Xiaolangdi Reservoir is a key control project to control the water and sediment in the lower Yellow River, and a timely and accurate grasp of the reservoir’s water storage status is essential for the function of the reservoir. This study used all available Landsat images (789 scenes) and adopted the modified normalized difference water index, enhanced vegetation index, and normalized difference vegetation index to map the surface water from 1999 to 2019 in Google Earth Engine (GEE) cloud platform. The spatiotemporal characteristics of the surface water body area changes in the Xiaolangdi Reservoir in the past 21 years are analyzed from the water body type division, area change, type conversion, and the driving force of the Xiaolangdi water body area changes was analyzed. The results showed that (1) the overall accuracy of the water body extraction method was 98.86%, and the kappa coefficient was 0.96; (2) the maximum water body area of the Xiaolangdi Reservoir varies greatly between inter-annual and intra-annual, and seasonal water body and permanent water body have uneven spatiotemporal distribution; (3) in the conversion of water body types, the increased seasonal water body area of the Xiaolangdi Reservoir from 1999 to 2019 was mainly formed by the conversion of permanent water body, and the reduced permanent water body area was mainly caused by non-water conversion; and (4) the change of the water body area of the Xiaolangdi Reservoir has a weak negative correlation with natural factors such as precipitation and temperature, and population. It is positively correlated with seven indicators such as runoff and regional gross domestic product (GDP). The findings of the research will provide necessary data support for the management and planning of soil and water resources in the Xiaolangdi Reservoir.
Surface water refers to rivers, lakes, ponds, reservoirs, swamps, glaciers, and other water bodies that exist on the Earth’s surface. They are tremendously important water resources for agriculture, aquaculture, industrial production, and terrestrial ecosystems. Xiaolangdi Reservoir is located in the key position of controlling water and sediment in the lower reaches of the Yellow River, and it is the only control project under Sanmenxia that can obtain large storage capacity [1,2,3]. Xiaolangdi Reservoir is a comprehensive water control project for flood control, ice jam prevention, sediment reduction, runoff regulation, water supply, and power generation in the lower reaches of the reservoir [1,2,4]. It has superior natural conditions and an important strategic position. Timely and accurate acquisition of the reservoir water storage situation is essential to the function of the reservoir [1,3,5,6]. The changes in the scope and area of the Xiaolangdi Reservoir can reflect the comprehensive role of climate and human activities in the water cycle and ecosystem .
Water body extraction methods include water body extraction algorithms based on band combination (single-band threshold method, multiband spectral relation method, water index method, and threshold method) and machine learning algorithm (support vector machines, random forest, deep learning) [44,45,46,47]. The water body index can quickly obtain the distribution range of the water body through simple band calculation and threshold processing. For example, Mefeeters et al.  proposed the normalized difference vegetation index (NDVI) index, which suppresses vegetation and highlights water information but the suppression effect in buildings was not good. Xu  proposed that the modified normalized difference water index (MNDWI) can reduce the interference of soil and buildings, but it is not effective in distinguishing water bodies from vegetation. This study uses a combination of MNDWI, normalized difference vegetation index (NDVI) , and enhanced vegetation index (EVI), which can eliminate land noise and decrease wetlands mixed with water bodies and vegetation to improve the accuracy of the water body index.
2 Materials and methods
2.1 Study area and data
2.1.1 Study area
Xiaolangdi Reservoir is the last Gorge Reservoir in the middle reaches of the Yellow River (Figure 1a); it is 40 km north of Luoyang City, 130 km from the Sanmenxia water control project, and 128 km from Huayuankou Station in Zhengzhou, Henan Province (Figure 1b–d) [1,4,5,6,7,32]. Xiaolangdi Reservoir is a large-scale comprehensive water conservancy project, which mainly focuses on flood control, ice jam prevention, and silt reduction, as well as water supply, irrigation, power generation, harm elimination, and benefit elimination [4,6]. The main project of the Xiaolangdi Reservoir started in September 1991, began to store water in October 1999, and completed the main project by the end of 2001. The total storage capacity of the Xiaolangdi Reservoir is 12.65 × 108 m3, the long-term effective storage capacity is 5.1 × 108 m3, and the control basin area is 69.4 × 104 km2, accounting for 92.3% of the Yellow River Basin Area [3,5]. Xiaolangdi Reservoir’s average annual accumulated precipitation from 1999 to 2019 was 613 mm, and the annual average temperature was 14.14°C.
2.1.2 Data sources
This study used all available Landsat TM, ETM+, and OLI surface reflectance images of the entire Xiaolangdi Reservoir in the GEE platform from 1999 to 2019. The Landsat TM and ETM+ surface reflectance datasets were generated from the Landsat ecosystem disturbance adaptive processing system algorithm, and the Landsat OLI surface reflectance products were generated from the Landsat surface reflectance code algorithm [51,52]. For each scene image, the function of the mask algorithm method is used to mask the cloud, cloud shadow, and snow pixels in the data. The high-quality image data after the mask has a good effect and is suitable for change detection of the land satellite data. The total number of these images was 789, including 224 Landsat TM from 1999 to 2013, 326 Landsat ETM+ from 1999 to 2019, and 239 Landsat OLI from 2013 to 2019. The spatial distribution (Figure 2a and b), temporal distribution (Figure 2c), and seasonal distribution (Figure 2d) of the Xiaolangdi Reservoir total observation count from 1999 to 2019.
Moreover, the Sentinel-2 images with a spatial resolution of 10 m were used to evaluate the accuracy of the extracted water bodies in the Xiaolangdi Reservoir. The 3 h Global Land Data Assimilation System (GLDAS) from 1999 to 2019 with a spatial resolution of 0.25° was used to analyze the temporal trend of precipitation and temperature in the Xiaolangdi Reservoir . The 2010 Global 30 m land cover remote sensing data product (GlobeLand30) (http://www.globallandcover.com) was used to classify water bodies and non-water bodies [54,55].
2.2.1 Water body extraction algorithm
The study used an MNDWI (equation (1)), NDVI (equation (2)), and EVI (equation (3)). If a pixel meets the following criteria: EVI < 0.1 and (MNDWI > EVI or MNDWI > NDVI), then it was classified as a water body [14,16,36,56]:
where ρ Red, ρ Green, ρ Blue, ρ Nir, and ρ Swirl are the reflectance of the red band (0.63–0.69 μm), green band (0.52–0.6 μm), blue band (0.45–0.52 μm), near-infrared band 1 (0.77–0.9 μm), and shortwave infrared band 1 (1.55–1.75 μm), respectively.
2.2.2 Change analysis of the surface water body
According to the long-term surface water bodies in the Xiaolangdi Reservoir, the water frequency (WF, equation (1)) ranges from 0 to 100%, According to WF, surface water body can be divided into three types: maximum water body (WF ≥ 25%), seasonal water body (25% < WF ≤ 75%), and permanent water body (WF ≥ 75%) [14,36,40].
where represents the number of times that all images were identified as water bodies during the year, and represents the number of high-quality images during the year.
2.2.3 Dynamic degree of the surface water body
The changes in the surface water body mainly include type, area, dynamic degree, and time and space. The dynamic degree of a single surface water body expresses the change of a given surface water body type within a certain period. The surface water body change transfer matrix describes the surface water body area change .
The dynamic degree of a single surface water body is expressed by the formula (equation (5))(5)
where S a and S b are the areas of surface water body types at the beginning and end of the study, and T is the length of the study period.
Surface water body transfer matrix.
The transfer matrix of the surface water body is the mutual conversion relationship between surface water body types in the same area in different periods. By establishing the transfer matrix of the surface water body in two periods, the area changes in different surface water body types in this period are analyzed (equation (6)):
where S ij is the state of the surface water body at the beginning and end of the study, and n is the number of surface water body types.
2.2.4 Accuracy assessment
The confusion matrix is widely used in the accurate evaluation of remote sensing classification data, and it is an important method for the accurate evaluation of the surface water body data . The confusion matrix with the producer accuracy (PA, equation (7)), the user accuracy (UA, equation (8)), the overall accuracy (OA, equation (9)), and the Kappa coefficient (Kappa, equation (10)) are as follows:
where S total is the sum of correctly classified pixels, n is the sum of validation pixels, r is the number of rows, is the observation in row i, column j; is a marginal total of row i; and is a marginal total of column j.
3.1 Accuracy of water body mapping in the Xiaolangdi Reservoir
In this study, Sentinel-2 MSI images with a spatial resolution of 10 m were used to verify the water body extraction results of Landsat 30 m spatial resolution. We have considered the 2 km buffer zone of the Landsat Xiaolangdi Reservoir in 2019 as the verification area to ensure the uniform distribution of verification points in water and non-water bodies. We used 2010 Global 30 m land cover remote sensing data product (GlobeLand30) (http://www.globallandcover.com) for the classification of data. The surface cover was divided into the water body and non-water body, and 5,000 verification points were randomly generated, including 1,072 water and 3,928 non-water verification points (Figure 3). Table 1 shows the water body extraction accuracy of the Xiaolangdi Reservoir. The overall accuracy was 98.86%, the producer’s accuracy was 95.20%, and the Kappa coefficient was 0.96. It shows that the water body extraction accuracy was relatively high and can be further analyzed.
|Total||1,021||3,979||5,000||OA = 98.86%|
|PA||95.20%||99.80%||Kappa = 0.96|
3.2 Spatiotemporal distribution of the maximum water body in the Xiaolangdi Reservoir
3.2.1 Inter-annual variation of the maximum water body area
The maximum water body area of the Xiaolangdi Reservoir changes greatly from 1999 to 2019. Figure 4 shows that the minimum value of the maximum water body area of the Xiaolangdi Reservoir from 1999 to 2019 was 29.14 km2 (1999), and the maximum was 241.41 km2 (2017), with an area ratio of 1:8.28 and a difference of 212.27 km2. From 1999 to 2005, the area of the maximum water body showed an upward trend of 30 km2/year. From 2005 to 2019, the area of the maximum water body showed a relatively flat change, showing an upward trend of 2.57 km2/year. Since the construction of the Xiaolangdi Reservoir was completed at the end of 2001, the area of the maximum water body around 2002 was quite different. The average water body area was divided into two phases: 1999–2019 and 2002–2019. This study was consistent with the changing trend of the Xiaolangdi water body researched by Yang et al. .
3.2.2 Monthly variation of the maximum water body area
Figure 5a and b shows the monthly distribution of the average maximum water body from February to July and August to January from 2002 to 2019. The maximum water body area of the Xiaolangdi Reservoir has a minimum value of August (106.72 km2) and a maximum value of March (230.52 km2). The area ratio was 1:2.16, with a difference of 123.8 km2. It shows that the area of the largest water body fluctuates greatly during the year.
3.3 Spatiotemporal distribution of the seasonal and permanent water bodies in Xiaolangdi Reservoir
Before and after the completion of the Xiaolangdi Reservoir, the seasonal and permanent water bodies have undergone major changes. Before the completion of the Xiaolangdi Reservoir in 1999, the seasonal water body area and permanent water body area were the smallest, 10.51 and 18.64 km2, respectively (Figure 6a). After the completion of the Xiaolangdi Reservoir in 2019, the seasonal water body area and permanent water body area were 61.39 and 145.76 km2, respectively (Figure 6b). Table 2 shows the comparison of the water body area changes in the Xiaolangdi Reservoir in the four periods of 1999–2004, 2005–2009, 2010–2014, and 2015–2019 in this study. The maximum of seasonal and permanent water body area was 2015–2019, and the minimum of seasonal and permanent water body area was 1999–2004. The area of seasonal and permanent water bodies from 1999 to 2019 showed an upward trend, with an increase of 41.32 and 85.35%, respectively. The area of the permanent water body has increased significantly, and the total water body area of the Xiaolangdi Reservoir shows an upward trend, with an increase of 74.29%.
|Type||Area (km2)||Percentage change|
3.4 Conversions of water bodies in the Xiaolangdi Reservoir
Seasonal and permanent water bodies are wetland ecologically staggered zones, and changes in water bodies are indispensable for the movement, living range, and migration of species, and for sustainable development. Figure 7a and b shows the conversion among non-water body, seasonal water body, and permanent water body in the Xiaolangdi Reservoir. During the period of 1999–2004 and 2005–2009, the increased seasonal water body was mainly transformed from the permanent water body, with a conversion area of 7.29 km2, of which 10.42 km2 of the seasonal water body was converted into a non-water body. In the period 2005–2009 and 2010–2014, the increased seasonal water bodies were mainly the conversion of permanent water bodies. The conversion area was 6.70 km2, and only 3.29 km2 of non-water bodies were converted into seasonal water bodies. The increased permanent water body was mainly transformed from non-water body and seasonal water body, with conversion areas of 2.69 and 2.27 km2, respectively. During the period of 2010–2014 and 2015–2019, the increased seasonal water bodies were mainly due to the transformation of permanent water bodies, with a conversion area of 1.98 km2, and only 0.96 km2 of non-water bodies converted into seasonal water bodies (Figure 7c). The seasonal water body area increase from 1999 to 2019 was 15.97 km2, which was mainly formed by the permanent water body transformation. The reduced area of seasonal water bodies was 14.80 km2, of which 67.56% of seasonal water bodies were converted into non-water bodies; the reduced permanent water body area was 19.53 km2, of which 75.39% was caused by non-water bodies.
Many factors affect the water body area changes in the reservoir, which are mainly the changes in precipitation and recharge water in the area and the influence of water consumption for production and domestic use. To determine the changes in the water body area and related factors of the Xiaolangdi Reservoir, we selected 11 indicators that may affect the water body area change (Table 3). It includes three items of nature and seven items of human factors. Pearson correlation analysis is carried out by using SPSS software. The spatial scope includes the Henan Province (Mengjin County, Xin’an County, Mianchi County, and Jiyuan City) and the Shanxi Province (Xia County and Yuanqu County) around the reservoir. The natural factor data are derived from the GLDAS data in the GEE platform and the annual runoff data are from the Yellow River Conservancy Commission of the Ministry of Water Resources. The data of anthropogenic factors mainly refer to the statistical yearbooks of Henan and Shanxi provinces from 1999 to 2019 to analyze the correlation between the reservoir body area and the main economic and social indicators.
|Water body area of the Xiaolangdi Reservoir|
|Gross value of the primary industry||0.684***||0.001||21|
|Gross value of the secondary industry||0.700***||0.000||21|
|Gross value of the tertiary industry||0.532**||0.013||21|
|Per capita GDP||0.661***||0.001||21|
|Grain sown area||0.237||0.301||21|
Note: “**” and “***” Significance levels at 0.05, and 0.01, respectively. N = 15 (2005–2019); N = 21 (1999–2019).
As shown in Table 3, (1) the water body area of the Xiaolangdi Reservoir is weakly, negatively correlated with precipitation and temperature, which shows that the influence of natural factors on the change of the water body area is not obvious. (2) The annual variation of runoff of the Xiaolangdi water station shows that the average runoff from 1987 to 2015 is 244.4 × 108 m3, the runoff from 2005 is 221.3 × 108 m3, and the runoff from 2019 is 459.2 × 108 m3. The runoff into the reservoir increased gradually from 2005 to 2019. The direct reason for the increasing area of the Xiaolangdi Reservoir in recent 20 years is the gradual increase of the runoff into the reservoir. (3) There was a weak positive correlation between the sown area of grain and the water body area. The correlation analysis showed that the increase of the sown area of grains and the water body area were associated, which indicated that the important reason for the change of the water area was the change of the cultivated area. (4) With the rapid development of society and economy, the demand for water resources in the Xiaolangdi area is increasing. Irrigation accounts for most of the total water withdrawals because wheat and corn are the main crops, and irrigation requires a lot of groundwater. The total population of the Xiaolangdi area in 2019 is nearly 74,000 less than that in 1999. There is a positive correlation between total population and water consumption. The decrease of population means the decrease of water consumption, which is the indirect driving force of the increase of water body area; (5) the water body area of the Xiaolangdi Reservoir is strongly and positively correlated with the gross domestic product (GDP), the gross value of the primary industry, the gross value of the secondary industry, the gross value of the tertiary industry, and per capita GDP of the Xiaolangdi area. The GDP of the Xiaolangdi Reservoir area was 30.83 × 108 yuan in 1999, 113.17 × 108 yuan in 2010, and 157.72 × 108 yuan in 2019, which is about five times that of 1999. After the completion of the Xiaolangdi Reservoir, the secondary industry developed rapidly, accounting for 58.81% in 1999, 68.18% in 2010, and 80.13% in 2019. The development of the Xiaolangdi tourism industry has driven the development of the surrounding economy, increased the surrounding fiscal revenue, promoted local employment, and improved the living standard of surrounding residents.
This study investigated the inter-annual and intra-annual spatial changes in the water body area in the Xiaolangdi Reservoir from 1999 to 2019 based on all available Landsat TM, ETM+, and OLI images in the GEE platform. The overall accuracy of Xiaolangdi Reservoir’s water body extraction was 98.86%, the producer’s accuracy was 95.20%, and the Kappa coefficient was 0.96, indicating that the water body extraction results are more accurate and suitable for water body extraction in the Xiaolangdi Reservoir. The water bodies were classified into the maximum water body, seasonal water body, and permanent water body according to the inundation frequency. The maximum water body area of the Xiaolangdi Reservoir varies greatly between inter-annual and intra-annual, and seasonal and permanent water bodies have uneven spatiotemporal distribution. In the conversion of water body types, the seasonal water body area increased by the Xiaolangdi Reservoir from 1999 to 2019 was 15.97 km2, which was mainly formed by permanent water body conversion, and the reduced permanent water body area was 19.53 km2, of which 75.39% was caused by non-water conversion. The change of the water body area of the Xiaolangdi Reservoir has a weak negative correlation with natural factors such as precipitation and temperature, and population, and it was positively correlated with seven indicators such as runoff and regional GDP.
The results of this study provide a scientific basis for the ecological protection, water resources management, and decision-making planning of the Xiaolangdi Reservoir in the Yellow River Basin. However, there are some shortcomings. The mixed pixels at the edge of the water body are the main reason for the classification error of the pixels. The next step can be combined with deep learning, neural network, and other algorithms for water body research.
We thank the editors and the anonymous reviewers for their valuable comments and suggestions.
Funding information: This research was funded by “Henan Provincial Department of Science and Technology Research Project (212102310019),” “Natural Science Foundation of Henan (202300410531),” “Youth Science Foundation Program of Henan Natural Science Foundation (202300410077),” “the major project of Collaborative Innovation Center on Yellow River Civilization jointly built by Henan Province and Ministry of Education (2020M19),” “College Students’ Innovative Entrepreneurial Training Plan Program (202010475141),” “Dabieshan National Observation and Research Field Station of Forest Ecosystem at Henan,” and “Data Center of Middle & Lower Yellow River Regions, National Earth System Science Data Center, National Science & Technology Infrastructure of China (http://henu.geodata.cn, accessed on 14 October 2020).”
Author contributions: This research was carried out under the cooperation of all authors. H.X., and T.C. provided the writing ideas for the research; R.W., and H.X. completed data collection, analysis and wrote the paper; and L.P., W.N., R.L., X.Z., X.B., C.Y., T.C., Y.S., and H.X. contributed to the discussion and revision of the paper. All authors have read and agreed to the published version of the manuscript.
Conflict of interest: Authors state no conflict of interest.
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