Integrated Zoning and Spatial Heterogeneity of Coastal Watershed-Nearshore Waters

Abstract

Integrated land-sea development and protection are important for the sustainable development of coastal areas. To promote the transition from separate land-sea management to integrated land-sea governance, the scientific delineation of the integrated spatial zoning units of coastal watershed-nearshore waters is an important tool for integrated land-sea management. From the perspective of physical geography, this study uses digital hydrological analysis based on DEM data to determine the coastal basin range and generate multilevel watersheds and river networks using different thresholds, and establishes spatial correspondence among physical geospatial spaces, such as watershed zones, coastlines, and nearshore waters, after considering the boundaries of major estuaries and bays. On this basis, the coastal zone is divided into several integrated spatial zoning units of coastal watershed-nearshore waters, and a physical geography-based integrated spatial zoning method is developed to form a physical geography-based land-sea integrated spatial zoning scheme. This study conducted spatial heterogeneity research of the integrated spatial zoning units, from the perspectives of topography analysis, land use analysis, socioeconomic analysis of the watershed, sea use analysis, seawater quality analysis of the nearshore waters, and load pressure analysis of the watershed-nearshore waters. The elemental differences of zoning units are studied in detail, which can provide a data reference for establishing the relationship between watersheds and nearshore waters. Applying this research method to the Guangxi Zhuang Autonomous Region in China, where the land-sea linkage pattern is forming, can provide a spatial reference and scientific basis for land-sea integrated management for ecological protection and sustainable resource utilization in the coastal zone.

1. Introduction

The land and the ocean are a community of life. The land provides important support for the development of the marine economy, and the ocean is an ecological barrier for the terrestrial ecosystem that maintains balance and stability. In the mutual integration of land and ocean, a dynamic and complex coastal ecosystem is formed [1,2,3]. The coastal zone is a key area for the development of human society from land to sea and is under the impact of high-intensity human activities. The coastal zone is intensively developed and utilized, with severe resource and environmental pressure and frequent ecological disasters. Therefore, balancing the use of coastal zone resources and ecological protection has become an important issue for the sustainable development of coastal areas [4,5].

Coastal zone management should focus on the uniqueness and integrity of land and sea ecosystems and promote land-sea integrated development and protection [6,7,8]. To seek solutions for the sustainable development of the coastal zone, an ecosystem-based integrated coastal zone management methodology has been proposed internationally, and China has gradually developed a land-sea integrated management strategy [9,10,11,12,13]. Land-sea integration improves land-sea social, economic, and ecological functions by scientifically planning and coordinating the elements of the coastal zone, such as resources, ecology, environment, and industry. Land-sea integration is a scientific understanding and application of land-sea interaction mechanisms and a concrete approach to improving sustainable development and adaptive ecosystem management in coastal areas. By adhering to the land-sea integration approach and giving full play to the role of the land and sea as a space coupling carrier in the coastal zone, a land-sea integrated coastal zone ecological environmental protection and governance system can be established, and the transformation from separate land-sea management to integrated land-sea governance can be realized [14,15,16,17].

From the perspective of physical geography, in the coastal zone, the land and sea are connected by the flow of coastal rivers. The coastal watershed is the “source” of freshwater, sediment, nutrients, pollutants, and other substances, which are transported through the “corridors”, i.e., coastal rivers, thus entering the “sink” areas of nearshore waters, such as estuaries and bays. The land-sea relationship is characterized by the pattern of the pressure on land and the response of the sea [8]. To realize the sustainable development of the coastal zone, the integrated land-sea complex ecosystem of the coastal watershed-nearshore waters should be taken as the management unit to form a land-sea integrated decision-making and governance system [18,19]. Lawrence [20] proposed that a common challenge within coastal management is how to best integrate watershed planning into regional efforts intended to address critical coastal issues, especially those related to water resources and water quality. Sreeja et al. [21] noted that incorporating coastal stretches as an integral part of the watershed becomes necessary due to the reciprocal relationships that exist between coastal ‘tail-end’ ecosystems and upstream stretches. Santana et al. [22] proposed that international society has begun to consider river watersheds and the coastal zone as one management unit that requires an ecosystem approach.

The integrated spatial zoning of coastal watershed-nearshore waters is an important starting point for promoting ecosystem-based integrated coastal management [23]. According to the land terrain data, each river forms a relatively fixed watershed range, and the same for coastal rivers. Basin has a wider scope compared to the watershed. In this study, the sum watershed ranges of all coastal rivers in the coastal zone area are defined as coastal basins [24]. The coastal basin has a relatively fixed range because of the relatively fixed coastal watershed ranges. Watershed refers to a river’s watershed or the internal zones of a coastal basin. From the perspective of physical geography, the coastal watershed influences the sea in a specific space across the coastline. There is a relatively fixed spatial correspondence for the coastal basin range and its internal watershed zones, coastal rivers, river estuaries, coastlines, and nearshore waters. In this study, a physical geography-based integrated spatial zoning method for coastal watershed-nearshore waters is developed to form a land-sea integrated spatial zoning scheme. For the land, the watershed zones are used as the basic unit, and for the sea, the nearshore waters, such as estuaries and bays, are used as the basic unit. Using the coastline as the correlation line between the coastal watershed and nearshore waters, the coastal zone is divided into several integrated spatial zoning units of coastal watershed-nearshore waters. With the wide application of geographic information system (GIS) technology [25,26,27], the extraction of two geomorphological features of watershed boundaries and river networks from Digital Elevation Model (DEM) data enhances the process of identifying the land-sea integrated spatial zoning of coastal zones [28,29,30,31]. Because ArcGIS software has been widely used worldwide due to its powerful functions, it meets the data processing needs of this study.

On this basis, the spatial heterogeneity of the integrated spatial zoning units is studied from different aspects, i.e., topography analysis, land use analysis, socioeconomic analysis of the watershed and sea use analysis, seawater quality analysis of nearshore waters, and load pressure analysis of watershed-nearshore waters [32]. The differences in the elements of zoning units are understood in detail, which can provide baseline data for establishing the spatial correlation between the coastal watershed and nearshore waters and provide a scientific basis for integrated land-sea management in the coastal zone. The overall framework is shown in Figure 1.

This study selects the Guangxi Zhuang Autonomous Region (abbreviated as Guangxi Region or Guangxi) in southern China as the study area. Driven by the policy of Integrated Coastal Zone Management, the Guangxi Region integrates the elements of land and sea resources to build a seaward economic belt. Therefore, this study is important and necessary. The research results can provide a spatial reference that can improve integrated land-sea management for coastal ecological protection and sustainable resource utilization in Guangxi’s coastal zone and promote the development of Guangxi’s seaward economy.

2. Materials and Methods

2.1. Study Area

The Guangxi Region is located at the southern end of China between 104°28′E–112°04′E and 20°54′N–26°23′N. The Guangxi Region is a provincial administrative region of China, connected with Guangdong Province to the east, Yunnan Province to the west, Hunan Province to the northeast, Guizhou Province to the northwest, Vietnam to the southwest, and Beibu Gulf to the south, as shown in Figure 2.

Guangxi has a land area of approximately 237,600 km² and a sea area of approximately 40,000 km². In 2021, the permanent population of Guangxi was 50.37 million, and the regional GDP was 2474.086 billion yuan, of which the marine GDP was 182.82 billion yuan. The Guangxi land area is mostly surrounded by mountains and plateaus, and the central and southern parts are mostly hilly and flat lands.

The land area of Guangxi is mostly surrounded by mountains and plateaus, and the central and southern regions are mostly hilly and flat. A dendritic river system with the Hongshui River and the Xijiang River as mainstreams running through the central part and the tributaries on both sides is formed, and there are four major water systems: the Pearl River, the Yangtze River, the Baidu River, and the Dujiang River. The land morphological features and main rivers of the study area are shown in Figure 3. Guangxi has abundant marine resources and an excellent marine ecological environment. The ocean is an important strategic space for economic and social development. There are many bays, including Qinzhou Bay, Lianzhou Bay, and Tieshan Harbor, along the coast. Approximately 120 coastal rivers flow into the Beibu Gulf, including the Maoling River, Qinjiang River, and Nanliu River. Guangxi is rich in coastlines, fisheries, tourism, oil and gas, minerals, marine energy, and other resources, and it has one of the four major fishing grounds in China and one of the six major coastal oil-bearing basins.

Guangxi has the regional advantages of coasts, riversides, and borders, and it has the most convenient access to the South China Sea. Currently, Guangxi is improving the region through the development of the seaward economy, implementing land-sea integration, and strengthening the development of resources and environmental protection in the coastal zone of Guangxi, which is of great significance for promoting the high-quality development of the seaward economy in Guangxi.

2.2. Data Source

The data used in this study mainly include DEM data, basic geographic information data, coastline data, land use data, socioeconomic data, sea use data, and seawater quality data of Guangxi. The specific data source information is shown in

Table 1. Information on data sources.

Number	Data Name	Data Content	Data Source
1	DEM data	Google Earth elevation data with a resolution of 70 m	Download from Google Earth
2	Basic geographic information data	China basic geographic information data (public version at 1:1 million scale) in 2021, including 4 tiles: G48, G49, F48, and F49	National Catalogue Service For Geographic Information of China, https://www.webmap.cn/ (accessed on 6 July 2022)
3	Coastline data	Guangxi coastline survey results in 2020	Bureau of Oceanography, Guangxi Region, http://hyj.gxzf.gov.cn/ (accessed on 8 April 2022)
4	Land use data	GlobeLand30 data with a resolution of 30 m in 2020, including 2 tiles: N48-20 and N49-20	GlobeLand30, http://www.globallandcover.com/ (accessed on 14 December 2022)
5	Socioeconomic data	Guangxi data from the 2020 Chinese Census and Guangxi Statistical Yearbook 2021	Bureau of Statistics, Guangxi Region, http://tjj.gxzf.gov.cn/ (accessed on 23 February 2023)
6	Sea use data	Guangxi Seause Right Confirmation data in 2020	Bureau of Oceanography, Guangxi Region, http://hyj.gxzf.gov.cn/ (accessed on 16 March 2022)
7	Seawater quality data	Guangxi Marine Environmental Quality Report in 2020	The People’s Government of Beihai Municipality, http://xxgk.beihai.gov.cn/ (accessed on 8 February 2023)

.

2.3. Integrated Spatial Zoning Method for Coastal Watershed-Nearshore Waters

The integrated spatial zoning of coastal watershed-nearshore waters includes land space and sea space. For the land space, the coastal watershed zones are used as the basic unit. For the sea space, the estuaries and bays are used as the basic unit. The coastline is the correlation line between these two spaces. It can be realized through the following three processes: the determination of the coastal basin range, the extraction of coastal watershed zones within the coastal basin range, and the delineation of the integrated spatial zoning units of coastal watershed-nearshore waters.

This study determines the coastal basin range by performing digital hydrological analyses, including Fill, Flow Direction, and Basin on the platform of ArcGIS, using the DEM data of the coastal area. First, Fill calculation is performed on the original DEM data to eliminate errors caused by data interpolation and other factors. Flow Direction calculation is used to determine the direction of the water flow as it leaves each grid cell, which is the basis for Basin extraction. The Basin operation analyzes the grid data of the water flow direction to identify adjacent grids that belong to the same basins and whose water outlets are at the edge of the DEM data. According to the basin data, the flow directions of major rivers, and the locations of river estuaries and coastlines in the study area, the coastal area can be divided into large-scale basins. The coastal basin whose outlet is on the coastline is identified, and its outer boundary delineates the coastal basin range. Various substances in this range enter the ocean through coastal rivers and then affect the ocean, which is the key land space in land-sea integration.

To establish the spatial correspondence between watershed zones and the coastline within the coastal zone, the extraction and refinement of watersheds and river networks within the coastal basin range are carried out. By superimposing the original DEM data with the coastal basin range, the DEM data for the coastal basin is obtained. After performing digital hydrological analyses, including Fill, Flow Direction, and Flow Accumulation, and based on the grid data from Flow Direction and Flow Accumulation, the generation of multilevel watersheds and river networks with different thresholds are generated. River network extraction is the premise of watershed extraction. First, the threshold of the river network is set. All of the grids where the cumulative amount of confluence is greater than the preset river network threshold are potential flow paths, and the grid set composed of these water flow paths are the river networks. When extracting a watershed, the stream links are first divided according to the intersection relationship of the river networks, and then, the watershed corresponding to each stream link is divided according to the flow direction. The watershed is the basic unit for watershed extraction, and the combination of watersheds belonging to the same river network forms the watershed. The size of the river network threshold determines the density of the river networks. The larger the set threshold is, the sparser the river networks, and only larger watersheds can be generated. As the threshold decreases, the river networks become denser, and small watersheds can be gradually generated. Through the extraction of watersheds and river networks with multilevel thresholds, the spatial correspondence between watershed zones, river networks, and coastline segments within the coastal zone is finally established.

For the land-sea integrated zoning of the coastal zone, the spatial correspondence between multilevel watershed zones and coastline segments is the basis of watershed zoning. For the complexity of the marine hydrodynamic environment that should be considered in sea areas, this study focuses on the key nearshore water spaces (such as estuaries and bays) that are affected by the coastal watershed. In this study, the outer boundary of the nearshore water is defined as follows: 1 km outward to the seaside from the boundary points of the main estuaries and bays on the coastline, thus performing the spatial zoning of the nearshore waters. Small bays are merged into the adjacent large bays. The long straight coast between two large bays is divided into independent nearshore spaces. Taking the coastline segments occupied by the divided nearshore water space units as the correlation line, the spatial correspondence among the watershed zones, the coastline segments, and the nearshore water zones is established, and the corresponding watershed zones and the nearshore water zones of coastline segments are merged, thus obtaining several integrated spatial zoning units of coastal watershed-nearshore waters.

2.4. Spatial Heterogeneity Analysis Method for the Integrated Spatial Zoning Units

Taking the integrated spatial zoning unit of coastal watershed-nearshore waters as the analysis unit, spatial heterogeneity analysis of the watershed, spatial heterogeneity of the nearshore waters, and load pressure analysis of the coastal watershed-nearshore waters are carried out.

2.4.1. Spatial Heterogeneity Analysis of the Watershed

The analysis of the watershed’s spatial heterogeneity includes three items: topography analysis, land use analysis, and socioeconomic analysis.

The topography analysis of the watershed is carried out from two aspects: elevation analysis and slope analysis. Depending on the applicable characteristics of different elevations, the watershed is divided into four categories according to the elevation: plain with an elevation <200 m, hilly land with an elevation of 200–500 m, mountain with an elevation of 500–1000 m, and high mountain with an elevation >1000 m. Regarding the Code for engineering surveying (GB50026-93), the National Standard of China, the watershed is divided into four categories according to the slope: flat slope with a slope <3°, gentle slope with a slope of 3–10°, middle slope with a slope of 10–25°, and steep slope with a slope >25°. Elevation classification and slope analysis of the DEM data for the watersheds are performed, and the area proportions of various types of topography in the watersheds are calculated.

The land use analysis of the watersheds is expressed by the spatial difference of the land use classification, which includes cultivated land, woodland, grassland, shrubland, wetland, water body, artificial surface, and bare land, as shown in

Table 2. Land use type classification and definition.

Land Use Type	Definition
Cultivated land	Land used for growing crops.
Woodland	Land covered by trees with more than 30% canopy cover, and open woodland with 10–30% canopy cover.
Grassland	Land covered by natural herbaceous vegetation with cover greater than 10%.
Shrubland	Land covered by shrubs and with a shrub cover greater than 30%.
Wetland	Located at the junction of land and water, the water is shallow or the soil is too wet with many swamps or wet plants.
Waterbody	Area covered by liquid water on the land.
Artificial surface	The surface formed by artificial construction activities, including various residential areas such as towns, industrial and mining areas, and transportation facilities.
Bare land	Natural land with vegetation covered less than 10%.

. The area occupied proportions by land use type in the watersheds are calculated.

The socioeconomic analysis of the watersheds includes the population density of the watershed (PDW), gross domestic product (GDP) of the watershed (GDPW), and per capita GDP of the watershed (PGDPW).

PDW refers to the population per unit land area in the watershed, which is mainly directly related to the artificial surface. The calculation method is as follows:

$$PDW=\frac{{{\displaystyle \sum }}_{i=1}^n\frac{T{P}_i}{TAS{A}_i}\times ASA{W}_i}{WA}$$

(1)

where TP represents the total population of the administrative region, TASA represents the total artificial surface area of the administrative region, ASAW represents the artificial surface area of each administrative region in the watershed, WA represents the watershed area, and $i$ represents the number of administrative regions in the watershed.

GDPW refers to the total economic value of the social final products and labor services produced in a certain period in the watershed and is an important indicator of the economic status and development level of a region. The calculation method is as follows:

$$GDPW={\displaystyle \sum }_{i=1}^n\frac{GD{P}_i}{T{A}_i}\times AA{W}_i$$

(2)

where GDP represents the gross domestic product of the administrative region, TA represents the total area of the administrative region, AAW represents the area of each administrative region in the watershed, and $i$ represents the number of administrative regions in the watershed.

PGDPW refers to the total economic value of the social final products and labor services produced in a certain period in the watershed, calculated based on the average population. The calculation method is as follows:

$$PGDPW=\frac{GDPW}{PDW\times WA}$$

(3)

where the definitions of GDPW, PDW, and WA are the same as above.

2.4.2. Spatial Heterogeneity Analysis of the Nearshore Waters

The analysis of the nearshore waters’ spatial heterogeneity includes two items: sea use analysis and seawater quality analysis.

Sea use analysis in nearshore waters is expressed by the spatial differences in sea use classification. The classification of sea use in China includes protection zone, fishery zone, industrial zone, transportation zone, recreational zone, special zone, and unused zone, as shown in

Table 3. Sea use type classification and definition.

Sea Use Type	Definition
Protection zone	Refers to natural marine areas that have special important ecological functions or are ecologically sensitive and fragile and must be strictly protected.
Fishery zone	Refers to the sea area used for the development and utilization of fishery resources and the development of marine fishery production.
Industrial zone	Refers to the sea area used for the development of coastal industrial production, etc.
Transportation zone	Refers to the sea area used for transportation constructions such as ports, shipping, roads, and bridges.
Recreational zone	Refers to the sea area for marine entertainment activities by the development and utilization of coastal and marine tourism resources.
Special zone	Refers to the sea area used for scientific research and teaching, military and coastal protection projects, dumping, sewage, etc.
Unused zone	Refers to the undeveloped sea area.

. The area proportions of sea use types in the nearshore waters are calculated.

Seawater quality analysis in nearshore waters is expressed by the spatial differences in seawater quality classification. According to the National Standard of the People’s Republic of China, seawater quality standard (GB 3097-1997), which is used for different use functions and protection objectives, seawater quality can be divided into Class I water quality, Class II water quality, Class III water quality, and Class IV water quality. Class I water quality is the best, and the water quality gradually worsens from Class I to Class IV. The area proportions of the seawater quality class in the nearshore waters are calculated.

2.4.3. Load Pressure Analysis of the Coastal Watershed-Nearshore Waters

The load pressure analysis of the coastal watershed-nearshore waters includes the load pressure of the watershed on the nearshore (LPWN), the load pressure of the watershed on the coastline (LPWC), and the load pressure of the nearshore on the coastline (LPNC). The calculation methods are as follows:

$$LPWN=\frac{WA}{NA}$$

(4)

$$LPWC=\frac{WA}{CL}$$

(5)

$$LPNC=\frac{NA}{CL}$$

(6)

where WA represents the watershed area, NA represents the nearshore area, and CL represents the coastline length.

3. Results

After performing digital hydrological analysis on the DEM data of Guangxi, including Fill, Flow Direction, and Basin, the layout of the basins in the whole region can be obtained. In Guangxi, there are four major river basins: the Pearl River Basin, the Yangtze River Basin, the Red River Basin, and the Coastal Basin, as shown in Figure 4. Among them, the Pearl River Basin runs from the northwest to the east and traverses the whole region, through the Xijiang River and Beijiang River out of Wuzhou, and runs into the South China Sea through Guangdong Province. The Pearl River Basin has the largest basin area of 204,768.24 km², accounting for 86.6% of Guangxi’s land area. The Yangtze River Basin is in the northeast of Guangxi, and the mainstream Xiangjiang River is in the upper reaches of the Dongting Lake system and joins the Yangtze River through Hunan Province, with a basin area of 8371.58 km², accounting for 3.5% of Guangxi’s land area. The Red River Basin is in the west of Guangxi, mainly formed by the tributary of the Baidu River that runs out of Guangxi into Vietnam, and the basin area is the smallest at 1483.82 km², accounting for 0.6% of Guangxi’s land area. The Coastal Basin is in the south of Guangxi and is mainly formed by the coastal rivers running into Beibu Bay, including the Nanliu River, Dafeng River, and Qinjiang River, with a basin area of 21,951.54 km², accounting for 9.3% of Guangxi’s land area.

According to the Coastal Basin size in Guangxi, five thresholds are set for the extraction of multilevel watersheds and river networks, i.e., 200 km², 100 km², 50 km², 10 km², and 1 km². First, a threshold of 200 km² is used. Eight major watersheds are formed in the Guangxi coastal zone, and from west to east, they are the Beilun River Watershed, Nasuo River Watershed, Fangcheng River Watershed, Maoling River Watershed, Qin River Watershed, Dafeng River Watershed, Nanliu River Watershed, and Baisha River Watershed. There is nearshore land space between adjacent watersheds and the coastline that has not been classified into any watershed. When 100 km² is used as the threshold, in the undivided nearshore land space during the first-level extraction, the Naliang River Watershed, Danzhu River Watershed, and Nankang River Watershed are formed. When 50 km² is used as the threshold, in the undivided nearshore land space during the second-level extraction, the Huangzhu River Watershed, Fengjia River Watershed, and Zhakouhe River Watershed are formed. When 10 km² is used as the threshold, in the undivided nearshore land space during the third-level extraction, the Zhupai River Watershed, Nali River Watershed, and Fozi River Watershed are formed. When the threshold is 1 km², in the remaining undivided nearshore land space, smaller watersheds and river networks are formed, and the spatial correspondence between the watershed zones at all levels and the coastline segments is essentially determined, as shown in Figure 5.

There are seven main bays along the coast of Guangxi from west to east, including the Beilun Estuary, Pearl Bay, Fangcheng Harbor, Qinzhou Bay, Dafeng Estuary, Lianzhou Bay, and Tieshan Harbor. Based on these seven bays and the flat coast of Beihai Silver Beach between Lianzhou Bay and Tieshan Harbor, this study first determines the boundary point between the seven bays and the flat coast on the coastline and then delineates the eight major nearshore water spaces after expanding 1 km outward to the seaside from the boundary. On the landward side, according to the results of the five-level watershed zoning of the coastal zone of Guangxi, eight major integrated spatial zoning units of coastal watershed-nearshore waters in Guangxi are formed after combining the watershed zones that converge into the same nearshore water space, namely, Beilun Estuary, Pearl Bay, Fangcheng Harbor, Qinzhou Bay, Dafeng Estuary, Lianzhou Bay, Beihai Silver Beach, and Tieshan Harbor integrated spatial zoning unit of coastal watershed-nearshore waters, as shown in Figure 6.

The total land-sea area of the Guangxi coastal zone delineated in this study is 23,636.36 km², with a watershed area of 21,951.54 km², a nearshore area of 1684.82 km², and a coastline length of 1618.897 km. The Lianzhou Bay Watershed-Nearshore integrated spatial zoning unit has the largest area of 9932.66 km², followed by the Qinzhou Bay integrated zoning unit with an area of 6276.23 km², and the Pearl Bay integrated zoning unit has the smallest area of 610.02 km². The Qinzhou Bay integrated zoning unit has the longest coastline length of 478.624 km, and the Beilun Estuary integrated zoning unit has the shortest coastline length of 27.387 km. The LPWN, LPWC, and LPNC of the eight integrated spatial zoning units are analyzed. The LPWN of the Beilun Estuary integrated zoning unit is the largest at 40.25, and the LPWN of the Tieshan Harbor integrated zoning unit is the smallest at 4.41. The LPWC and LPNC of the Lianzhou Bay integrated zoning unit are the largest, with values of 81.2 and 2.77, respectively. The LPWC of the Fangcheng Harbor integrated zoning unit is the smallest at 4.89. The LPNC of the Dafeng Estuary integrated zoning unit is the smallest at 0.25, as shown in Figure 6 and

Table 4. Statistical table for integrated spatial zoning units of the Guangxi coastal watershed-nearshore waters.

Number	Name of Integrated Spatial Zoning Units	Area (km2)			Coastline Length (CL) (km)	LPWN	LPWC	LPNC
		Watershed Area (WA)	Nearshore Area (NA)	Total Area (TA)
①	Beilun Estuary watershed-nearshore waters	1005.90	24.99	1030.89	27.387	40.25	36.73	0.91
②	Pearl Bay watershed-nearshore waters	515.00	95.02	610.02	78.978	5.42	6.52	1.20
③	Fangcheng Harbor watershed-nearshore waters	1051.78	186.32	1238.10	215.156	5.65	4.89	0.87
④	Qinzhou Bay watershed-nearshore waters	5722.79	553.44	6276.23	478.624	10.34	11.96	1.16
⑤	Dafeng Estuary watershed-nearshore waters	1836.17	77.71	1913.88	307.070	23.63	5.98	0.25
⑥	Lianzhou Bay watershed-nearshore waters	9605.29	327.37	9932.66	118.297	29.34	81.20	2.77
⑦	Beihai Silver Beach watershed-nearshore waters	808.10	100.72	908.82	107.402	8.02	7.52	0.94
⑧	Tieshan Harbor watershed-nearshore waters	1406.51	319.25	1725.76	285.983	4.41	4.92	1.12
Total		21,951.54	1684.82	23,636.36	1618.897

. The larger the watershed area is and the smaller the nearshore area, the larger the LPWN. The larger the watershed area and nearshore area are, and the shorter the coastline length is, the larger the LPWC and LPNC are.

For the Guangxi coastal zone, plains (elevation < 200 m) are dominant, followed by hilly land (elevation of 200–500 m), and high mountain (elevation > 1000 m) accounts for the least. For the eight integrated spatial zoning units of coastal watershed-nearshore waters, the Dafeng Estuary Watershed, Beihai Silver Beach Watershed, and Tieshan Harbor Watershed are dominated by plains, and only the Beilun Estuary Watershed and Fangcheng Harbor Watershed have high mountains, accounting for less than 1%. The elevation decreases gradually from the inland boundary of the coastal watershed to the coastline, as shown in

Table 5. Statistical table of elevation analysis and slope analysis in the Guangxi coastal watersheds.

Watershed Number	The Proportion of Each Elevation Level (%)				The Proportion of Each Slope Level (%)
	<200 m	200–500 m	500–1000 m	>1000 m	<3°	3–10°	10–25°	>25°
①	44.2	32.1	22.8	0.9	15.7	22.7	44.9	16.7
②	79.4	14.9	5.7	0.0	22.4	40.8	32.1	4.7
③	70.6	18.7	9.9	0.8	31.4	29.0	27.9	11.7
④	88.9	10.2	0.9	0.0	35.4	36.8	25.8	2.0
⑤	99.6	0.4	0.0	0.0	42.5	50.0	7.4	0.1
⑥	83.7	14.5	1.8	0.0	37.0	34.7	25.5	2.8
⑦	100.0	0.0	0.0	0.0	91.2	8.3	0.5	0.0
⑧	95.4	4.5	0.1	0.0	53.6	32.3	13.4	0.7

and Figure 7a. For the Guangxi coastal zone, flat slope (slope < 3°) and gentle slope (slope of 3–10°) are dominant, followed by middle slope (slope of 10–25°), and steep slope (slope > 25°) are the least common. The areas with larger slopes are mainly near the inland boundary of the coastal watershed, and the slopes are small in the areas near the coastline. The Dafeng Estuary Watershed and Beihai Silver Beach Watershed are closer to the coastline with small slopes, as shown in Table 5 and Figure 7b. For the Guangxi coastal watersheds, the higher the elevation is, the steeper the slope is, which shows the topographical characteristics of the watershed covered by the rivers running into the sea.

In 2020, woodland was the main land use type in the Guangxi coastal watersheds, followed by cultivated land, and artificial surface ranked third, with other types accounting for small proportions, as shown in

Table 6. Statistical table of land use analysis and socioeconomic analysis in the Guangxi coastal watersheds.

Watershed Number.	The Proportion of Land Use Type (%)							Socioeconomic Statistics
	Cultivated Land	Woodland	Grassland	Shrubland	Wetland	Water Body	Artificial Surface	PDW (Person/km2)	GDPW (100 Million Yuan)	PGDPW (Thousand Yuan)
①	11.7	81.8	0.7	3.1	0.1	0.8	1.8	140	124	88
②	8.8	87.2	0.2	0.5	0.1	1.9	1.3	101	63	121
③	15.3	73.0	0.1	0.4	0.0	2.4	8.8	671	129	18
④	33.2	59.1	0.1	0.0	0.0	2.1	5.5	446	764	30
⑤	38.8	59.5	0.0	0.0	0.0	1.0	0.7	55	279	276
⑥	33.8	58.4	0.0	0.0	0.0	2.1	5.7	452	1683	39
⑦	58.5	20.7	0.3	0.0	0.1	5.7	14.7	903	313	43
⑧	33.8	55.3	0.0	0.0	0.3	4.2	6.4	431	394	65

and Figure 7c. For the eight integrated spatial zoning units of coastal watershed-nearshore waters in Guangxi, the PDW of the Beihai Silver Beach Watershed is the largest at 903 persons/km², with many urban built-up areas in this watershed. The PDW of the Dafeng Estuary Watershed is the smallest at 55 people/km². The PDW and the proportion of artificial surface area are positively correlated, with a correlation coefficient of 0.99. The GDPW of the Lianzhou Bay Watershed is the largest at 168,300 million yuan, and the GDPW of the Pearl Bay Watershed is the smallest at 6300 million yuan. The GDPW and the size of the watershed are positively correlated, with a correlation coefficient of 0.97. The PGDPW of the Dafeng Estuary Watershed is the largest at 276 thousand yuan, which is due to the low population density in this watershed. The PGDPW of the Fangcheng Harbor Watershed is the smallest at 18 thousand yuan, as shown in Table 6 and Figure 7d.

The sea use status quo of the nearshore waters in Guangxi is dominated by protection zones and unused zone areas, indicating that the development and utilization of the Guangxi nearshore waters are low. For other sea use types, the proportion of fishery zones is high in the nearshore waters of Lianzhou Bay and Dafeng Estuary, at 8.3% and 7.1%, respectively. The proportion of industrial zones is high in the nearshore waters of Fangcheng Harbor and Qinzhou Bay, at 5.2% and 2.2%, respectively. The proportion of transportation zones is relatively high in the nearshore waters of Tieshan Harbor, at 3.2%, as shown in

Table 7. Statistical table of sea use analysis and seawater quality c analysis in the Guangxi nearshore waters.

Nearshore Waters Number	The Proportion of Sea Use Type (%)							The Proportion of Seawater Quality Type (%)
	Protection Zone	Fishery Zone	Industrial Zone	Transportation Zone	Recreational Zone	Special Zone	Unused Zone	Class I	Class II	Class III	Class IV
①	55.6	0.0	0.0	0.2	0.0	0.2	44.0	100.0	0.0	0.0	0.0
②	32.1	0.0	0.0	0.1	0.0	0.0	67.8	100.0	0.0	0.0	0.0
③	5.8	0.0	5.2	2.0	1.4	0.1	85.5	74.6	25.4	0.0	0.0
④	20.8	1.4	2.2	0.6	0.1	0.1	74.8	43.5	14.5	6.0	36.0
⑤	44.0	7.1	0.0	0.0	0.0	0.0	48.9	0.0	58.2	41.8	0.0
⑥	44.0	8.3	0.0	1.8	0.0	0.0	45.9	95.8	4.2	0.0	0.0
⑦	59.3	0.6	0.0	0.1	0.9	0.0	39.1	99.8	0.2	0.0	0.0
⑧	38.9	1.4	0.4	3.2	0.0	0.0	56.1	89.7	10.3	0.0	0.0

and Figure 7c. The seawater quality in the nearshore waters of Guangxi is generally excellent, with Class I seawater quality being dominant, followed by Class II seawater quality, and Class III and IV seawater qualities accounting for small proportions. The Class III and Class IV seawater qualities are mainly concentrated in the coastal waters of Qinzhou Bay and the Dafeng Estuary, indicating that these nearshore waters are seriously polluted, and the seawater quality must be improved, as shown in Table 7 and Figure 7d.

4. Discussion

The land and sea in the coastal zone are a complete ecosystem. The main governance across the land-sea interface challenges includes determining boundaries, addressing cross-scale effects, and accessing knowledge [14]. International society has begun to consider river basins and the coastal zone as one management unit [22]. There are usually gaps and overlaps at the edges of the separate terrestrial, estuarine, and marine-realm maps, and often no clarity on which specific coastal boundary was used [2]. Watersheds are the basic unit of Earth’s terrestrial systems [18]. DEM is one of the important sources in the extraction of watersheds and river networks [26,30]. The integrated spatial zoning method based on DEM data for coastal watershed-nearshore waters constructed in this study may be the most convenient method for determining the coastal basin range, which solves the problem of determining the research scope of coastal land areas in coastal zone planning. The multilevel watershed and river network extraction method using different thresholds proposed in this study can provide technical means for watershed zoning in coastal zones and improve the establishment of the corresponding relationship between watersheds and coastline segments. This study uses the integrated spatial zoning method for coastal watershed-nearshore waters, which breaks the boundaries of administrative districts, to establish a physical geography-based land-sea integrated spatial management method for the coastal zone. Apart from these natural boundaries, various administrative/political units present additional boundary concerns for sustainable coastal management. Taking these multiple boundaries of the coastal zone into account within the river basin regime becomes essential for resource sharing in upstream basin decisions that damage the coastal ecosystem [21].

The integrated zoning and spatial heterogeneity analysis method of coastal watershed-nearshore waters developed by this research has been applied in Guangxi of China. The coastal basin range and watershed zoning are consistent with the coverage of coastal rivers, and the integrated spatial zoning units of coastal watershed-nearshore waters delineated by this research are accurate and reliable, which meet the needs of integrated management of land and sea in coastal zones. This technical method can be optimized and applied in coastal areas worldwide, providing a spatial reference for integrated coastal zone management.

Based on the integrated spatial zoning units, the spatial heterogeneity analysis can reveal the element differences in zoning units of coastal watershed-nearshore waters in detail and provide a data basis for establishing the spatial correlation between coastal watersheds and nearshore waters. However, the elements of spatial heterogeneity analysis are still insufficient to fully establish the correlation between land and sea. A richer conceptual framework of governance is required to improve our ability to navigate the rapid social and environmental change occurring across the land-sea interface [7]. In the next step of research, using the land-sea integrated spatial zoning units in the coastal zone as the basis, the correlation between coastal watersheds and nearshore waters can be studied from the perspectives of physical geography and socioeconomics. For example, based on the monitoring results of the seawater quality status in nearshore waters, the relationship between the spatial differentiation of seawater environment quality in estuaries and bays and that of the upstream watershed is analyzed. Combined with the health requirements of the ecological environment in estuaries and bays, the socioeconomic and ecological environment regulation strategies of the upstream watersheds to maintain the ecosystem health of the nearshore waters are explored [33], including freshwater demand in estuaries, reduction of pollutants in the watersheds [34], sediment erosion and sedimentation, and fish migration, to research integrated land-sea environmental system management. In addition, multidimensional land-sea integration studies of the coastal zone, such as the construction of land-sea ecological security patterns and the linkage of land and sea functions, can be carried out to provide technical support and research ideas for territorial spatial planning [3,6,15], integrated protection and utilization planning, and ecosystem-based integrated management of coastal areas [11].

5. Conclusions

From the perspective of physical geography, based on the relatively fixed spatial correspondence for the coastal basin range and their internal watershed zones, coastal rivers, river estuaries, coastline segments, and nearshore waters, this study constructs a physical geography-based integrated spatial zoning method for coastal watershed-nearshore waters that can divide the coastal zone into several integrated spatial zoning units. On this basis, the spatial heterogeneity of coastal watersheds is studied through topography analysis, land use analysis, and socioeconomic analysis. The topography analysis includes elevation analysis and slope analysis, and the socioeconomic analysis includes the population density of the watershed (PDW), the GDP of the watershed (GDPW), and the per capita GDP of the watershed (PGDPW). Moreover, the spatial heterogeneity of nearshore waters is studied through sea use analysis and seawater quality analysis. In addition, the load pressure of the watershed on the nearshore (LPWN), the load pressure of the watershed on the coastline (LPWC), and the load pressure of the nearshore on the coastline (LPNC) are performed.

The proposed method has been applied in the Guangxi Region of China. Guangxi can be divided into four major basins: the Pearl River Basin, the Yangtze River Basin, the Red River Basin, and the Coastal Basin. Five-level threshold watershed and river network extraction are carried out in the Guangxi coastal zone, and the generated river networks are essentially consistent with the water system information provided by the basic geographic information. According to the results of the multilevel watershed zoning of the Guangxi coastal zone and the nearshore waters divided by the main estuaries and bays, the Guangxi coastal zone is divided into eight integrated spatial zoning units of coastal watershed-nearshore waters. In the spatial heterogeneity analysis of the integrated spatial zoning units, the larger the watershed area is, the smaller the nearshore area is, and the larger the LPWN is. The larger the watershed area and nearshore area are, and the shorter the coastline length is, the larger the LPWC and LPNC are. For the Guangxi coastal watersheds, the elevation decreases gradually from the inland boundary of the coastal watershed to the coastline. The areas with larger slopes are mainly near the inland boundary of the coastal watershed, and the slopes are small for the areas near the coastline. The higher the elevation is, the steeper the slope is, which shows the topographical characteristics of the watershed covered by the rivers running into the sea. The PDW and the proportion of artificial surface area are positively correlated, with a correlation coefficient of 0.99. The GDPW and the size of the watershed are positively correlated, with a correlation coefficient of 0.97. These results provide a spatial reference and scientific basis to improve integrated land-sea management for ecological protection and sustainable resource utilization in the coastal zone, thus promoting the development of Guangxi’s seaward economy. The proposed method is scientific, practicable, and operable and can be promoted and applied in coastal areas for integrated land-sea management.