yellow River every year. Otherwise, the salinity of Wuliangsuhai Lake water will continue to rise, which will lead to salinization of lakes, deterioration of habitat quality of aquatic organisms and birds, and major changes in the structure and function of the ecosystem of Wuliangsuhai Lake. Therefore, the ecological supplement of the Wuliangsuhai Lake is a fundamental measure to avoid the salinization of water body and the huge changes in the structure and function of the ecological system.

The urgent task of rescuing the Wuliangsuhai Lake is to reduce the pollutants accumulated in the lake area for a long time, improve the lake habitat and restore its ecological function. With the continuous decrease of water inflow, even if exogenous pollution is effectively controlled, due to the role of water surface transpiration and leakage, the concentration of pollutants in the water body of Wuliangsuhai Lake will continue to increase, and the problem of eutrophication is difficult to change. Therefore, timely ecological replenishment of Wuliangsuhai Lake is an effective measure to control the continuous increase of pollutant concentration in the lake area. It was calculated in Table 3 that the ecological water supplement reaches different water quality standards at different times. Therefore, it is necessary and urgent to supplement the ecological water shortage to maintain the survival and ecological function of Wuliangsuhai Lake.

#### **5. Conclusions**

In this paper, according to the current situation of water quality and the goal of environmental protection, and the ecological water demand of Wuliangsuhai Lake is estimated by using the dynamic viewpoint. The water requirement of dilution of ammonia nitrogen is determined as the standard of water demand of pollutant dilution. Finally, the ecological water demand and timely ecological water supplement of Wuliangsuhai Lake with different water quality targets and different time of reaching the standards were obtained. The ecological water supply in Wuliangsuhai Lake is insufficient under the current conditions. The water demand of Wuliangsuhai Lake under different periods and water quality standards can be met through the four solutions in this paper, which have reduced the contradiction between supply and demand and ensured that the storage capacity of Wuliangsuhai Lake is within the range of reasonable ecological protection objectives. Moreover, the water balance of Wuliangsuhai Lake with a water equilibrium difference of 0.2 hundred million m<sup>3</sup> in 2020 was reached. While self-purification and assimilative capacity are improved, it has laid an effective foundation for further controlling the salinization of Hetao area in Wuliangsuhai Lake. It not only maintains the water needed for ecosystem balance, but also creates good ecological effects and ecological benefits in the process of ecological restoration and reconstruction. At the same time, theoretically, it provides constructive suggestions for the study of maintaining ecological balance and

ensuring ecological function in Wuliangsuhai Lake, and it also has provided a favorable basis for the ecological operation and the water ecological environment restoration of Wuliangsuhai Lake. The water balance of Wuliangsuhai Lake can be achieved through the implementation of water diversion and transfer projects, but restoring water quality is a long-term, complex. and challenging task. In order for the quality and safety of the ecological environment in Wuliangsuhai Lake to be maintained, exploring the water-saving potential, strengthening the construction of purified lakes, and remedying pollution sources will be effective measures.

**Author Contributions:** Conceptualization and ideas, F.W.; theory and formulation, F.W. and F.Z.; investigation and data curation, F.W. and L.X. simulations and results analysis, F.W. and X.Z.; writing draft, review and editing, F.W., F.Z. and L.X.; supervision, F.W. and X.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by Henan Science and Technology Department. The funded projects are Major Science and Technology Special Projects in Henan Province (201300311400) and the General Project of Science Foundation in Henan Province (222300420491).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Thank all the authors for their contributions. All authors have read and agreed to the published version of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

