Land Cover

Environmental Monitoring and Assessment (****) ***: 349 359

c Springer 2006

DOI: 10.1007/s10661-006-6559-z

LAND-COVER CHANGES IN AN URBAN LAKE WATERSHED

IN A MEGA-CITY, CENTRAL CHINA

YAN LI1, SHUQING ZHAO1, KUN ZHAO1, PING XIE2 and JINGYUN FANG1,

1

Department of Ecology, College of Environmental Sciences, and Key Laboratory for Earth Surface

Processes of the Ministry of Education, Peking University, Beijing, China;

2

Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, China

( author for correspondence, e-mail: ******@*****.***.***.**)

(Received 4 March 2005; accepted 26 April 2005)

Abstract. Urbanization can exert a profound in uence on land covers and landscape characteristics.

In this study, we characterize the impact of urbanization on land cover and lacustrine landscape and

their consequences in a large urban lake watershed, Donghu Lake watershed (the largest urban lake in

China), Central China, by using Landsat TM satellite images of three periods of 1987, 1993 and 1999

and ground-based information. We grouped the land covers into six categories: water body, vegetable

land, forested land, shrub-grass land, open area and urban land, and calculated patch-related landscape

indices to analyze the effects of urbanization on landscape features. We overlaid the land cover maps

of the three periods to track the land cover change processes. The results indicated that urban land

continuously expanded from 9.1% of the total watershed area in 1987, to 19.4% in 1993, and to

29.6% in 1999. The vegetable land increased from 7.0% in 1987, 11.9% in 1993, to 13.9% in 1999 to

sustain the demands of vegetable for increased urban population. Concurrently, continuous reduction

of other land cover types occurred between 1987 and 1999: water body decreased from 30.4% to

23.8%, and forested land from 33.6% to 24.3%. We found that the expansion of urban land has at

least in part caused a decrease in relatively wild habitats, such as urban forest and lake water area.

These alterations had resulted in signi cant negative environmental consequences, including decline

of lakes, deterioration of water and air quality, and loss of biodiversity.

Keywords: Central China, Donghu Lake, land cover change, landscape characteristics, remote

sensing, urbanization, urban lake watershed

1. Introduction

Human activities have greatly altered the earth s land surface (Turner et al., 1990;

Vitousek et al., 1997). Urbanization is one of the most pervasive anthropogenic land

conversions, which can signi cantly affect the land cover and landscape charac-

teristics in considerably short period of time. Urban expansion generally occupies

arable land (Lopez et al., 2001; Chen et al., 2003). However, with the accelerat-

ing of urban growth, relatively wild habitats, such as urban forest and lake area,

also became the target of urban expansion. Despite limited size of forest and wa-

ter body in urban ecosystems, they provide important services, such as habitats

for wildlife, water supply, and air ltering (Bolund and Hunhammar, 1999). How

urban expansion in uences relatively wild components of urban ecosystems and

350 Y. LI ET AL.

what consequences these impacts will bring to urban ecosystems are becoming

interesting topics (Pickett et al., 1997).

Since the economic reform in the late 1970s, especially after the land reform

launched in 1987, rapid urbanization has taken place across China (Cheng and

Masser, 2003). As the largest mega-city in Central China, Wuhan City along Central

Yangtze River has experienced an extensive urban growth. Donghu Lake, located in

Wuhan City, is the largest urban lake in China. In the Donghu Lake watershed, lake

water, urban forests, city, arable land, and other land cover types co-exist, and there-

fore the watershed has provided an ideal area to explore the effects of urbanization

on relatively wild habitats (lake and urban forest) and their consequences.

In this study, we integrate satellite remote sensing data and geographic infor-

mation system (GIS) to de ne the boundary of Donghu Lake watershed and then

to quantify land cover changes for 1987, 1993 and 1999 in the watershed. Our

objectives are to (1) estimate the changes in land cover and lacustrine landscape

under the impacts of the urbanization, and (2) assess impacts of these changes on

urban forest and lake water dynamics and their environmental consequences.

2. Data and Methods

2.1. S TUDY AREA

Donghu Lake (114-**-***-** E, 30 31 30 36 N) is situated at the eastern part

of Wuhan City, the capital of Hubei Province, Central China, with a surface area

of 33 km2 at the maximum water level of 21 m. The lake area has a subtropical

climate, with an annual precipitation of 1160.3 mm, and annual mean temperature

16.7 C (Liu, 1995). The lake and its watershed provide habitats for a wide variety

of wildlife, such as hydroplants and shes. As a typical urban lake, Donghu Lake

also provides other important ecological services, including urban water supply,

aquaculture, and aesthetic value.

2.2. B OUNDARY OF LAKE WATERSHED

Donghu Lake watershed was estimated as an area of 190 km2 (e.g. Gong et al.,

1965), but its boundary has never been explicitly examined. In order to de ne the

explicit boundary, we digitalized topographical maps covering this area with a scale

of 1:10,000 by using a GIS software (Founder drawing 5.5), and then obtained the

digital elevation model (DEM) for the watershed. The DEM and existed boundary of

water body were used to examine the boundary of the watershed using ArcInfo GIS

software. As a result, we de ned the watershed between 114 20 and 114 30 E

in longitude and 30 29 and 30 37 N in latitude with an area of 177.3 km2 . Its

western part includes Shahu Lake, another main water body linked to Donghu Lake

(Figure 1). As part of Jianghan plain in Central Yangtze, the watershed is fairly at,

with an altitudinal range of 20 157 m, and 85% of total area is below 35 m.

LAND-COVER CHANGES IN AN URBAN LAKE WATERSHED 351

Figure 1. Sketch map of Donghu Lake watershed.

2.3. L AND COVER INFORMATION

We used the image processing software IDRISI (Clark University, USA) to classify

the land cover types for the three periods of 1987, 1993, and 1999, using a data set

of a 3-band RGB combination (Bands 4, 5, and 3) of Landsat Thematic Mapper

(TM) imagery. In order to minimize the impact of possible interpretation errors due

to survey date of images, we acquired the images of the three periods with similar

survey dates (September 26 for 1987, October 12 for 1993, and September 27 for

1999). We geo-referenced all these images according to 1:50,000 topographical

maps and then, using the maximum likelihood method, classi ed the land covers

into six categories: water body, vegetable land, forested land, shrub-grass land, open

land, and urban land (Table I and Figure 2a c). In order to determine the accuracy

of the image classi cation, we used the strati ed random sampling method (Jensen,

1996) to generate reference points, and obtained a total of 230 reference points for

TABLE I

Descriptions for land cover types in the study area

Land cover

category General description

Water body Areas that lake and ponds used to aquaculture.

Vegetable land Mainly vegetable land, plus some farmland.

Forest Area covered conifers, broadleaved trees in hilly land and orchard in the plain.

Shrub-grass land Areas with shrub or grass vegetation, including urban lawn.

Open land Open lands in urban fringe with little vegetation cover.

Urban land Built-up land.

352 Y. LI ET AL.

Figure 2. Land cover types of Donghu Lake watershed in 1987 (a), 1993 (b), 1999 (c), and spatial

distribution of urban expansion between 1987 1999 (d).

each of classi ed images. The accuracy of three classi ed products was assessed by

verifying general land cover delineations on topography maps, municipal maps and

eld survey. The resulting overall classi cation accuracy was 89, 85, and 89% for

the three images, respectively. Through overlaying the classi ed land cover maps

for 1987, 1993 and 1999, we achieved transition matrix and spatial pattern of urban

expansion to understand the land cover change processes between 1987 1993 and

1993 1999.

2.4. PARAMETERS OF PATCHES

Patch characteristics can be used to describe landscape features (Forman, 1995). We

exported the classi cation maps for 1987, 1993 and 1999 to the PatchGrid Module

LAND-COVER CHANGES IN AN URBAN LAKE WATERSHED 353

of ArcView GIS software to obtain each patch area and its perimeter for all the

land cover category of the three periods. We used the area and number of patches

and mean patch size of all the land covers for all three periods to characterize

the changes of landscape characteristics. Mean patch size (MPS) can serve as a

fragmentation index of habitat at the landscape level, because a patch type with a

smaller MPS might be considered more fragmented.

n

MPS = ai j / n i

j =1

where i is the i th land cover type, j is the j th patch of the i th land cover type, ai j

is the j th patch area (ha) of the i th land cover type, and n i is the patch numbers of

the i th land cover type.

3. Results and Discussion

3.1. L AND COVER CHANGES

The results indicated that urbanization since 1987 in the Donghu Lake watershed

has led to a signi cant increase in coverage of urban land and vegetable land while to

a decrease in that of other land cover types. The area of urban land rapidly increased

by 223% from 16.2 km2 in 1987 to 52.4 km2 in 1999 (Figure 3a). Concurrently,

vegetable land increased from 12.5 km2 in 1987 to 24.7 km2 in 1999.

Since the economic reformation of China in 1978, especially after the land

reform in 1987, rapid urban growth has taken place in China and urban population

has remarkably expanded. As one of the most developed cities in China, Wuhan has

experienced rapid urban expansion and its population increased from 6.29 million

in 1987, to 6.92 million in 1993, and 7.40 million in 1999 (Wuhan Statistical Year

Book, 1990, 1994, 2000), which led to the rapid increase of urban land in the Donghu

Lake watershed. Meanwhile, suburban vegetable land was expanded to meet the

demands of vegetables for enhanced urban population. The area of vegetable land

in Wuhan City increased from 556.7 km2 in 1987 to 1191.7 km2 to 1999 (Figure 4),

which evidenced our ndings. On the other land, forest cover decreased from 59.5

km2 in 1987 to 43.1 km2 in 1999, and water body from 53.8 km2 to 42.2 km2 in

1999. Shrub-grass land and open land changes also tended to a decline.

A transition matrix (Table II) can help understand the land cover change pro-

cesses and the change rate among land cover categories between two neighboring

periods under the impacts of urbanization. The increase of urban land between

1987 1993 was primarily contributed by conversion from open land, shrub-grass

land, and forest to urban land, and the contribution rate of these three land conver-

sions to the coverage of urban land in 1993 were 25.3, 14.0 and 13.7%, respectively.

The land conversion from water body to urban land also contributed to the increase

354 Y. LI ET AL.

Figure 3. Area and percentage of (a) land area, (b) number of patches, and (c) mean patch size for

the land cover types during the period of 1987 1999.

of urban land in 1993 (5.3%). Urban land area further increased between 1993

and 1999 primarily by land transformations from forest, shrub-grass land and open

area to urban land at respective rate of 15.1%, 11.3% and 8.3%. These suggest

that forested land, shrub-grass land, and open land are the major sources of newly

increased urban land during 1987 1999. In the period of 1987 1993, urban sprawl

occupied mainly remaining suburban bare areas, while in the latter period of 1993

1999, urban forest became the target for further urban expansion due to no more

open area available. As indicated by the classi ed satellite images and spatial distri-

bution maps of urban expansion (Figure 2a d), between 1987 and 1993, urban land

LAND-COVER CHANGES IN AN URBAN LAKE WATERSHED 355

TABLE II

Land cover transition and contribution rate between 1987 1993 and 1993 1999

Water Vegetable Shrub-grass Open Urban

body land Forest land area land

Land cover a (b) % a (b) % a (b) % a (b) % a (b) % a (b) %

1987 1993

Water body 79.4 (92.6) 2.1 (5.5) 12.8 (12.8) 1.3 (4.8) 1.0 (7.8) 3.4 (5.3)

Vegetable land 2.5 (0.7) 49.4 (29.4) 31.6 (7.3) 8.0 (6.8) 5.3 (9.3) 3.1 (1.1)

Forested land 4.4 (5.6) 17.5 (49.4) 58.1 (64.0) 8.4 (33.9) 3.8 (31.2) 7.9 (13.7)

Shrub-grass land 1.1 (0.4) 12.1 (11.1) 29.6 (10.5) 24.6 (32.3) 7.6 (20.3) 25.1 (14.0)

Open area 0.6 (0.2) 4.8 (3.6) 11.9 (3.5) 16.1 (17.6) 12.3 (27.3) 54.4 (25.3)

Urban land 1.3 (0.4) 1.1 (0.9) 6.0 (1.8) 3.9 (4.3) 1.8 (4.0) 85.9 (40.5)

1993 1999

Water body 84.2 (92.1) 3.9 (7.2) 6.1 (6.5) 0.5 (2.6) 2.3 (18.0) 3.1 (2.7)

Vegetable land 1.0 (0.5) 47.7 (40.5) 25.6 (12.4) 7.8 (18.0) 5.3 (18.8) 12.7 (5.1)

Forest 4.1 (5.2) 16.2 (35.3) 55.8 (69.8) 6.5 (38.5) 2.8 (25.6) 14.7 (15.1)

Shrub-grass land 1.2 (0.4) 15.1 (9.0) 20.4 (6.9) 16.6 (26.9) 6.2 (15.2) 40.4 (11.3)

Open area 0.5 (0.1) 12.2 (3.5) 5.6 (0.9) 7.7 (6.1) 13.4 (16.2) 60.5 (8.3)

Urban land 2.0 (1.6) 3.1 (4.3) 4.2 (3.3) 2.0 (7.7) 1.0 (5.9) 87.7 (57.4)

a: Transition rate is the percentage of the area of transition type from one land cover to another to

the area of certain land cover in the beginning period

b: Contribution rate is the percentage of the area of transition type from one land cover to another to

the area of different another land cover in the end period.

Figure 4. Changes in the size of orchard and area of vegetable land in Wuhan City between 1987 and

1999 (based on Wuhan Statistical Year Book, 1990, 1994, 2000).

growth mainly occurred in open land along the western boundary of the watershed

(the bank of the Yangtze River) and around Shahu Lake. In the period of 1993 1999,

urban land further expanded resulted mainly from a connection of scattered urban

lands. Additionally, during the latter period the spread of urbanization encroached

356 Y. LI ET AL.

into southern hilly part of the watershed around the Donghu Lake scenic area, where

forested land mainly distributed.

The increased vegetable land in 1993 was primarily caused by land transforma-

tion from forested land to vegetable land between 1987 1993, at a contribution rate

of 49.4%. Simultaneously, the land transformations of shrub-grass land and water

body to vegetable land between 1987 1993 contributed to 11.1% and 5.5% to the

coverage of vegetable land in 1993. The same land conversions between 1993 1999

as in the period of 1987 1993 have resulted in a continuous increase of vegetable

land in 1999, with a contribution rate of 35.3, 9.0 and 7.2%, respectively.

The continuous decrease of water body between 1987 1993 and 1993 1999

was mainly resulted from the land transformation from water body to forested land,

at the transition rate of 12.8% in the former period and 6.1% in the latter period,

which probably resulted from the reclamation of small water body to orchard (a

component of forested land) to provide more fruits for enhanced urban population.

This was mirrored by a census of Wuhan socioeconomic development (Wuhan

Statistical Yearbooks, 1990, 1994, 2000), which indicated that the area of orchard

in Wuhan City increased from 54.3 km2 in 1987 to 92.8 km2 to 1999 (Figure 4).

The retention rate (the percentage of the area that keep the same land cover

types between two periods) of forested land between 1987 and 1993 was 64%

(Table II). The reduction of forested land in this period was mainly caused by

the land conversions of the forest to vegetable land, shrub-grass land and urban

land, and the transition rate was 17.5, 8.4 and 7.9% respectively. During the period

of 1993 1999, the retention rate of forested land amounted to 69.8%, but land

conversions of forested land to vegetable land and urban land led to the continuous

reduction of forest cover. These results implied that the drop of forest cover in

the Donghu Lake watershed was principally resulted from the urban growth and

vegetable land expansion in this area.

3.2. LANDSCAPE FEATURE ALTERATIONS

As indicated in Figure 3, urban expansion had signi cantly altered landscape char-

acteristics in Donghu Lake watershed, but its impacts varied with land cover cate-

gories.

Urban land continuously expanded from 9.1% of total watershed area in 1987,

to 19.4% in 1993, and to 29.6% in 1999, and became the most dominant land cover

type in the watershed area by 1999. Compared to this change, forest and water

body were the most dominant land cover types in 1987, which occupied 33.6% and

30.4% of total watershed area, but reduced to 24.3 and 23.8% by 1999, respectively

(Figure 3a).

As showed in Figure 3b and c, the patch numbers of urban land increased from

398 in 1987 to 525 in 1993, then decreased to 373 by 1999. The mean patch size

of urban land continuously increased from 4.1 ha in 1987 to 14.1 ha in 1999,

indicating urban land became more congregated. The results also suggested that

LAND-COVER CHANGES IN AN URBAN LAKE WATERSHED 357

urban sprawl between 1987 1993 was accomplish by building up more newly

urban land, while redeveloping and connecting built-up scattered urban lands was

the way of urban growth in 1993 1999. This was coincided with the planning policy

of Chinese Cities. The early policy of city planning was to build more newly urban

land before the 1990s; while after the 1990s, urban growth was the mixture of urban

expansion and redevelopment (Cheng and Masser, 2003). Although more vegetable

land patches developed between 1987 and 1999, the vegetable land became less

fragmented following the urbanization process for the increase of total vegetable

land. The patch numbers of water body rapidly decreased between 1987 and 1993

then slightly increased from 1993 to 1999. The mean patch size of water body

increased from 9.2 ha in 1987 to 28.8 ha in 1993, then dropped to 18.9 ha in 1999.

As illustrated by classi ed satellite images (Figure 2a c), many small water bodies

near the city center disappeared between 1987 and 1993, which led to the increase

of the mean patch size of water body despite the decrease of water body coverage in

this period. On the other hand, the patch numbers of forested land decreased from

1808 in 1987 to 1705 in 1993, then increased to 2016 in 1999. Forested land became

more fragmented, as indicated by the decline of mean patch size, especially between

1993 1999. Shrub-grass land also became more fragmented; the mean patch size

declined from 1.3 ha in 1987 to 0.6 ha in 1999.

3.3. C ONSEQUENCES OF URBAN SPREADING INTO RELATIVELY WILD

HABITATS

Rapid urban growth and concurrent expansion of vegetable land were accomplished

at the cost of reduction and fragmentation of forested and shrub-grass land and the

loss of small water patch and decrease of large water patch. These alterations

appear to signi cantly restrict the functions of relatively wild components in the

lake watershed, such as preserving water quality, providing wildlife habitats, and

air ltering.

The number of lakes in Wuhan City in 1988 was 35, and declined to 27 in

1998 due to the draining and reclaiming the lakes to urban and vegetable land

(Liu et al., 2004). Chemical oxygen demand (COD), an index of water quality

in Donghu Lake, was 4.5 mg/L in 1987 (Jin, 1995), and increased to 5.4 mg/L

in 1993 (Zhu and Chen, 2001), then to 8.0 mg/L in 1999 (Zhou et al., 2002),

indicating a signi cant deterioration in water quality of Donghu Lake over the past

12 years.

The loss of biodiversity is increasing at an alarming rate. For example, the

number of aquatic vascular plants in Donghu Lake decreased from 50 in 1987

1988 (Yao et al., 1990), to 47 in 1994 (Yu et al., 1998), then to 33 in 2001 (Wu

et al., 2003). Of which the number of submerged plants, which play a critical role in

preserving water quality of aquatic ecosystems (Melzer, 1999), also declined from

18 to 10, then to 9 in the corresponding periods (Wu et al., 2003). Additionally,

Hydrilla verticillata, an indicative species of water quality was recorded in Donghu

358 Y. LI ET AL.

Lake in 1987 1988, but it has disappeared based on a long-term survey of 1994 to

2001 (Yao et al., 1990; Yu et al., 1998; Wu et al., 2003).

The invasion of urban into forested land may have reduced the capacity of forest s

air ltering. The nitrogen oxides (NOx ) emissions in Wuhan City increased during

the past 12 years; NOx concentration was 0.054mg/m3 in 1987 (Yang et al., 1991),

and increased to 0.068 mg/m3 in 1993, then to 0.1 mg/m3 in 1999 (Hu et al., 2002).

A study on land-use and land-cover change and its impacts in Atlanta Metropolitan

Area in Georgia indicated that forest coverage correlates strongly and negatively

with nitrogen oxides (NOx ) emissions (Lo and Quattrochi, 2003), which support

our results.

4. Conclusions

This study documented that urbanization could greatly alter the land cover pat-

terns and modify lacustrine landscapes of urban lake watershed in a considerably

short period, characterizing by the spread of urbanization into relatively wild land

cover types, such as urban forest and lake. These alterations have resulted in sig-

ni cant negative consequences, including decline of lakes, loss of biodiversity, and

deterioration of water and air quality.

Urban planners and environmental policy makers are more concerned about how

to balance the protection of natural ecosystems with the inevitable urban growth.

The ndings documented in this study not only elucidate the impact of urbanization

on land cover changes and landscape characteristics in urban lake watershed and

their consequences, but also provide some guide for future urban planning in the

similar areas.

Acknowledgment

This study was funded by the State Key Basic Research and Development Plan

(#G2000046801), WWF-Beijing Of ce (CN0079) and Donghu Experimental Sta-

tion of Lake Ecosystems, Chinese Academy of Sciences. We are grateful to S. L.

Piao and S. Rao for remote sensing and GIS technical assistance, and D. Flynn for

helpful writing suggestions.

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