Detection and mapping of vegetal cover depletion of a catchment areas using Remote Sensing
Mustapha K. Idris & Hamirdin I.
Department of Geography University Malaya 59100 Kuala Lumpur, Malaysia
1. Introduction
Vegetal cover , especially types of vegetation, plays a crucial role in the hydrological cycle of an areas where water resources are being exploited. Durian Tungal dam, in the state of Melaka, Malaysia became completely dried up and rendered inoperative in January 1991. The inclement dry spells in the state and errors in calculating the depth-level of the reservoir were tendered as major causes but the development of events was far too gradual as to ignore the importance of soil canopy cover over the dam watershed area. Recent studies showed that the watershed areas have been gradually depleted of forest cover over a period of years in lieu of developmental projects. This paper focuses on mapping the depletion of vegetation cover and the consequences that resulted in the inability of the dam to widstand the prolonged dry spells.
2. Study Area
The study area is located in the north-eastern part of the state of Melaka. Being one of the smaller states in Malaysia, Melaka is experiencing an expanding population and rapid industrialization. The dam, located near the town of Simpang Gading, is an earth-filled impounding dm. It was envisaged that water from the dam is discharged regularly into the intake plant on sungei Melaka before being processed for domestic consumption.
The watershed covers an area of about 41 km2 and is drained by Sungai Machap Umbor, Sungei Ayer Pasir, and other tributaries. In 1974 the vegetation cover comprised mainly of rubber plantations and smallholdings, some patches of secondary forests and belukar, while the upper reaches was mainly under the Bukit Sedanan forest reserve. The topography is generally low and undulating in the lower regions while the headwaters consist of hilly and dissected terrain. The highest point, Bt. Sedanan, at 1093 metres also marked the northern boundary of the watershed.
3. Materials and Methods
Temporal detection of thematic features based solely on remote sensing data, especially in humid tropical region, has been established. Problems relating to cloud cover and non-availability of time-specific images, amongst others, have been recognized. This study is no exception . A two-stage procedure was followed. Firstly, topographic maps of 1959 and 1974 were combinely utilized to derived a vegetation map of the study area prior to the construction of the dam as shown in Fig. 1. The watershed boundary was also delineated based on contours present on the same maps. Reference to aerial photographs taken around this period was made to ascertain the land cover then present.
Figure 1. Land cover of Durian Tunggal Catchment Area. 1974
Tow Landsat TM images of the area were available for this study. The 1990 image was found unsuitable as most of the watershed areas was under heavy cloud cover. Another Landsat TM image of the same area, path/row 126/58, taken on 02.07.89 was selected, despite the presence of clouds, for the second stage. A 512 x 512 pixel subset of the image was inputted for processing using IDRISI, a grid-based geographic analysis system, which also incorporates an image processing module. A variety of modules was used to generate a
landcover classification of the area using normalized ratio, principal component and clustering. Visual interpretation of the grey-scale display of TM4 was also pursued for mapping purposes. The same band was also used to digitize the boundaries of the watershed and the impounded lake.
A field study to update land cover types was undertaken in late 1990 and early 1991 and the observations were incorporated with those derived from the satellite data, as in figure 2. Measurements relating to stream characteristics within the watershed area were also made for further analysis.
Figure 2. Land cover of Durian Tunggal Catchment Area. 1989
