By Nguyen Viet Chien*, Lam Dao Nguyen* and Pham Bach Viet**
* Information and Remote Sensing Division
Ho Chi Minh City Institute of Physics
National Center for Science and Technology
175 Hai Ba Trung St., District 3, Ho Chi Minh City
Tel.: (84-8) 823-4915, fax: (84-8) 823-4133
email: vientham@hcm.vnn.vn
** Institute of Tropical Biology
National Center for Science and Technology
ABSTRACT
This pilot study made use of digital satellite data of SPOT (1997)
and MESSR (1992) to analyze land-use/land-cover changes of Can Gio mangrove
area, belonging to Ho Chi Minh City in Viet Nam. The analysis was done
to re-estimate the situation of the coastal zone environment, then to
recommend a management model related to environmental factors by application
of GIS technology. The approach applied was based on the integrated management
of coastal zone, emphasizing mangroves. The results show a trend of forest
quality and define a spatial distribution of management zones to enable
sustainable use of resources.
Content
1. Introduction
In recent years, the environmental quality of Viet Nam's coastal zone
has received more attention than previously, because its environmental
problems have been occurring on a wide scope and to a serious degree.
These problems have considerable effects on economic development and environmental
protection. The mangrove ecosystem, one of the more important and sensitive
ecosystems in the coastal zone, has been affected by various activities,
and these cause the extent of mangrove to be reduced, the quality of environment
to be degraded, and biological productivity and diversity to decline.
Other environmental problems also occur, such as shoreline erosion, salinity
intrusion, and degradation of the quality of water and land resources.
According to statistical data from the Forest Inventory and Planning Institute
(FIPI), during 1950 to 1987, 150,000 ha of mangrove were lost; and within
the period of 1987-1995, less than 10 years, another 100,000 ha were clear
cut to convert mangrove land to cultivated land for paddy and shrimp ponds.
This is an unavoidable consequence due to unreasonable management of environment
and resources.
Therefore, to be able to implement an environmentally sound management
plan for sustainable development of coastal zones, particularly mangrove
areas, this pilot study was done, based on the approach of integrated
management of coastal zones, assisted by remote sensing and GIS technology
to analyze environmental trends, particularly land use/land cover, with
the aim of proposing an appropriate model for management. The Can Gio
mangrove area was selected for this pilot study. Satellite digital data
of two different dates, in 1992 and 1997, were used in analyses of changes
of land use/land cover. The results were then combined with other data
from the environment to analyze and define the spatial distribution of
management zones, based on considered management objectives of mangrove
ecosystem.
Content
2. Description of the study area
Can Gio district, a southern peripheral district of Ho Chi Minh City,
is a wetland area limited by the mouths of Sai Gon and Dong Nai rivers,
with an area of 71,400 ha (figure 1).
The area is characterized by 7-10 km per sq km of streams and channels
traversing the area and a semi-diurnal tidal regime with tidal ambient
of 3.5-4 m. Since the average elevation is lower than 1.5 m, two thirds
of the area is submerged under high tide, creating an estuarine saline
wetland. Two main saline zones are defined based on the isoline of 4 per
cent in rainy season (salinity constraint for agriculture) (figure 2).
There are four main soil types (figure 3), including mangrove saline soil,
acid sulphate soil with pH from 4.5 to 6.5, marine sandy soil and sand
dune soil, which all were developed from new deposits of marine and riverine
alluvia in the Quaternary period (figure 4) . Typical species of mangrove
were found in this area, including Rhizophora spp., Avicennia spp., Sonneratia
spp., Ceriops sp., Lumnitzera racemose, Phoenix paludosa, Nipa and others,
for a total of 51 species. The annual average rainfall is about 1,336
mm, with most of the rainfall concentrated in June and August; the rainy
season begins in late May and ends in late October.
In the wartime before 1975, most of the forests were destroyed by herbicides
and defoliants. This was the most serious ecocide (ecological destruction)
in the past, during the period of 1966-1969. A campaign of reforestation
in this area has been implemented by the government since 1978 and it
is continuing to this day.
Currently, there are around 60,000 people in the study area living mainly
on cultivation activities, fishing, logging and charcoal processing, and
shrimp cultivation (shrimp ponds). Although part of this area has been
declared a protected area (a nature reserve), resource use demands a choice
between economic development and environmental protection.
Therefore, environment and resource management of Can Gio mangrove area
has to be planned well in order to satisfy local development, which is
under the influence of urbanization and industrialization. Re-estimation
of land-use trends and mangrove resources and zoning for management are
essential.
Content
3. Geographic database contents
3.1 Maps
Topographic maps at a scale of 1:25,000 were used as base maps. The type
and source of thematic maps (see figures 2-6) that were collected for
this study include the following:
Figures 5
Figures 6
(a) Administrative boundaries (1:25,000, 1991, General Department of
Land Administration of Viet Nam);
(b) Roads and other infrastructure (1:25,000, 1991, General Department
of Land Administration of Viet Nam);
(c) Hydrology: rivers and lakes (1:25,000, 1991, General Department
of Land Administration of Viet Nam);
(d) Soil map (1:50,000, 1986, Department of Agriculture and Rural Development
of Ho Chi Minh City);
(e) Geology map (1:100,000, 1989, General Department of Mines and Geology
of Viet Nam);
(f) Salinity map (1:50,000, 1990, Institute for Economic Research of
Ho Chi Minh City);
(g) Existing land-use map (1:25,000, 1992, Institute for Economic Research
of Ho Chi Minh City);
(h) Environment station map (1:50,000, 1995, Department of Science,
Technology and Environment of Ho Chi Minh City).
3.2 Images
The following satellite images were collected: (a) SPOT: bands 1, 2,
3; path/row 277/329; date 26 February 1997; resolution 20 m; (b) MESSR
: bands 1, 2, 3, 4; lat./long. 10.54/106.94 (Center); date 7 January 1992;
resolution 50 m (see figures 7 and 8)
The coverage of the SPOT image (path/row 276/329) taken in 1987 is only
about 15 per cent of the study area, so this data could not be used for
the study.
In addition, black and white aerial photographs at a scale of 1:14,000,
taken in January 1991, were used as reference data.
Content
4. Methodology
The flow chart in figure 9 describes the overall methodology used in
the study.
4.1 Digital image processing of satellite data
Multispectral data of SPOT (1997) and MESSR (1992) were analyzed and
interpreted to assess changes of land use and land cover (see the flow
chart in figure 10). To enhance visual interpretability and distinction
of features, a linear contrast stretch technique was employed:
where BVin is the original input brightness value and BVr is the range
of the brightness value that can be displayed on a cathode ray tube.
Normalized Difference Vegetation Index (NDVI) was calculated for discrimination
of water, vegetation and non-vegetation in the scene. In the case of MESSR
and SPOT XS, it was calculated by the following formulas:
Nine ground control points (GCP) from base maps of the GAUSS coordinate
system were selected for geometric correction.
Supervised classification with the selection of training areas for each
of the land-cover classes was applied to interpret studied objects. Interpretation
keys were constructed from 21 selected training sites based on field surveys
combined with aerial photos. Objects were classified and grouped into
eight expected land-cover classes.
Results of the interpretation were classified into eight different land-cover
types, and they were then post-classified using a 5 x 5 mean filter to
show only the dominant classifications.
Stratified random sampling, in which each land-cover category may be
considered a stratum, was selected for classification accuracy assessment
of the classified output above. The previous land-use map and ground truthing
data were chosen to make the accuracy assessment.
4.2 GIS analysis
In this study, spatial and tabular analysis were performed to identify
suitable areas for the environment management zone on the basis of weighting
factor of each layer; the process is shown in figure 11.
Relationships between thematic data were analyzed to assign weight, which
is the relative significance of considered factors. This was based on
the requirement of appropriate use of mangrove resources with three major
objectives, including (a) maintenance and protection of mangroves, (b)
protection with intervention for sustainable use of forest resources and
(c) sustainable use for agro-aquacultural activities. Thematic data input
into GIS were analyzed to find their suitability for management objectives.
They were overlaid to define spatial distribution of management zones
following a weighted - linear combination model:
Content
5. Model analysis
In management of coastal mangrove areas, ecological and socio-economic
factors cannot be separated, so they have to be combined when considering
a management model.
Certain ecological factors were regarded as important: (a) estuarine
areas; (b) true mangrove species in terms of vegetation composition; (c)
salinity in water and soils as a constraint factor that supports a good
growth of mangrove species or cultivation activities; (d) forest stands
and cover-rate, which reflect forest status (which is relevant to soil
properties); and (e) tidal regime (frequency and length of submerged time).
It is suggested that those areas of mangroves located near or adjacent
to populated areas be conserved for the utilization of local people. These
areas are prioritized for sustainable local use of mangrove resources.
Extraction of resources is controlled and managed for sustainable use.
This practice is based on ecological management principles that aim to
preserve a balance between human and environment needs, since humans are
part of ecosystems.
There were three management zones recommended:
- Zone 1 = Reserved mangrove zone
- Zone 2 = Intervention mangrove zone
- Zone 3 = Altered mangrove zone
Types and classes of land use, soil, salinity (rainy season), and geology
were ranked based on their suitability for each zone. Ratings were assigned
in three classes (low suitability, medium suitability, and high suitability)
with scores of 1, 2 and 3, respectively. Management objectives of each
zone are fundamental for ranking.
Relative significance among factors (land use, soil, geology, and salinity
in the rainy season) was considered integratively. The given weights were
4, 3, 2 and 1, that is, very high significance, high, medium, and low
significance, respectively (see table 1).
In the overlay process, if an area has a value that is equal to the others,
priority is given in this order: zone 1 (reserved mangrove), zone 2 (intervention
mangrove), and zone 3 (altered mangrove). In other words, if there is
an overlap of suitability between two or three zones, zone 2 and 3 are
eliminated if they are defined within zone 1 or zone 2 (mathematically,
the Boolean AND and OR operator was employed in this process). The following
sections explain the objectives of each zone.
5.1 Reserved mangrove zone
The main objectives in the reserved mangrove zone are (a) to maintain
ecological balance and preserve the mangrove ecosystem and (b) to protect
the coastal area against shore erosion and storms and to regulate local
climate. Intervention is applied only for improving forest quality and
health of the ecosystem. The succession process is accelerated through
replanting and thinning.
5.2 Intervention mangrove zone
The objectives of creating an intervention mangrove zone are (a) expansion
of habitats for birds and other animals looking for food, (b) sustainable
development of aqua-silviculture, (c) protection of river banks and (d)
restoration of ecologically degraded areas. Local use of mangrove resources
is controlled and managed through the approach of silviculture reforestation
and the extraction of timber and firewood through suitable rotation. Settlement
of local people is permitted and activities of aqua-silviculture are undertaken
based on a forest handover policy by local forest managers.
5.3 Altered mangrove zone
The objectives of the altered mangrove zone model are (a) to maintain
activities of agriculture with stable productivity and (b) to prevent
negative impacts on the mangrove ecosystem. A multiple use model of agro-aqua-silviculture
is suggested and it is prioritized for the utilization of local people,
who depend on mangrove resources. An intensive use model is recommended
for local development in this area. Plantations are established on areas
of existing non-mangrove species (inland forest) and higher elevation
landforms (without diurnal or monthly tides) in order to supply fuelwood
and to create an environment for aquaculture. Intensification of rice
culture is suggested where the water supply is sufficient.
Content
6. Results
6.1 Land-use/land-cover types
Eight land-use/land-cover types were classified as follows (see figures
12 and 13):
- (a) agricultural land;
- (b) built-up;
- (c) forest with cover rate over 75 per cent;
- (d) forest with cover rate 50-75 per cent;
- (e) forest with cover rate less than 50 per cent;
- (f) water bodies;
- (g) mud, sand fields; and
- (h) others.
The accuracy of the land-use/land-cover map analyzed from the SPOT image
is 87.2 per cent, and from MESSR it is 84.8 per cent. The class "mangrove
<50 per cent" in the classified images of 1992 and 1997 have low
accuracy because of the confusion of this class with the "mangrove
50-75 per cent" class. Given the main land-use categories of the
study area, the classes "water bodies", "mud, sand field",
"waste land", "mangrove >75 per cent", and "mangrove
50-75 per cent" from MESSR data and most of the classes of SPOT data
have satisfactory accuracy (more than 80 per cent). In general, SPOT data
with 20-m resolution is preferred in coastal zone land-use/ land-cover
mapping.
6.2 Land-use changes
Changes of land use between 1992 and 1997 are mainly in forest, built-up
area and agricultural land. Forest area changes in cover rate from over
75 per cent to 50-75 per cent. This is the change of forest quality, which
is an alteration from high cover rate to medium cover rate. In 1992, the
area of mangrove with cover rate of 75 per cent was 9,945 ha, but in 1997
that coverage was found in only 3,585 ha; at the same time, the area of
mangrove with 50-75 per cent cover rate increased from 642 to 16,866 ha.
These changes are due to forest degradation, including logging and thinning
in mature forests.
The total area of agricultural land was reduced from 9,555 to 5,285 ha,
mainly changing into forest land (reforestation). Built-up land is in
the same situation as agricultural land, reducing from 969.1 to 433.9
ha. In general, the greatest changes are concentrated in area of forest
categories, agriculture and built-up land (see table 2 and figures 14-16).
However, in terms of land-use structure changes between 1992 and 1997,
the change in built-up area does not correspond to actual changes in Can
Gio District, that is, the land requirement for construction and new roads.
In addition, the change in forest quality is too great, and it is also
not of high consistency. This high difference is due to in part to the
use of satellite data sources with different spatial resolutions in the
processing stage; doing so causes confusion between built-up areas and
other categories, and between forest cover rate of 75 per cent and 50-75
per cent.
6.3 Management zones
Management zones were spatially defined based on structure of land use,
specific features of the mangrove area (vegetation and current forest
cover rate), environmental characteristics (soil, geology and salinity
distribution), present status and requirements of socio-economic development,
and the environmental role of the area for Ho Chi Minh City, as described
below (see figure 17; see also figures 18-25 for photographs of the study
area)
(a) Zone 1: Mangrove management for ecological equilibrium and environment
protection (prevention of shoreline erosion, reduction of saline intrusion,
climatic regulation);
(b) Zone 2: Mangrove management with intervention for sustainable development
of agro-aquaculture;
(c) Zone 3: Other economic activities in areas in which forests were
lost.
Forests with cover rate over 50 per cent are maintained for typical mangrove
species within zone 1, covering 62 per cent of the area, and also existing
in those areas with suitable conditions for growing mangrove. In terms
of management, intervention is done only to improve forest quality; this
is the minimum area to ensure ecological balance and environment protection.
Zone 2 is distributed in areas with cover rate less than 50 per cent and
areas experiencing physical limits for agriculture (water source and saline
soil). This zone covers 22 per cent of the area. Nature or semi-nature
approach is applied to extract resources; forest plantations are employed
for timber. Zone 3 is prioritized for agricultural activities and other
economic activities.
Content
7. Conclusions
Results of interpretation and analysis of satellite data on land-use/land-cover
changes showed actual changes in the area during 1992-1997. However, the
use of data sources at different resolutions has a slightly adverse effect
on the accuracy of classification of land cover (particularly forest).
This study also shows that SPOT data with high resolution are suitable
for estimating changes of land use/land cover, especially for mangroves.
NDVI shows good results in discriminating objects such as water bodies,
vegetation and non-vegetation. NDVI can support the orientation of mangrove
management and monitoring changes over time.
Environmental data and the results of remote sensed data analyzed in
GIS are useful in supporting decision-making. They help to define a spatially
suitable management plan for the Can Gio mangrove area.
Acknowledgements
The authors of this pilot study would like to express their gratitude
to the Government of France for its financial support. Acknowledgement
is also expressed to the Space Technology Applications Section of the
Environment and Natural Resources Development Division in ESCAP, the Department
of Geography at East China Normal University, the Institute of Economic
Research of Ho Chi Minh City, the Department of Science, Technology and
Environment of Ho Chi Minh City, and staff members of the Information
and Remote Sensing Division of Ho Chi Minh City for all their help in
this study.
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Annex I
Data dictionary
Annex II
Reference dates of data used in the study
1. Maps
1.1 Base maps
In this study, the following topographic maps (at scale 1:25,000) were
used as base maps for geographic data base of the study area: sheet Nos.
C-48-46Cd, C-48-46Dc, C-48-58Ab, C-48-58Ad, C-48-58Ba, C-48-58Bb, C-48-58Bc,
C-48-58Bd, C-48-58Cb, C-48-58Da, C-48-58Db, C-48-58Dc, C-48-58Dd, C-48-59Aa
and C-48-59Ac, dated 1991 in GAUSS projection (produced by the General
Department of Land Administration).
1.2 Thematic maps
The types and sources of thematic maps that were collected for this study
are listed in the table below.
2. Images
2.1 Satellite imagery
The type and characteristics of satellite images in CD-ROM form which
were collected for digital image processing are listed in the table below.
2.2 Aerial photographs
Black and white images at 1:14,000 scale taken in January 1991 have been
selected as reference data; the source is the General Department of Land
Administration of Viet Nam.
2.3 Ground photographs
Color images taken in October 1998 during a two-day field trip were selected
for the training sample.
3. Attribute data
Secondary data were collected to understand the existing conditions.
The main sources of data are from the Statistics Department of Ho Chi
Minh City and the Institute for Economic Research of Ho Chi Minh City.
The following statistical records were collected:
- Population statistics in 1994 at village level
- Agriculture area statistics of the study area in 1995
- Built-up area statistics of the study area in 1995
- Forest area statistics of the study area in 1995
- Water body area statistics of the study area in 1995
- Other land area statistics of the study area in 1995
- Soil characteristics of the study area
- Water quality data in 1997
- Socio-economic data of the study area in 1990-1997
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