The benefits of the water diversion on a densely populated fragmented lake area — A remote sensing view

Water diversion is crucial for maintaining sustainable development in populated lake areas. However, there has been no clear evaluation of how water diversion affects lake health in these areas, which are often limited by scarce in situ observations. To address this issue, we selected Baiyangdian, a fragmented freshwater shallow lake in Xiongan New Area, as our study area. We conducted a time series analysis of Baiyangdian’s water level and area changes, and identified the impact of water diversion on Baiyangdian. We used laser altimetry data (ICESat, ICESat-2), remotely-sensed lake extent, water diversion data, precipitation data, and in situ lake level data. Our results show that water diversion, rather than natural precipitation, has maintained healthy water levels in Baiyangdian in recent years. Baiyangdian’s water level has risen significantly and stabilized, moving from a drying-up level to a healthy level. The intensity of water diversion has increased from 0.91 million m3 in the 2000s to 7.17 million m3 in the 2020 s, with a corresponding water level rise from 4.93 m to 7.14 m, respectively. Moreover, a positive feedback loop between economic development, policy-making, and the lake’s ecological health can promote the sustainable development of both people and nature.


Introduction
Lakes are crucial freshwater reservoirs that are affected by a combination of climatic and anthropogenic factors. Extreme hydrological events, such as severe floods and droughts, often impact the lake area [1]. In addition, rising air temperatures and increased evaporation rates have dramatic effects on the lake [2,3]. Human activities, particularly rapid urbanization and unsustainable agricultural irrigation, have depleted water resources and harmed lake water quality [4]. In extreme water conditions, human activities are becoming increasingly important [5]. Among the various forms of human impact on lakes, water diversion has been proven to be an effective way to alleviate water use issues while simultaneously restoring lake ecological characteristics [6]. Therefore, it is crucial to study the impact of water diversion on lakes, and to gain a clear understanding of how it can keep lakes ecologically safe in densely populated areas.
Changes in lake storage can be studied in terms of the lake area and level, and it is feasible to combine the two to study the trend of lake changes [7,8]. Abundant research methods on lake area include in situ observation and remote sensing. The development of medium and high-resolution satellites in recent years has provided a better understanding of lake change. For example, the study of changes in the lake area and vegetation before and after 2. Study area and data sources 2.1. Study area overview Baiyangdian is located in central Hebei Province, in the Xiongan New Area's hinterland. It belongs to the Daqing River system and is the largest freshwater shallow lake-type wetland in the North China Plain [36]. Baiyangdian's interior is dominated by water bodies interspersed with reed fields, swamps, and villages [37]. Baiyangdian maintains North China's ecological balance, provides a necessary living environment for many plants and animals. Furthermore, Baiyangdian serves as a flood and drought prevention system, as well as a source of water for agricultural irrigation, human consumption, and industry in the Xiong'an New Area [38]. Moreover, Baiyangdian is sensitive to climate change and anthropogenic impacts [39]. The main factors influencing the amount of water entering Baiyangdian are precipitation and water diversion. The ecological water supplement in Hebei Province and Xiong'an New Area has been increased since 2000, to maintain the stability of Baiyangdian lake level and ensure the sustainable development of Baiyangdian and its surrounding environment. A series of water diversion measures have been taken, such as The Yellow River To Baiyangdian Lake Diversion Project and South-to-North Water Diversion Project [40,41]. Figure 1 depicts the study area and laser altimetry satellite point data. Reservoirs such as Angezhuang, Wangkuai, and Xidayang are built in the upper reaches of the Baiyangdian watershed ( figure 1(b)). These reservoirs provide Baiyangdian with a regular water diversion, which is an important part of its overall water diversion. This measured water level data was collected at the Xin'an water level station, Duancun water level station, Wangjiazhai water level station, and Shifangyuan water level station, as shown in figure 1(c).

Data source
The data used in this experiment are ICESat GLA14 product (2003 to 2009), ICESat-2 ATL08 product (from 2018), 30 m Earth surface water data, Sentinel-2A data, measured water level data obtained from four water level stations within Baiyangdian, precipitation data from the Baiyangdian watershed obtained by ERA5-land, and Baiyangdian water diversion data (table 1).
(2) ICESat-2 (Data used: 2018-2022): This experiment uses the parameters of latitude, longitude, time, and elevation values from ATL08, a class 3 product of ICESat-2. We chose the ICESat series of satellites to obtain water level changes in Baiyangdian because the ICESat series is a high-precision satellite-based laser altimetry system that can obtain water surface elevation with an accuracy of about 0.1 m. Additionally, reed fields, marshes, and villages are scattered throughout the interior of Baiyangdian, which has reduced the lake's integrity. The ICESat series of satellites' points are evenly distributed over Baiyangdian, and we can use them to obtain changes in the overall water level of Baiyangdian.  Figure 1(a) shows the location of the Baiyangdian watershed in the Xiongan New Area and its relative position to Beijing and Tianjin. The bottom image is 90 m SRTM DEM, and the Baiyangdian watershed extent data were obtained from the Baiyangdian watershed groundwater dataset [35]. Figure 1(b) shows the laser elevation points within the water body of Baiyangdian Lake. The blue area in the figure is the reference range of Baiyangdian given by the Baiyangdian watershed groundwater dataset. The green and orange points are the filtered laser points of ICESat and ICESat-2, respectively.
(3) Sentinel-2A (Data used: 2018-2022): We selected one Sentinel-2A image each month. We chose to use Sentinel-2A to extract the extent of the lake because its 10 m resolution allows us to obtain more accurate lake extent data. More accurate lake extent can help us extract ICESat-2 points on the lake surface and calculate the lake area. Additionally, Sentinel-2A has a revisit cycle of 10 days, ensuring that we have lake extent and area data within each month.
The 30 m global surface water data (Global Surface Water) dataset: The data is a global surface water dataset produced by Jean-François Pekel et al in an article published in the journal NATURE [42]. This dataset is divided into two phases from 1984 to 1999 and 2000 to 2020 (data are still being updated). This experiment requires access to the extent of the Baiyangdian water bodies from 2003 to 2009 and after 2018. Since a higher resolution water body extent can be extracted using Sentinel-2A after 2015, we selected only the latest version of the monthly global surface water history data from 2003 to 2009 for this experiment. In this experiment, we extract the monthly water body extent by attributes, filter and extract the ICESat laser points for the corresponding time and calculate the lake area.
Baiyangdian water level station actual measurement data: The time period of the measured water level data from the water level stations obtained for this experiment is from 2019 to 2021. The specific dates are the transit dates of the ICESat-2 satellite within the Baiyangdian range. There are four water level stations to obtain the measured water level data, namely Xin'an water level station, Wangjiazhai water level station, Shifanyuan water level station, and Duancun water level station (figure 1(c)). The four water level stations provide data from two different datums, the Dagu Datum and the Yellow Sea Datum, respectively. There is a conversion relationship between the Dagu and Yellow Sea elevation. This conversion relationship is not certain in the current reference data. Here, we take the converted Yellow Sea elevation from ICESat satellite data, compare it with the Dagu Datum corresponding to the station's water level, and find a fixed difference. We use this difference as the conversion constant between the Dagu and Yellow Sea elevations. In order to facilitate the unified analysis, this experiment uses the Yellow Sea Datum as the water level reference surface. In addition, the elevation values measured by ICESat and ICESat-2 will also be converted into the water level values of the Yellow Sea basal plane in this experiment.
ERA5-Land precipitation data of Baiyangdian watershed [43]: ERA5 data is a complete dataset produced by the European Centre for Medium-Range Weather Forecasts (ECMWF) by combining model data with observations from around the world. ERA5-Land is an enhanced global dataset generated by ECMWF for the land component. Compared to ERA5, ERA5-Land has an enhanced spatial resolution of 9 km and the same temporal resolution (1 h). ERA5-Land data from 2000 to 2021 were selected and cropped in this experiment using the Baiyangdian watershed boundary. We calculated monthly and yearly precipitation data using hourly precipitation data from the Baiyangdian watershed.
Baiyangdian Water diversion Data [44]. This experiment was studied from 2003 to 2009 (time of ICESat data) and from 2018 to 2022 (time of ICESat-2 data). The water diversion of Baiyangdian is mainly divided into permanent water diversion from upstream reservoirs such as Wangkuai, Xidayang, and Angezhuang and water supplement from Large-scale water supplement projects to Baiyangdian (the water diversion volumes used in this paper are the volumes of water that eventually enter Baiyangdian from the water supplement project). The overall situation of the water diversion of Baiyangdian can be summarized as follows: there was an occasional water supplement in the 1980s and 1990s; since 1997, the Angezhuang Reservoir began recharging the Baiyangdian; since 2000, the Wangkuai Reservoir began recharging the Baiyangdian; since 2002, the Xidayang Reservoir began recharging the Baiyangdian; in 2004, the Yuecheng Reservoir made a major emergency water transfer; water from the Yellow River started to enter Baiyangdian in 2006; water from the mid-term South-North Water Transfer Project entered Baiyangdian from 2018.

Methods
This study investigates the impact of precipitation and water diversion on the densely populated fragmented lake area of Baiyangdian, using changes in the water level and area of the lake. To analyze the influence factors of Baiyangdian lake, we combined the changes in the water level of Baiyangdian with the area of Baiyangdian lake, precipitation data, and water diversion data. We used ICESat and ICESat-2, the global laser altimetry satellites with higher spatial resolution and richer data than previous satellite altimetry data, to obtain the historical water level of Baiyangdian from 2003 to 2009 and from 2018 to 2022. We combined this with actual measurement data from water level stations to study the water level change. We used the 30 m global surface water dataset (Global Surface Water) and Sentinel-2A to extract the lake water surface to study the area change. We also used ERA5-Land data to obtain the monthly and yearly precipitation in the Baiyangdian watershed and study the influence of precipitation in combination with water level changes. Finally, we used water diversion data from Baiyangdian to study the effects of artificial intervention in conjunction with water level changes.
In this experiment, we used remote sensing data for lake level and area to assess the benefits of water diversion on a densely populated and fragmented lake area. By combining lake level and area, we can determine not only the change in lake storage but also which area is most affected by water diversion in space. We used ICESat and ICESat-2 satellite data to obtain the water level of Baiyangdian Lake. These satellites have higher spatial resolution and richer data than previous satellite altimetry data.
The concept of 'benefit' used in this paper is confined to the improvements in the volume of water in the lake, the internal structure of the lake, the vegetation around the lake, and the urban floor space around the lake. These factors are more related to remote sensing data, as this study focuses on remote sensing.

Validation of satellite-acquired water levels
We used the elevation accuracy evaluation standard root mean square error (RMSE) (equation (1)) to represent the deviation of the ICESat-2 water level and water level station water level, thus verify the accuracy of the laser altimetry satellite.
In the equation, H is the water level data obtained from the water level station corresponding to the satellite transit time, H icesat2 is the calculated average water level of the laser altimetry satellite for the day, and n is the number of laser altimetry satellite points for verification.

Reference indexs for the water level of Baiyangdian Lake
Ecological security can be defined as humanity's degree of assurance unaffected by ecological destruction and environmental pollution in all aspects [45]. In this experiment, we used changes in the water level of Baiyangdian to study the effect of water replenishment on the ecological security of the lake. In Baiyangdian, cultivated land, shallow lakes, and other surface features are distributed at different elevations. We used the drying up water level and healthy water level as reference indices of the multi-year water level changes in Baiyangdian lakes and indicate the improvement of the ecological environment of Baiyangdian lakes.
Drying up water level: When the Baiyangdian lake level falls below 6.5 m (Dagu Datum), the entire Baiyangdian water storage will be less than 6,000 cubic meters, and the Baiyangdian will dry up. The Baiyangdian ecosystem is very fragile at this point, and this water level is called the drying up water level [46]. If the water level decreases to 5.5 m (Dagu Datum), the Baiyangdian wetlands will disappear [47].
Healthy water level: Baiyangdian lakes can maintain a self-regulating ecological environment when the water level reaches 8.5 m (Dagu Datum). At this time, the Baiyangdian lakes and their surrounding ecological environment can complete the normal ecological cycle. This water level is called the healthy water level. However, the water level of the lake cannot keep rising. The warning level of Baiyangdian is 10 m (Dagu Datum), and exceeding the warning level will be a safety hazard [46].

The relationship between social and economic development and healthy water levels
The relationship between healthy water levels and socio-economic development has been demonstrated through the interplay between the main factors influencing lake levels and local GDP. The maintenance of healthy lake water levels is a key prerequisite for ecological development in and around lakes. To investigate the main factors influencing water levels in Baiyangdian, we utilized water diversion data from 2000-2021 to study the degree of anthropogenic influence on Baiyangdian lakes, as well as precipitation data from 2000-2021 in the Baiyangdian watershed to study the degree of natural influence on Baiyangdian lakes. By combining satellitederived lake levels with water diversion and precipitation data, we analyzed the main factors affecting water levels in Baiyangdian, with changes in these factors representing changes in the water levels of the lake. To assess socio-economic development, we employed GDP as an important indicator, reflecting such development. Specifically, we used the change in GDP in Hebei province from 1980 to 2020 to represent the socio-economic development of the region. Finally, we combined the main factors and the changes in GDP in Hebei Province from 1980 to 2021 to analyze the trends in both, with the relationship between the main factors and GDP in Hebei province representing the relationship between healthy water levels and socio-economic development.

Validation of laser altimetry water level using water level station measurements
We selected ICESat-2 water level data near the Duancun water level station to verify its accuracy (see figure 2). Table 2 displays the time and number of validated laser points. Over eight days, a total of 164 ICESat-2 validation points were recorded. The Duancun water level station data served as the true value. We obtained a relationship between the water level at the Duancun water level station and the ICESat-2 water level ( figure 2(b)). The R 2 between the two is 0.567, and the RMSE of the ICESat-2 satellite observed water level was 0.111 m. These results demonstrate that ICESat-2 satellite altimetry can achieve relatively high accuracy in measuring the water level of Baiyangdian Lake.
As water level station data was only available after 2018, we drew insights from previous studies to validate ICESat-2's accuracy. Prior research has confirmed that the ICESat series satellites perform well in acquiring lake water levels, and our results further reiterate their high accuracy in measuring Baiyangdian Lake's water level [48,49].
In contrast to the water level station data, ICESat-2 validation points are more uniformly distributed in spatial coverage (figure 2(a)). Although daily water level data obtained by ICESat-2 exhibit some extreme differences, the data can better reflect the entirety of the lake. The smallest difference was 0.417 m on 2021/8/12,  Figure 2(a) shows the laser elevation points within the water body of Baiyangdian Lake. Figure 2(b) shows the relationship between the measured water level (Duancun water level station) and laser point water level (ICESat-2, laser spots within the red dashed line in figure 2(a)). The fitted equation, coefficients of determination, 95% confidence bands, and 95% prediction bands are given in the figure.

Precipitation trend of baiyangdian watershed
The There are certain differences between the measured values of the four Baiyangdian water level stations (mean value of 0.977 m). However, these differences gradually decrease over time. Among the four water level stations, the Xin'an water level station had the lowest water level, while the Wangjiazhai water level station had the highest. The minimum difference between the water level stations is 0.445 m, and the maximum is 1.285 m. The water level difference between the stations gradually decreased in 2021 and has been reduced to 0.524 m by September of that year. The difference between the water level of each hydrological station is closely related to the water diversion. The water diversion in 2021 was 1.73 billion m3, which is significantly higher than the average annual water diversion of 125.6 million m3 from 2000 to 2020 ( figure 7). Additionally, the lake area in September 2021 increased by 5.73 km2 compared to 2020 ( figure 5(a)). The waterbody in the northern and central parts of Baiyangdian was almost connected in 2021 ( figure 5(b)), which led to closer water levels between these two areas.
The area of Baiyangdian lakes has been steadily increasing overall. Specifically, the area of the lakes has increased in the pre-flood months of 2000, 2005, 2010, 2015, and 2020. The change in the area between 2000, 2005, and 2010 was more pronounced than the change in extent in 2015 and 2020 (figures 6(a) to 6(e)). The most significant changes in Baiyangdian during these five years occurred in the west (Zaoza Lake) and north (Shaoche Lake). Additionally, the minimum and maximum values of the Baiyangdian lake area from 2019 to 2021 have increased compared to those from 2003 to 2009. The extreme difference in intra-year lake area of Baiyangdian decreases year by year between 2019 and 2021 (figure 4). Since the lake area and water level in Baiyangdian exhibit the same trend (figure 6(f)), the change in the lake area can reflect the change in the lake level. The expansion of the lake area indicates that the lake water level is increasing overall. Furthermore, the decrease in the extreme difference in lake area within a year suggests that the lake level of Baiyangdian is becoming more stable.

The effect of increased water diversion on the water level in Baiyangdian
The Baiyangdian water level is affected by both precipitation and water diversion, but water diversion has the greatest impact on the elevated water level of Baiyangdian. Between 2004 and 2009, the water level of Baiyangdian was maintained at the drying up water level. However, after 2018, the water level of Baiyangdian jumped to around a healthy level and was significantly elevated (figure 7). The annual average water diversion increased from 91 million m3 between 2004 and 2009 to 717 million m3 between 2018 and 2022. In contrast, the average annual precipitation of the Baiyangdian watershed for the two periods was only slightly different (580 mm and 603 mm), indicating that the increase in water diversion is the main reason for the rise in the water level of Baiyangdian lakes.
While precipitation is also important in maintaining healthy water levels in Baiyangdian, it is influenced by seasonal factors. For example, precipitation in the Baiyangdian watershed was only 469 mm in 2019, which is insufficient ( figure 3). Despite the entry of 308 million m3 of water diversion water in mid-2019, the water level after the flood season still dropped below the healthy water level line.
With the increase in water diversion, the water level of Baiyangdian has recovered to a higher level and stabilized in recent years. The overall Baiyangdian water level shows an increasing trend from the long-term Baiyangdian lake level obtained in this experiment. In addition, the standard deviations of water levels from illustrates the greater stability of Baiyangdian lake levels following the increased anthropogenic intervention. The lake's water level is maintained at a healthy level and remains stable, benefiting the lake and its surrounding ecology.
5. Discussion 5.1. Advantages and challenges of using laser altimetry satellites to analyze lake water levels Laser altimetry data has unique advantages and can complement water level station data. The traditional method of lake level detection is using the water level station ground observation system. The advantage of water level station observation is its continuous observation, high accuracy, and high temporal resolution, making it a vital reference for studying the trend of lake changes [50]. However, water level station data has its shortcomings. The fragmented lake weakens the connectivity between parts, and the stations are often widely spaced, leading to poor spatial coverage. Bandini et al mentioned in their paper that water level stations cannot satisfy larger hydrological observations, and remote sensing data sets are receiving more attention in the field of hydrology [51].
In this experiment, laser altimetry satellite points are more uniformly distributed throughout Baiyangdian (figure 1). The laser altimetry satellite can better capture trends in the water level of the entire Baiyangdian lake. Additionally, the accuracy of laser altimetry satellite observations is more suitable for water level observation. Therefore, this experiment chose to use laser altimetry data to analyze water level changes in Baiyangdian. However, laser altimetry satellite observation has a certain repetition period, which limits its ability to obtain continuous water level observations with a small temporal resolution like water level stations.
In summary, laser altimetry data can effectively monitor water level changes in the lake and serve as supplementary data to water level observations by water level stations.

Necessity of water diversion for ecological restoration of Baiyangdian lake
The characteristics of the intra-annual water level change in Baiyangdian differ from those of natural lakes without human regulation. This is mainly due to the influence of water diversion. For instance, the change in the area of Baiyangdian tends to be smaller in the flood season and larger in the non-flood season, as shown in figure 4. As a result, the annual flood water level of Baiyangdian is lower than the non-flood water level. However, Zohary T et al stated that the highest water level exists in natural lakes at the end of the rainy season or snowmelt. The lowest water level occurs at the end of drought [52]. This indicates that the trend of intra-annual water level change in natural lakes is the opposite of that in Baiyangdian.
Water diversion is important in maintaining healthy water levels in the Baiyangdian watershed, where precipitation has been decreasing and unstable for many years. Before water diversion projects were frequently implemented, the water level in Baiyangdian was mainly determined by precipitation with much uncertainty. This led to notable fluctuations in the water level of Baiyangdian. For example, Cheng et al indicated that the Baiyangdian lake area fluctuated greatly from 1984 to 2003, with dry spells in 1981-1988, 1994, 2002, and 2003 [53].
In this experiment, there was less water diversion and less artificial intervention from 2004 to 2009 compared to 2018-2022. Therefore, the standard deviation of the lake level was larger and less stable. Drastic changes in lake levels can impact the growing environment for plants and animals, affect the biodiversity around the lake, and increase the likelihood of extreme hydrological events such as droughts and floods, which can be harmful to humans [54,55].
Moreover, Sun et al concluded that precipitation in the North China Plain had decreased significantly at a rate of 5.1 mm per year since 1960 [56]. This decrease in precipitation may result in the natural water supply of Baiyangdian being insufficient to meet its water storage needs. In summary, the decrease and instability of precipitation in the Baiyangdian watershed can lead to low water levels and even dry lakes in Baiyangdian. Water diversion is necessary to keep the water level of Baiyangdian at a healthy level and to store water to meet water requirements.

Positive feedback relationship between human socioeconomic development and healthy water levels in lakes
Humans can take the initiative to regulate water resources in a certain area, improve the ecological environment, and promote socio-economic development. Since 1980, the GDP of Hebei Province, where Baiyangdian is located, has grown rapidly. Rapid economic development can provide an economic basis for artificially regulating the ecological environment of lakes. During this period, the water diversion of Baiyangdian gradually increased, and the form became more abundant (see figure 8). From 1980 to 1999, there was almost no water diversion in Baiyangdian. There were only a few small-scale recharges from upstream reservoirs when Baiyangdian was a natural lake with little human intervention. Since 2000, the Yue Cheng reservoir, the Yellow River, and the mid-term South-North Water Transfer Project have gradually started to transfer water to Baiyangdian. In particular, after the establishment of the Xiongnu New Area in 2017, the ecological water replenishment of Baiyangdian was raised to a new order of magnitude, even reaching 1.73 billion m3 of water in Baiyangdian in 2021. The lake's character is gradually changing, and anthropogenic factors now dominate Baiyangdian.
The increase in water diversion has maintained the Baiyangdian water level at a healthy level and improved the ecological environment of the lake. Other scholars have also illustrated the improvement of the ecological environment of Baiyangdian Lake in their studies. For example, Zhao et al mentioned in their paper that since the construction of the Xiongan New Area, the increase in water diversion and the variety of water sources entering the precipitation have maintained the lake water and groundwater at healthy levels [57]. In their article, Wang et al illustrated that the period from 1989 to 2000 was the degradation period of the ecological water level of Baiyangdian Lake, and the period from 2000 to 2018 was the stabilization and restoration period [30]. Not only Baiyangdian, but there are also many other examples of similar positive effects of water diversion on lakes. For example, Wu et al analyzed the effects of climate change and human activities on Taihu Lake. Then, they made recommendations for the diversion project of the Yangtze River to Taihu Lake and urban regulation [5]. Dai's study on the changes in the lake phytoplankton community after water transfer in Taihu Lake showed that seasonal water transfer activities reduced the concentration of organic pollutants and promoted phytoplankton diversity in Taihu Lake [58].
Positive human intervention can maintain the ecological health and stability of a certain area. A healthy and stable ecological environment can provide better living conditions for residents, a better growing environment for plants and animals, and promote the development of tourism, animal husbandry, and other industries. In addition, we should also note that national policies have dominated the formation of healthy water levels in Baiyangdian in recent years.
5.4. The benefits of water diversion for the densely populated fragmented lake area and their specificities A fragmented lake is distinct from an ordinary lake and cannot be considered as a whole. This experiment used Sentinel 2a and the lake remote sensing dataset to obtain changes in the lake area of Baiyangdian and the ICESat series of satellites to obtain changes in water levels. We took advantage of the remote sensing data to identify two particular benefits of water diversions to the densely populated and fragmented lake area: (1) There is spatial heterogeneity in the impact of water diversions on the extent and lake level of densely populated and fragmented lake areas. Our analysis of changes in the lake area found that water diversion affects the small lakes in Baiyangdian to varying degrees. For example, Baiyangdian's lake area is on an upward trend, with changes in lake area more pronounced in the west and north than in other regional lakes ( figure 6). We also compared the spatial heterogeneity of lake levels. We found that the water levels in the northern Baiyangdian lakes were mostly higher than those in the western lakes at the same time. The change in mean water level in the northern Baiyangdian lakes rose by 0.56 m more in two periods, 2003-2009 and 2018-2021, compared to the southwestern lakes ( figure 9(b)). Therefore, we speculate that the heterogeneous effects of water diversion on the various regions of the Baiyangdian may bring about uneven development. To further investigate this, we compared Landsat-8 imagery from May over two years, 2017 and 2021, and found that the greatest changes in urban floor area occurred where lake water growth was more pronounced in the west and north.
(2) Water diversion has increased connectivity between fragmented lakes. In 2021, the amount of water diverted was 1.73 billion m 3 , much higher than in previous years. Using lake extent data extracted from Sentinel 2a for 2021 and September 2021, we found that the lake area of Baiyangdian increased by 5.73 km 2 in 2021, and connected rivers appeared between small lakes in the central and northern parts of Baiyangdian ( figure 5). The increased connectivity of the lakes also improves their stability. From 2019 to 2021, the extreme difference in lake area within Baiyangdian decreased annually, and the lake extent became more stable ( figure 4). The standard deviation of water levels from 2003 to 2009 and from 2018 to 2022 is 0.309 m and 0.189 m, respectively, indicating that the lake level is becoming more stable ( figure 7). Overall, the increased connectivity of the lakes allows multiple regional water systems to complement each other, resulting in more stable lake levels and increased resilience to extreme hydrological events.
Water diversion has brought a variety of ecological impacts to the densely populated fragmented lake area. From 2004 to 2009, the water level of Baiyangdian had increased, but it remained around the lower dry lake level, fluctuating widely. Furthermore, the Baiyangdian Lake area was dry in 1981-1988, 1994, 2002, and 2003 [53], with natural factors being the main drivers of fluctuation. With increased water diversion, the water level of Baiyangdian has risen from the dry lake level to the healthy lake level and a more stable level from 2018-2022. The increase in the volume of water in the lake has positively impacted the lake's ecology. For instance, the vegetation cover in Baiyangdian has increased, and vegetation growth has become more vigorous, with the rate of increase in NDVI in the Baiyangdian watershed from 2001 to 2018 being 0.0031/a. Additionally, the NDVI of 65.46% of cultivated plants (crops) in the Baiyangdian watershed has shown an increasing trend, promoting prosperous agriculture in the area [59]. The number of species of aquatic vegetation, phytoplankton, zooplankton, macrobenthos, and fish in Baiyangdian has all shown a trend of first increasing and then decreasing, with continuous droughts damaging the biological growth environment and reducing species diversity. However, with the entry of water diversion and government management of the environment, biodiversity is gradually recovering [36]. Therefore, water diversion can restore ecological security in the densely populated fragmented lake area.
All of the above viewpoints, except for the description of biodiversity, are derived from remote sensing data. Remote sensing data has better spatial coverage compared to ground-based observations and can detect changes in the amount of water in the lake, as well as in the surrounding vegetation and urban area. This experiment suggests a reference method for studying the impact of water diversion in other similar regions. In addition, we can also draw on the findings of other studies to obtain changes in and around Baiyangdian. For example, between 2017 and 2021, the average annual PM2.5 concentration in Xiong'an New Area dropped from 67 μg/m 3 to 45 μg/m 3 , the resident population in Xiong'an New Area grew from 1092400 to 1259,600, and the water quality of Baiyangdian improved from Class V to Class III [60][61][62]. These conclusions cannot be drawn only from remote sensing data. It could be further studied with possible aids from other source data, such as water quality measurements and economic data.

Conclusion
In this study, we analyzed changes in Baiyangdian lakes using Baiyangdian lake level and area data, Baiyangdian watershed precipitation data, Baiyangdian water diversion data, and water level station measured water level data. We found that water resources in densely populated lake areas are influenced by both natural climatic and anthropogenic factors, with the latter being the main factor.
Specifically, we observed a significant increase in the annual average water diversion in Baiyangdian, from 91 million m3 (2004-2009) to 717 million m3 (2018-2022). This increase resulted in a rise in the average water level from 4.93 m to 7.14 m, respectively, a difference of 2.21 m. Water diversion had a more pronounced effect on the western and northern parts of Baiyangdian. The increase in water diversion raised the water level from a drying-up level to a healthy level. The larger standard deviation in lake levels between 2004 and 2009 illustrates the greater fluctuations in water levels and the poor stability of the lake in the absence of human intervention. In contrast, water diversion can strongly maintain the long-term stability of the lake water level and promote the ecological recovery of the lake. Additionally, we observed that water diversions increase the connectivity of the lake, and that there is spatial heterogeneity in their impact on the lake.
Our study suggests that combining the lake's individual circumstances with an appropriate water diversion plan is a realistic strategy to improve the lake's and its surroundings' ecological environment. Positive human intervention can make the lake's inter-and intra-annual water level more adequate and stable, improving the lakes' water storage capacity and risk resistance, and ultimately enhancing the lakes' ecological environment and their surroundings. This virtuous circle has a strong promotion effect on the lake and human society. In the equation, ( ) Band Green indicates the green light band, and ( ) Band NIR indicates the near-infrared band. This method mainly uses the different spectral reflection characteristics of water bodies and vegetation in the green and NIR bands to extract water bodies. After obtaining the extent of the water body, we can find the area of the water body at the corresponding time.
6.2. ICESat and ICESat-2 extract the water level of Baiyangdian lake Longitude, latitude, time, and elevation are the same parameters extracted from ICESat and ICESat-2 for this experiment. ICESat and ICESat-2 elevation parameters are geodetic heights with TOPEX/Poseidon ellipsoid and WGS84 ellipsoid as reference ellipsoid, respectively. To convert geodetic heights into water level data based on the base surface of the Yellow Sea elevation by calculation, the parameters of the difference between the TOPEX/Poseidon ellipsoid and the geoid must also be extracted. Equations (3) and (4)  In the equations (2) and (3), H icesat and H icesat2 are the calculated water levels, H elev and H fit are the laser point elevation measurements obtained from ICESat and ICESat-2, H gdHt is the difference between the TOPEX/Poseidon ellipsoid and the geoid, and 0.705 is the difference between the TOPEX/Poseidon ellipsoid and the WGS84.