3.3. Model Prediction Analysis
In this study, 2014 was taken as the simulation base year, and the prediction time was 2021–2050. Based on the simulation results obtained using the constructed dynamic system model, the trends of the six main influencing factors (i.e., the total population, total water consumption, domestic water consumption, agricultural water consumption, per capita water resources, and the total amount of sewage discharge) of the Ulan Suhai Lake system from 2021 to 2050 are shown in
Figure 3.
The model (
Figure 3d) predicts that agricultural water consumption slowly increases during 2021–2050 because Ulan Suhai Lake is in the Hetao irrigation district, in which the water resources are required and used for crop irrigation. With the increase in the urbanization level, the per capita water resource utilization remains low [
38] and domestic water consumption slowly increases (
Figure 3c), which is consistent with the population growth rate. The per capita water resources exhibit a decreasing trend [
39], but they remain positive (
Figure 3b), which means that this increase is not caused by the decrease in the total water resources and is most likely related to the rapid growth of the total population. With the growth of urbanization and the population increase, the sewage volume also increases dramatically (
Figure 3e). The growth of the sewage discharge causes domestic sewage pollution in the villages and towns to become an important source of water pollution in the region. In addition, Ulan Suhai Lake is an important component of the drainage system of the Hetao irrigation district in Inner Mongolia (
Figure 1). Ulan Suhai Lake has been receiving chemical fertilizer and pesticide residues from the Hetao irrigation district and discharging them into urban domestic sewage and industrial sewage for a long time [
20], which has aggravated the water pollution in Ulan Suhai Lake for a long time and has directly threatened the ecological water security of the Yellow River. With the rapid development of living standards, and the pressure on water resources increases, the main problem of which being how to address the maximum rate of water usage in the case of a limited water supply.
3.4. Model Scenario Simulation Analysis
The six main influencing factors (i.e., the total population, total water consumption, domestic water consumption, agricultural water consumption, per capita water consumption, and the total amount of sewage discharge) in the Ulan Suhai Lake system from 2021 to 2050 were simulated under different scenarios. The change trends of these indicators are shown in
Figure 4.
As can be seen from
Figure 4, under the five scenarios, the total population (
Figure 4a), domestic water consumption (
Figure 4c), agricultural water consumption (
Figure 4d), total water consumption (
Figure 4f), and the total amount of sewage discharge (
Figure 4e) all slowly increase, while the per capita water resources decrease (
Figure 4b). The specific data are presented in
Table 9. The status continuation mode and development continuation mode ensure stable economic development. Under the status quo continuation model and the development continuation model, the annual average per capita water resources decrease by 13.88 m
3 and 25.75 m
3, and the annual growth rate decreases by 0.004% and 0.98%, respectively. Under the conservation continuation mode, the per capita water resources basically remain in a stable state, indicating that the pressure on the water resources is the lowest, but this mode limits economic development. In contrast, the two comprehensive modes are superior in that the per capita water resources change slowly while ensuring stable economic development and slowing the increase in the total water consumption, which improves the water resources utilization in the short term. This shows that the balance of the per capita water resources can be guaranteed in the long term in the future through measures such as improving water use efficiency, reducing sewage discharge, and improving sewage treatment and utilization.
3.5. Analysis of the Variation Trends of the Bearing Capacity of Each Subsystem and the Collaborative Bearing Capacity under the Different Modes
The three subsystems of Ulan Suhai Lake (the water resources, water environment, and water ecology) and the large, complex system of the synergy of the three were simulated under the different scenarios, and the results were analyzed (
Figure 5 and
Figure 6).
As is shown in
Figure 5a, under the five modes, the state and level of the water resources carrying capacity are general, and the simulation results of the water resources carrying capacity are as follows: conservation continuation mode > comprehensive mode I > comprehensive mode II > status continuation mode > development continuation mode. The carrying capacity of the water resources subsystem is generally weak [
40] and in a fragile and generally critical state [
38]. It decreases under the development continuation mode and increases under the conservation continuation mode and the two comprehensive modes, and the growth is most obvious during 2021–2040. Under the status continuation mode, the slow growth of the population and gross domestic product (GDP) causes the water resources carrying capacity to increase more slowly. If the current situation continues, the water shortage will increase further after 2035, which is not conducive to the development of Bayannur City. If regulatory measures are adopted, the carrying capacity of the water resources in 2035 (2045) will be 0.597 (0.598) under the status continuation mode, while the relative ratios of the carrying capacity of the water resources in 2035 (2045) under the development continuation mode, conservation continuation mode, comprehensive mode Ⅰ, and comprehensive mode Ⅱ will be −14.4% (−17.2%), +29.5% (+34.4%), +26.8% (+31.75), and +21% (+25.8%). This demonstrates that the adoption of water conservation and water protection measures while ensuring economic development can alleviate the pressure on the water resources caused by economic and population development, and the comprehensive mode can largely reduce the pressure on the water resources subsystem. The conservation continuation mode takes more into account the protection of the water resources, so the water carrying capacity value increases significantly, but the GDP decreases by 3.5%, which inhibits the economic development of the region. The development continuation mode considers slowing down the growth rate of the GDP in 20355; that is, under the comprehensive mode I, as opposed to the status continuation mode, the growth rates of the primary and secondary industries are both reduced by 20%, and the growth rate of the tertiary industries reaches 55%. The effective utilization coefficient of agricultural irrigation will increase to 0.70, and the rate of industrial water use will decrease by 20% to improve the utilization rate of the water resources, which will more effectively relieve the pressure on the water resources subsystem [
41], improve the per capita water resources, and protect the water resources while ensuring economic development.
As can be seen from
Figure 5b, under the status continuation mode, the increases in the population and economic development increase the demand for water resources in all aspects, leading to increased pollution of the water environment and affecting the water ecology. The water environment carrying capacity initially exhibits a constant trend and then decreases [
42], and is mainly affected by the total amount of water resources and other factors [
43]. The carrying capacity of the water environment decreases under both the status continuation mode and development continuation mode, but it decreases faster under the development continuation mode. Under the other three modes, it increases and remains average over time. Compared with the water environment carrying capacity value of 0.610 (0.621) under the status continuation mode in 2035 (2045), the water environment carrying capacity values in 2035 (2045) under the development continuation mode, conservation continuation mode, comprehensive mode Ⅰ, and comprehensive mode Ⅱ are −11.2% (−33.6%), +30.9% (+51.0%), +27.3% (+47.2), and +16.2% (+35.3%). Under the status continuation mode, the wastewater treatment rate increases to 95%, the domestic wastewater discharge coefficient decreases by 20%, and the industrial wastewater discharge coefficient decreases by 50%. The COD and NH
3-N emission concentrations of the industrial, agricultural, and domestic water decrease by 50%, 30%, and 10%, and the growth rates of the primary, secondary, and tertiary industries all increase to 65%. The effluent standard of the wastewater plant is increased to Class III or Ⅳ type water standards, to support the development of water-saving agriculture, and other measures are taken to protect the water environment. Thus, all three modes can alleviate the water environment pollution, the pressure on the water environment subsystem is correspondingly reduced. However, the conservation continuation mode limits local development. synergistic economic and environmental development is ensured while the water environment is effectively protected, so comprehensive mode I performed best.
- b.
Analysis of variations in the carrying capacity of the water ecological subsystem
Figure 5c shows that the water ecological carrying capacity value is 0.6673 in 2035 under the status continuation mode, and it exhibits a gradual upward trend [
44] and has a good carrying capacity [
45]. However, with the growth of the population and GDP, the change in the water ecological carrying capacity is not obvious, which means that the water ecological subsystem is less influenced by the population and GDP. It is most strongly influenced by the eutrophication index, biodiversity index, and wetland area. If regulation measures are taken, the water ecological carrying capacity of 0.667 (0.698) in 2035 (2045) will be less changed under the development continuation mode and conservation continuation mode than under the status continuation mode, −12.7% and +15.7%, respectively. Under the comprehensive mode, measures such as keeping the population growth rate constant, reducing the total nitrogen and total phosphorus concentrations, chlorophyll concentration, and submerged plant cover in the lake area and increasing the biodiversity index and wetland area put the least pressure on the aquatic ecosystem and protect the aquatic ecology while allowing economic development.
Based on the analysis of the change trends of the bearing capacities of the three subsystems, the collaborative bearing capacity simulation (
Figure 5d) results are as follows: conservation continuation mode > comprehensive mode I > comprehensive mode II > status continuation mode > development continuation mode. Under the five modes, considering the economic and population growth alone, the collaborative carrying capacity will face greater pressure, and the carrying capacity will be low and in a fragile state. If the conservation continuation mode is adopted alone, the respective carrying capacity values of the water resources, water environment, and water ecology will improve, the carrying status will improve, and the water resources gap will improve, but local economic development will be limited. In contrast, the status continuation mode, the development continuation mode, and comprehensive mode II result in a long-term fragile state. The conservation continuation mode and the comprehensive mode I have bearing capacity values of greater than 0.5 in 2043, the bearing status changes from fragile to general, and the bearing capacity level increases to general, which effectively improves the collaborative bearing capacity. It was found that the collaborative bearing capacity is more comprehensive and systematic than the single carrying capacity. By setting and comparing the different modes, comprehensive model I was selected, which can effectively improve the carrying efficiency of Ulan Suhai Lake.
As is shown in
Figure 6, the simulations under the five modes from 2021 to 2050 show that the water resources carrying capacity, water environment carrying capacity, water ecology carrying capacity, and collaborative bearing capacity are higher under the conservation continuation mode, comprehensive mode I, and comprehensive mode II than under the other two modes, and the changes in the water resources carrying capacity and water environment carrying capacity of Ulan Suhai Lake are small, while the changes in the water ecological carrying capacity are significant. The collaborative bearing capacity of Ulan Suhai Lake does not change significantly under the status continuation mode (
Figure 6a) and remains in a fragile state for a long time. Under the development continuation mode (
Figure 6b), economic development is accelerated. However, the lake’s carrying capacity gradually weakens, its carrying status changes from fragile to collapse, and its carrying level is extremely poor. Its collaborative bearing capacity gradually increases under the conservation continuation mode (
Figure 6c), and in 2040 its bearing status changes from fragile to average, while its bearing level changes from poor to average. Its state increases under the two comprehensive modes, but changes slowly under comprehensive mode II (
Figure 6e)and remains in a fragile state until 2049. Under comprehensive mode I (
Figure 6d), the lake’s bearing capacity value is greater than 0.5 after 2043, its bearing status changes from fragile to average, and its bearing level increases from poor to average. In comparison, under the conservation continuation mode, the lake’s collaborative bearing capacity increases, but in terms of the local development strategy, this mode is not suitable for improving the lake’s carrying capacity. Under the development continuation mode, the lake’s collaborative bearing capacity decreases, which is not conducive to the protection of the water resources, water environment, and water ecology. Comprehensive mode I is more conducive than other modes to improving the lake’s carrying capacity value, and is thus the optimal solution.