Abstract: The present study assess spatial change in the mangrove cover over the last three decades in the Godavari delta, using Landsat MSS, TM, ETM + and IRS P6 LISS III images. From 1977 to 1988, the mangrove vegetation cover had a drastic change and lost about 24.6 km2, which was mostly recovered during the next period (1988-2000). Over the entire three decade period the net change was negative, which was quantified as 8.8 km2. It is also observed that plantation cover shows increase of additional 7.6 km2 during 1977 to 2005. In Coringa Wildlife Sanctuary the net change was positive with an increase in mangrove area of 107.2 to 114.2 km2. The study concluded that mangrove loss is mainly due to encroachment by aquaculture, agriculture and coastal erosion. The gain in the forest cover is attributed to the accretion, restoration and protection.
INTRODUCTION
Mangroves are a group of highly adapted halophytes occupying the intertidal zone in estuaries, lagoons and coastal mud flats in tropical and subtropical regions of the world. Mangroves are socioeconomically important ecosystems, especially for the inhabitants of coastal regions, who depend on them as their primary source of income, fuel, food, medicine and other basic necessities and also safeguard the coastal environments.
Mangrove forests are undergoing constant changes-short term and/or successional/long term-due to it's dynamic nature itself and to a greater extent through various natural and biotic influences. Growing industrial areas along the coastlines and discharge of domestic and industrial sewage are polluting these areas. The expansion of agriculture and aquaculture farms in the coastal areas has led to conversion of mangroves in the recent past (Barbier and Cox, 2004). The extent of mangroves has also changed due to the accretion near river mouths, leading to the formation of new mangrove areas.
An accurate and up-to-date information on the status of mangrove vegetation, continually over time, is pre-requisite for a sustainable management of mangrove forest. Traditional field survey inside the mangrove swamps is extremely difficult. Remote Sensing (RS) and Geographic Information System (GIS) provides valuable aids for the purpose because RS and GIS is a fast, efficient and accurate mean of information retrieval to detect such changes continually over time (Ricketts, 1992; Hashiba et al., 2000). The information gained can be utilized for effective planning and management of mangrove forest.
The mangrove forests comprise 15.8 million hectares, roughly less than half the original mangrove forest cover and are fast declining further at an assumed rate of 2 to 8% per year in the world (Spalding et al., 1997). India has a total area of 4461 km2 under mangroves which is 0.14% of the countries total geographic area. It account for about 5% of the world’s mangrove vegetation (Anonymous, 2003). Nearly 57% mangroves are found along the east coast (Kathiresan, 2003). There is a decline of 59.18 km2 of mangrove between 1972-1975 and 1980-1982 (Anonymous, 1983). According to the Government of India Report (1987), India lost 40% of its mangrove area during the last century (Kumar, 2000).
Mangroves of Godavari Delta
The mangrove forests in Andhra Pradesh are located in the estuaries of the
Godavari and the Krishna rivers. The Godavari mangroves are lies between the
latitudes 16°30’ and 17°00’ N and the longitudes 82°11’
and 82°21’ E in East Godavari District, East Coast of India. The mean
annual rainfall in this region is 110 cm and the annual mean temperature is
28°C. The Godavari River is connected to Kakinada Bay by two major canals,
namely the Coringa and the Gaderu. There are numerous other small canals branching
from these canals, supplying tidal water to the mangroves. The Godavari River
flows into the sea through two mouths, one near Bhairavapalem in the north and
the other near Kottapalem in the south.
The presence of numerous canals, river tributaries along with mangroves makes Godavari delta as a unique habitat (Tripathy et al., 2005). Anthropogenic activities are such as discharge of industrial and municipal sewage, aquaculture, tourism, maritime transport, offshore oil exploration and production, dumping at sea degrading the marine environment of Godavari.
Vadlapudi (1999) carried out change analysis on mangroves of Godavari using IRS 1B LISS II data and observed that during a span of one year period (1994-1995), the spread of aquaculture has caused destruction of mangrove up to 22.69 ha. Ramasubramanian et al. (2006) analysed the remote sensing images of 1986 and 2001 and reported that the changes in the vegetation due to forest restoration and natural regeneration are significant.
Realizing the importance of Godavari delta and mangroves, the present study was undertaken to assess the three decade spatial changes in vegetation dynamics using multi-temporal satellite data and Geographical Information System (GIS).
Coringa Wildlife Sanctuary
Coringa Wildlife Sanctuary, a part of the Godavari mangroves was declared
as a sanctuary in July 1978 to conserve the mangrove vegetation of the estuary,
extending in an area of about 236 km2 Coringa Wildlife Sanctuary
has three Reserve Forests, namely Coringa R.F., Coringa Extn. R.F. and Bhairavapalem
R.F. Most of the mangroves in the Sanctuary are not directly connected with
the Bay of Bengal. The Gaderu and Coringa rivers are the distributaries of the
River Godavari. The mangroves of Coringa Wildlife Sanctuary receives tidal flushing
through Matlapalem canal, Coringa river and Gaderu river. The other six Reserve
Forests namely Rathikalava RF, Masanitippa RF, Matlatippa RF, Balusutippa RF,
Kothapalem RF and Kandikuppa RF are situated on the southern side of Nilarevu
River and fall under non-sanctuary area. The Sanctuary has an unique distinction
of having a 18 km long sand spit in the north eastern side (Hope island), where
the species of olive Ridley sea turtle (endangered species) nests during January-March
of every year. The sand spit of Hope Island has changed with time and has grown
nearly 2.6 km between 1937 and 2001 (Ramasubramanian et al., 2006).
MATERIALS AND METHODS Field Survey
Field survey was carried out in the study area during April 2006 to study the vegetation and other land cover. False colour composite satellite images and Survey of India topographical maps (65 H and 65 L) were used to collect ground information. Ground truth data was collected using GPS, at various locations before the image classification.
Remote Sensing Techniques Data Used
The study was carried out using temporal satellite images of Landsat MSS
(Landsat Multi Spectral Scanner-path/row: 152/48; date 8th January, 1977), Landsat
TM (Landsat Thematic Mapper-path/row: 141/48, 141/49; date: 12th October, 1988),
Landsat ETM + (Landsat Enhanced Thematic Mapper+-path/row: 141/48, 141/49; date:
8th December, 2000) and IRS P6 LISS III (Indian Remote Sensing P6 Linear Imaging
Self Scanner-path/row: 103/61; date: 13th December, 2005). The orthorectified
Landsat data was downloaded from GLCF website (http://www.glcf.umiacs.umd.edu/).
The images were processed using the ERDAS Imagine 9.0 software.
The Landsat TM, ETM+ and IRS P6 LISS III images was geometrically corrected in relation to the MSS image to obtain the same spatial resolution (since Landsat Multi Spectral Scanner (MSS) image downloaded at 57 m resolution from http://www.glcf.umiacs.umd.edu.) i.e., from 30 to 57 m (in case of IRS LISS III from 23.5 to 57 m).
Study Area Image Extraction
Mosaicing of satellite data, corresponding to the study area was done. To
understand spatial patterns with in mangroves and also surroundings towards
landward side, a buffer of 5 km was generated from mangrove boundary using January
1977 data as a base. By using 5 km buffer as mask, study area has been extracted
from Landsat MSS, TM, ETM+ and IRS P6 LISS-III scenes (Fig. 1).
Classification
Supervised digital classification method is used for mapping of vegetation
and land cover of the study area. This has been the most frequent method for
remotely sensed data classification. In supervised classification the samples
of known identity were used to classify pixels of unknown identity. Training
sites in the images are generated to represent the typical spectral information
of the land cover classes (dense mangroves, open mangroves, plantations, agricultural
lands, built up area, aquaculture, sand, mudflats and water bodies). After the
selection of training sites, the classification was run on the images using
Maximum likelihood classifier in ERDAS Imagine 9.0 software. Prior to the classification,
signature separability analysis was carried out on the training signatures for
better classification.
| Fig. 1: | Multi-temporal false colour composite images of Godavari mangroves |
To determine the accuracy of the thematic maps obtained using the supervised classification, an accuracy assessment was carried out using the GCPs recorded during the field survey. Overall accuracy was calculated, based on error matrix method (Congalton and Green, 1999) that included mangrove, non mangrove and water classes, as the percentage of agreement between GCP reference points and the classification results.
Change Detection
After the images of the four time periods (T1, T2,
T3 and T4) were classified independently, the post classification
change detection method was applied to determine the changes and to see the
nature of changes. The change in the spatial extent of mangroves and other land
cover types detected in the three decades were analysed. From the change map
the change area matrix table was generated for the result and discussion.
RESULTS AND DISCUSSION
Delta of Godavari River with rich alluvial soil, abundant water supply and highly favorable geomorphic terrain, predominantly support agricultural lands. Nine major land cover types were delineated using satellite data viz: Dense Mangroves, Open Mangroves, Plantations, Agriculture, Built up area, Aquaculture, Water bodies, Mudflat and Sand (Table 1).
From land cover map of the year 2005, it may be seen that water bodies (sea/rivers/streams/canals/reservoir/tank) in the study area constitutes 44.1% of the area. Another major feature is most of the inland area is under intense agriculture (20.2%). In 1977, the forest cover (Dense Mangroves and Open Mangroves) accounted for about 194.8 km2 (Table 1, Fig. 2).
The changes in the mangrove cover have been assessed and the details were given in Table 1-4. There were inter-period variations (1977-1988, 1988-2000, 2000-2005), both positive and negative, those are related mainly to the dynamics of the natural mangrove vegetation structure, accretion, erosion and anthropogenic influence (Fig. 3).
| Table 1: | Areal extent of vegetation and other land cover in Godavari mangroves: study area (area in km2) |
| Table 2: | Change area matrix of vegetation and other land cover in Godavari mangroves: study area during January 1977 to December 2005 (area in km2) |
| Fig. 2: | Classified vegetation and land cover maps of Godavari mangroves |
| Fig. 3: | Vegetation and land cover Change maps of Godavari mangroves |
In addition to it, variations in the extension of mangrove as a result of between-date tidal differences, especially in the area of the bay mouth bars were observed. But these changes are not large enough to consider as the main source of error in the classification.
From 1977 to 1988, the mangrove vegetation cover had a negative change and lost about 24.6 km2, which was mostly recovered during the next period (1988-2000), giving a total balance of 185.2 km2 from 1988 to 2000. In the last period (2000-2005) there was gain with an area of 0.9 km2, but over the entire three decade period the net change was negative, which was quantified as 8.8 km2. During January 1977 to December 2005, the important positive instances to be mention are an increase of Plantations cover (7.6 km2) and formation of new mangroves as a result of accretion and restoration. In any case, it was not possible to analyze the change in species composition and our results take into account only the mangrove/non mangrove covers.
| Table 3: | Areal extent of Vegetation and other land cover in Coringa Wildlife Sanctuary (area in km2) |
| Table 4: | Change area matrix of vegetation and other land cover in Coringa Wildlife Sanctuary during January 1977 to December 2005 (area in km2) |
The change analysis map shows that the major changes have been taking place in the proximity of agricultural lands as a result of high anthropogenic pressure (Fig. 2). It is noticeable that 13.2 km2 of mangrove have been encroached upon by commercial aquaculture (8.2 km2) and agriculture (5 km2) during the 1977-2005. Due to intense anthropogenic pressure 34.8 km2 of mudflats have been converted to human land use (aquaculture and agriculture). Due to erosion and tidal activities, around 15.2 km2 of mangrove cover has undergone inundation in three decades (Table 2).
As part of plantation efforts by Andhra Pradesh State Forest Department, there has been a significant increase in the area (7.6 km2) under forest plantations, which is mostly constituted by Casuarina equisetifolia.
The changes in the vegetation due to forest restoration, natural regeneration and accretion are appreciable during 1988 to 2005. In this period, the gain of 15.8 km2 area of mangrove cover was noticed. Most of the recent studies in other parts of country reported a loss of mangrove cover due to anthropogenic pressure and natural factors (Samant, 2002; Barnali, 2006; Reddy et al., 2007). Until now, it seems that this is one of the few studies of the India that reports an increase in the mangrove cover since 1988 (Singh et al., 2004). The sand spit of Hope Island has grown drastically and now bears plantations and new mangrove formations in the northern part (Ramasubramanian et al., 2006).
Accuracy assessment was carried out with total 90 ground control points to assess the accuracy of the supervised classification of different periods. The overall accuracy assessment stands at 95.5% for latest data of 2005 (Table 5).
Coringa Wildlife Sanctuary
Analysis of vegetation and land cover inside Coringa Wildlife Sanctuary
reveals that during 2005, mangrove cover occupies 114.4 km2 (48.4%)
area of the sanctuary. The second highest land cover class is water bodies,
which represents 43.4% of area. An increase in mangrove area of 7.1 km2
was observed compared to 1977 (Table 3).
| Table 5: | Accuracy assessment of vegetation and other land cover classification for Godavari mangroves: study area |
| *: UA: User's Accuracy (%); **: PA: Producer's Accuracy (%) | |
In addition, plantation activity by State Forest Department has resulted 1.8 km2 area under plantation cover by planting of Casuarina equisetifolia. Mudflats are mainly contributes to the area for regeneration and restoration of the mangroves in the sanctuary.
It is observed that an area of 2.9 km2 was converted to aquaculture ponds by 2000, was under recovery and reduced due to conservation approaches to an area of 1.2 km2 by 2005 (Table 4).
It is clear that the mangrove vegetation cover can display great variability depending on its dynamics, environmental events and developmental activities in the area.
CONCLUSIONS
An estimate of spatial extent of mangrove and other land cover classes have been worked out. Three decadal time series data reveals that the period of 1988 was critical with reference to mangrove cover. By 1988, there is a drastic reduction in mangrove forest, which was quantified as 24.6 km2. In spite of various causes of depletion of mangrove cover, it has been found that there is an overall increase in mangroves as compared with 1988 by 2000 and 2005. But over the entire three decade period the net change was negative, which was quantified as 8.8 km2.
During the past three decades accretion and restoration have played a major role in the formation of mangrove areas as well as improvement of mangrove cover. Overexploitation, conversion of mangrove into other land uses and coastal erosion were found to be the three major threats.
It is observed that there is a significant positive change in the mangrove cover of Coringa wildlife sanctuary in terms of area. Due to natural growth in terms of restoration and regeneration and protection efforts by the forest department the area under mangrove has increased in Coringa Wildlife Sanctuary. But being a dynamic system there has been lot of changes in the mangrove cover as evident by the change matrix table.
ACKNOWLEDGMENTS
We thank Dr. P.S. Roy, Deputy Director and Dr. M.S.R. Murthy, Head, Forestry and Ecology Division, National Remote-Sensing Agency for constant encouragement and Dr. K.N. Reddy, Laila Impex, Vijayawada for field support. We also thank Global land cover Facility Programme, University of Mary Land, USA for providing Landsat data and Forest Department of Andhra Pradesh for their necessary support.