**5. Discussion**

#### *5.1. Qualitative Analysis of the Recharge Process*

There are two possible sources of groundwater for recharge to lakes. One of them, it comes from local precipitation. The local precipitation assimilates into the groundwater by soil infiltration. The other one, groundwater of recharge lakes comes from external water. Because the lakes are located on the fault zones with transmission channels of water, the exogenous water continuously transports to the lakes and surrounding aquifers through the fault zones.

Generally, the common lake will expand to a maximum in the annual rainy season and shrink to a minimum in the dry season. However, we used remote sensing to make an experiment. Here, the experiment also used the above-mentioned the deep learning method to extract lakes in the target region. Based on the number of remote sensing images in the same year, the randomly selected sample year is 2000. Then the work selected one day of observation month as the representative image of the month, and continued to select the representative image of the next month about one month apart. This work found that the opposite phenomenon has occurred in the Daihai Lake. The lake area is smallest in the rainy season (June to September) and largest in the dry season (January to May and October to December) (Figure 10). In dry season, there was no precipitation to supplement the lake water while the lake water area increased. This indicates that the evaporation of the lake surface is greater than the amount of water entering the lake and the main recharge of the lake is derived from groundwater. The ground water recharge comes from the leakage of the fault zone in this area. However, the above situation did not occur in the experiment of the Huangqihai Lake. During the previous dry season, the lake area continued to decline and the arrival of the rainy season eased the situation. In the next dry season, there was no rainfall and the lake area fell again (Figure 10). This shows that the water supply of the Huangqihai Lake mainly comes from the precipitation and runo ff in the basin. The recharge method of the Daihai Lake belongs to the second case and this of Huangqihai Lake is the first case. This is an important evidence to indicate that Daihai Lake receives external water supply to alleviate the lake shrinkage.

**Figure 10.** Lake area change in 2000.

In order to further verify our experimental results, a hydrological method was also used. The soil infiltration test showed that the local precipitation cannot enter the groundwater in Daihai basin, because the unsaturated soil water does not reach the maximum water-holding capacity of field (WHCF) and the soil moisture content is in a loss state. In other word, the necessary condition for precipitation infiltration is above the maximum WHCF [14]. We studied the evapotranspiration (ET) of this region is about 395 mm [31] (www.cnern.org.cn), which is basically the equal to the local average annual precipitation of 384 mm (DH) and 374mm (HQH). That is to say, the precipitation of the basin is basically evaporated, and cannot form groundwater of the basin through infiltration. We used remote sensing method to analyze the causes of the lake area change, and we also used soil chemistry and surface evapotranspiration method to verify the results. These experimental results indicated that there is exogenous water supply to Daihai Lake (Figure 11).

**Figure 11.** Daihai Lake receives external groundwater recharge.

The above three methods confirm the existence of external groundwater recharge in Daihai Lake, and whether there will be external water recharge in Huangqihai Lake, which is only 64 km away from Daihai Lake. If so, why did the Huangqihai Lake eventually dry up in 2008? Hydrological and isotope experiments showed that the deep underground wells and springs in the Huangqihai basin are the same as those in the Daihai Basin, and the surface springs eventually converge into rivers to supply the Huangqihai Lake. However, because of the artificial construction of river dams to intercept water sources for daily life, economic and agricultural, the lake has lost its recharge.

#### *5.2. Quantitative Analysis of Water Supply and Consumption*

The above analysis shows that the Daihai Lake received external groundwater recharge while the Huangqihai Lake received no external groundwater supply. The quantitative value of groundwater recharge was calculated along with the balance relationship between recharge and emission, based on the analysis [32]. If the water content of a basin maintains a dynamic balance, it must be equal to the water flowing in and out. Since the Daihai basin is a closed watershed, the water volume changes of the basin can be expressed through the Daihai Lake.

Firstly, we introduced the related indicators in the next work. The region is special and the crops species are relatively scarce. We use the annual statistical yearbooks of the region to estimate the agricultural water consumption (AWC) for the di fferent crop areas in these years [33–35]. Flood irrigation is the main practice for farmland in Daihai basin, so the water requirement of crops needs to be converted into the total irrigation water consumption. The irrigation e fficiency of this area is about 62.7% [36]. We used agricultural irrigation consumption (AIC) as one of the groundwater estimation indicators. Agricultural water consumption in the region accounted for 54.3% of the total water consumption of human activity (TWCHA) in 2003, which makes us to estimate TWCHA by AIC (excluding power consumption of power plants). The local governmen<sup>t</sup> introduced a power plant that needed to use the lake water for water cooling for economic development after 2005, which increased the evaporation of the lake water directly. Annual water consumption (PPWC) reached 1.206\*107m3 according to the estimation. In this experiment, the annual water volume reduction (WVR) was estimated using the annual water level relationship and the area obtained by remote sensing.

For the water flowing into the Daihai Lake, external water, precipitation, and runo ff are the main resources. The hypothesis that rainfall infiltrates into groundwater recharge lakes has already been negated through the special phenomenon of change of the lake area in di fferent seasons by remote sensing and has proven that this conclusion is a reliable the work of water chemistry. The runo ff volume of the basin is only 6\*103m3 yearly [13]. The supply of lake water is less, so it can be ignored. Therefore, water flowing into the lake can be reduced to surface precipitation and external groundwater (DGR). The water flowing out of the Daihai Lake, first of all the direct annual evaporation water consumption (EWC) the lake was calculated by evaporation of the basin, which is need to multiply conversion coe fficient (0.55) [37]. Then, the Daihai Power Plant needs to consume lake water every year. Finally, there is about 9.12\*107m3 of groundwater in the basin itself and this part of the groundwater has been extracted by people (TWCHA). Although the surface precipitation cannot infiltrate to form groundwater, the lake replenishes lost water of the basin to maintain the water balance in the basin. So, this element of artificial consumption is also part of the lake water consumption. The calculation method of parts of the indicators are more intuitive in Figure 12.

**Figure 12.** Water consumption and recharge in Daihai Basin.

All of these inflows and outflows are change of lake water volume (WVR). The resulting equation is

$$\text{WVR} = \text{PPWC} + \text{EWC} + \text{TWCHA} - \text{DGR} \tag{3}$$

$$\text{DGR} = \text{PPWC} + \text{EWC} + \text{TWCHA} - \text{WVR} \tag{4}$$

The recharge volume of exogenous groundwater is not a fixed value calculated by the water chemistry method Table 6. At the same time, in order to better observe the proportion of water consumption and the variation, we presented the whole calculation results in the form of histogram (Figure 12). The reason of inconsistency is that the calculation method is to use the surface area to calculate EWC in this paper. The area of the Daihai Lake after 2000 year is about 100 km<sup>2</sup> less than that of 1960s, so the calculation of evaporation loss will also be reduced. However, this value is maintained range 0.89\*10<sup>8</sup> m<sup>3</sup> to 2.68\*10<sup>8</sup> m<sup>3</sup> and the average annual replenishment is 1.81\*10<sup>8</sup> m3, which is close to the 1.8\*10 8m3 calculated by the water chemistry method. The method just measured the recharge amount for one year while our method measured the recharge amount for many years. The experimental results in this paper are more reliable. Finally, the total annual consumption of groundwater is calculated to be an average of 2.0\*108m3. This work quantitatively calculated the recharge of groundwater to Daihai Lake, which provided substantial evidence for groundwater to alleviate the shrinkage of the lake. This annual groundwater consumption is slightly larger than the average annual external groundwater recharge. This imbalance state indirectly proves that lakes are shrinking every year.


**Table 6.** Water consumption and recharge in Daihai Basin (107m3)

1 PPWC: power plant water consumption; 2 WVR: water volume reduction of lake yearly; 3 DGR: direct groundwater recharge; 4 TGC: total groundwater consumption.

Although, we concluded that the area change of the Huangqihai Lake is mainly related to the huge evaporation in the region. Next, this result needs further proof. Because of the irregular lake shape and the lack of the lake water level data, we cannot ge<sup>t</sup> the WVR value. So, we can still explain from other aspects. Based on the agricultural planting data and the classification results of this experiment, we obtained AWC value. Then, TWCHA was estimated by the proportion of agricultural water usage in Ulanchab city for many years. Based on the annual runo ff data of Jining Hydrological Station of Bawang River (Figure 1), it is concluded that the average annual runo ff of the Bawang River has been about 6.3\*10<sup>6</sup> m<sup>3</sup> in the past 30 years. Due to many dams in the upper reaches of the river, the inflow of the river into the lake is much lower than this value and other secondary inflow runo ff is similar. Therefore, the runo ff into the lake can be neglected.

It can be seen from the Table 7 that the water consumption of human activities has been increasing since 1993. In 2001, the unexpected planting events accelerated the consumption of groundwater resources, which prevented the rapid recharge of Huangqihai Lake. In addition, the increase of human activities has also increased the consumption of groundwater resources. Ultimately, the groundwater level will decrease. Because the average depth of the lake was only three m [38], the lake cannot be recharged by groundwater. Besides, saline-alkali land hindered the infiltration of groundwater, which led to the drying up of the lake eventually in 2008. Due to the rapid decrease of lake surface area in the lake, it is obviously di fferent from Daihai Lake and there is no direct groundwater to recharge to it.


**Table 7.** Water consumption and recharge in Huangqihai Basin (107m3)

#### *5.3. Reasons for Lake Degradation*

There are two mainstream views on lake shrinkage. The one is that climate change has mainly led to varying degrees of shrinkage of lakes [39–42], and the other one is excessive human activities is main reason [43]. As we know from the above, the reasons for the decline of these two lakes are climate and perceived factors. Using the above experimental data, we calculated the proportion of the annual human water consumption to the two basins.

Evaporation precipitation difference (EPD) is evaporation minus precipitation (Figure 13). Next, we replaced Daihai Basin's EPD and Huangqihai Basin's EPD with DH EPD and HQH EPD solely. Through the investigation of evaporation and rainfall data, we can clearly find that the DH and HQH EPD value are all positive. In other words, the evaporation has always been greater than precipitation in this area, so the water of lakes is in continuous consumption state. Furthermore, we calculated the area of lakes and Pearson correlation coefficients of EPD for 0.83(DH) and 0.77(HQH). Therefore, it is concluded that evaporation is one of the causes of lake water shrinkage. Because the area of Daihai Lake is not consistent with the trend of broken line of DH EPD before 2000, we thought that the reduction of the Daihai Lake water is not only related to the evaporation in the basin, but also may have other factors after 2000. HQH EPD is different from DH EPD. Compared with Daihai Lake, the changes of the Huangqihai Lake area and the trend of the HQH EPD are more undulate. The evaporation loss value is huge in Huangqihai basin. Since 1984, the EPD has increased and the lake area has gradually decreased. In 1990, the EPD value reached the maximum and the lake area presented, correspondingly, the lowest value in this period. After 1990, the EPD decreased and the lake area gradually increased. In 1999, the EPD reached its peak again and the lake area responded very closely to the change. Later, the EPD values remained low, but it seemed that the lake could not stop shrinking and eventually dried up completely in 2008. Excessive evaporation in the basin was the main reason of the drying up of the lake before 2001, but other factors may be lead to the changes after 2001.

The recharge period of groundwater determined is about 30 years by tritium [44]. Since the seasonal variation of recharge flow has been homogenized for 30 years, the recharge flow rate of external water can be regarded as a constant. Therefore, the source of the lake can be simplified as precipitation and relatively stable external water. Furthermore, precipitation and groundwater are stable for recharge source of the lake, and evaporation is stable for consumption source of the lake in the three decades. So, excluding the above two stabilizing factors, the reduction of two lake area should have other fluctuation reasons. It should be related to the large amount of groundwater pumped for agricultural irrigation, industrial production and domestic water.

Before 1998, the climate change in Daihai basin was the main factor leading to the reduction of Daihai Lake area Table 8. After 1998, with the development of economy and the expansion of population, human activities have become the main factor affecting the change of Daihai Lake, and the situation is getting worse and worse. Comparatively speaking, the factors of Huangqihai Lake are more complex. Among them, human activities consume more water than climate before 2001, and climate consumes more

water than human activities after 2001 except 2006. This conclusion is consistent with the phenomenon of EPD, and indicates that human activities have more influence on the lake changes since 2001. Therefore, on the whole, the shrinkage of the Huangqihai Lake is a combination of human activities and climate.

**Figure 13.** Area, precipitation, and evaporation of the lakes difference in basin.


**Table 8.** Proportion of water consumption in two basins (%)

#### *5.4. Environment E*ff*ects and Measures*

The inland closed lake environment is very sensitive to the feedback of climate change [45], we need to assess the impact of lakes on the surrounding environment. It can be seen Figure 5 that although both lakes have suffered from different degrees of shrinkage, the feedback from the Daihai Lake area to the whole basin is positive because the lake can maintain more water (Figure 14). We calculated these proportions and the proportion of buildings of Daihai basin is 1.19% lower than that of Huangqihai basin average year. In other words, that is the water consumption of agriculture and industry in Daihai basin will be lower than that of Huangqihai basin. The proportion of the total water area of Daihai basin is 2.47% higher than that of Huangqihai basin yearly. That is to say, the Daihai basin has more abundant water resources than the Huangqihai basin. More water resources directly led to an increase 2.64% and 2.70% of the farmland and woodland of Daihai basin compared to the Huangqihai Basin average every year. There is more woodland and farmland, so the proportion of nudation of Daihai basin is 11.80% less than the average of Huangqihai yearly. All in all, more water maintains a better ecological environment in the basins.

**Figure 14.** Proportion of various land types in the basin.

Many lakes are shrinking in arid and semi-arid areas [46]. People try to use various methods to alleviate and even change the process. It is not clear whether the effect of the method is effective or ineffective. We found out the reasons for the shrinkage of lakes, in order to implement effective measures rather than carrying out transformation over rules of nature. Therefore, the behavior of returning farmland and saving local water usage can alleviate the shrinkage of the lake. Fortunately, the local governmen<sup>t</sup> seems to be aware of the crisis of the lake shrinkage and has been carrying out the whole basin activities of returning farmland and woodland since 2016. We still need to observe how it works. Although this plan can alleviate the shrinkage of Daihai Lake, it seems that the surface area of Daihai Lake cannot be restored to former area without more foreign water supply. Hence, the local governmen<sup>t</sup> proposed to divert the Yellow River to supply Daihai Lake. Whether this project will affect the ecological environment of other places once it is implemented deserves our attention.

The results of the above analysis indicated that excessive groundwater extraction for agricultural irrigation and industrial production is the main reason for the lake decline. Although exogenous groundwater recharge to Daihai Lake, it cannot maintain such a huge consumption of people. Let alone Huangqihai Lake, which has no huge amount of exogenous water supplement. Cognac is normal state for the lake in the future. Therefore, reducing or even stopping the extraction of groundwater is an important measure to alleviate this trend.
