Identifying Optimum Sites for locating Reservoirs employing Remotely Sensed Data and Geographical Information Systems.

Identifying Optimum Sites for locating Reservoirs employing Remotely Sensed Data and Geographical Information Systems.

K. Wan Yusof¹ and Serwan M. J. Baban ²
¹Email: apx158@coventry.ac.uk
GRRU Group, Geography, School of Natural and Environmental Sciences, Priory Street, CV1 5FB. U.K.
Tel. no: 44 024 76 838444 Fax. no: 44 024 76 838409

²Email:sbaban2001@yahoo.com
Department of Surveying & Land Information, The University of the West Indies, St. Augustine, Coventry University, Coventry, Trinidad, West Indies
Tel: 001 868 662 2002 ext. 2108 Fax: 001 868 6624414

Abstract
Langkawi Island in Malaysia, faces the possibility of having water shortages in the future, building reservoirs have been promoted as a possible solution to meet future demands for water supply. In this study, a criteria was developed to locate reservoirs taking into consideration all relevant factors including; topography, geology, hydrology, location in relation to both abstraction and supply points, land use/cover types and settlements.

A satellite imagery and digitised geological and elevation maps were analysed and used to generate the necessary data layers to satisfy various conditions within the established criteria. Then, IDRISI, a raster based GIS was employed to implement the criteria using two different methods to combine the information layers. First, the Boolean method which considered all the layers as being equally important to the process and gave them an equal weight. Second, theWeighted Linear Combination (WLC) method which grouped the layers and graded them according to their perceive importance. The Boolean method produced five reservoir sites located at the northern, southern and eastern of the Island. Whilst, the WLC method produced five sites located mainly towards the central area of the Island.

Comparing these outcomes with a field based study with a similar objective on the Island, which identified six suitable reservoir sites, showed that two of the sites located using the Boolean method and four of the sites identified using the WLC method have corresponded well with the field-based study sites.

This study indicates that the developed criteria for locating reservoirs are sensitive to the physical, environmental and economical settings on the Langkawi Island. Furthermore, GIS and remote sensing can be useful tools for generating, manipulating and handling relevant data layers, leading eventually to identifying a number of optimum sites for locating reservoirs and ultimately providing options and, assisting with the planning process.

1. Introduction
A study was conducted in Langkawi Island which faces the possibility of having water shortages in the future as a result of rapid development. The study concluded that the total water demand may increase three times greater in 2010 compared to 1993 (Syed, 1992). Building reservoirs has been promoted as a possible solution to meet the future demand of water supply (Baban and Wan Yusof, 2000).

This paper aims to develop and implement a set of criteria to locate reservoir sites on the Langkawi Island using remotely sensed data and GIS.

2. The study area
Langkawi Island lies off the northwest coast of Peninsular Malaysia and located approximately at latitude 6o20'N and longitude 99o40'E (Fig.1). Its annual average rainfall is 2430mm (Shaaban and Sahat, 1995; Wan Yusof and Baban, 1999). Most streams have small catchment areas and they tend to flow downstream rapidly from the mountain to sea. As such their flow tend to cease during periods of dry weather (Syed, 1992).

3. Methodology
Factors influencing reservoir site selection includes topography, hydrology, geology, soil, land use/land cover, road network as well as socio-economic and environmental factors (Gismala et.al., 1996; Murphy, 1977). Based on these factors and the development guidelines by Langkawi District Council (1992), criteria for locating reservoir in the Island was developed (Table 1). The reservoir sites should not be in the vicinity of densely populated areas, must be located on land providing a strong foundation and there should not be any development in the forest reserve areas, hence constraint criteria 1, 2 and 3. Constraint criteria include numbers 4, 5 and 6 will ensure the necessary degree of suitability within the classified areas such as avoiding high grade agricultural land, having the pre-requisite altitude and slope. Constraint 7 was included to account for the projected demand for water supply in the Island. Total water demand was projected to be 70 megalitre/day (Mld) in 2010 (Syed, 1992). As a general guideline, a common dam height is between 15 and 30 m (Morris and Fan, 1997).

Table 1. Reservoir location constraints criteria

Criteria	Consideration
The dam and reservoir site must:
(1) not be located in or within settlement areas	Safety
(2) be on granite and/or metamorphic rock	Safety
(3) avoid forest reserved areas	Resources/Environment
(4) avoid high grade agricultural land value areas	Resources/Environment
(5) be at an altitute of between 25-90 m	Hydraulic/ Economic
(6) be on a gentle slope of 0^o-11^o	Environmental/Safety
(7) have a sufficient surface area to provide the necessary volume	Consumption/Economic

If two reservoirs were developed, the water demand would each reduce to 35 Mld in 2010. This would require a water storage capacity of 16,425 x 103 m3 yr-1 with a maximum surface area of 85.2 ha for dam height of 15 m and a minimum surface area of 25.5 ha for dam heights 50 m.

4. Use of remote sensing and GIS in the site selection process
A land use/cover map of the Langkawi Island was produced from satellite data of 1995 and field data using a Maximum Likelihood Supervised classification (Baban and Wan Yusof, 2000). The land use/cover map was used to extract information and construct layers of information for constraints 1, 4 and 5, i.e. to identify urban areas, to protect agricultural and forest areas respectively. IDRISI, a raster based, GIS was employed to create information layers corresponding to each constraint. In implementing the criteria, these relevant information layers were combined using two different methods. First, the Boolean method which considered all the layers as being equally important and gave them an equal weight. Second, the Weighted Linear Combination (WLC) method which grouped the layers and graded them according to their perceive importance.

In the Boolean method, all the criteria are reduced to constraint Boolean images of areas which are suitable and not suitable. The constraint layers were subsequently overlayed consecutively, by using the OVERLAY multiply function to produce a single suitability Boolean image. Subsequently, the suitable areas in the last Boolean image were regrouped to determine the surface area. The output image was achieved by using the GROUP and AREA functions of IDRISI. The GROUP function was used to assign identifiers to unite groups of pixels and the AREA function was used to calculate the surface area for each group. Finally, the RECLASS function was used to select only those areas (groups) that can comply with the specified surface areas (Table 1) (Fig.2).

In the WLC technique, the criteria are standardised to a continuous scale of suitability from the least to the most suitable, thus giving a flexibility in the site selection (Eastman, 1997). The procedure in the WLC required that the principal eigenvector of the pairwise comparison matrix be computed to produce the best-fit set of weights. Subsequently, the acceptable best fit of the respective weights were used in the Multi Criteria Evaluation function (MCE) to calculate the weighted linear combination (WLC) using the factors (slope, height, land value, and erosion) and constraints (forest reserve, settlement zone, and geological foundation) images. Then using the GROUP, AREA and RECLASS functions again, potential sites with the highest suitability were produced (Fig.3).

5. Results and discussions
A field study to locate reservoir sites on the Langkawi Island based on topography, catchment areas, hydrology, land use/cover, accessibility and socio-economic factors has identified six possible reservoir sites (Fig.4) (Syed, 1992). These areas are the Limbong, Ulu Melaka, Upper Ayer Tawar, Lower Ayer Tawar, Batu Asah and Langkanah. This information has been used to evaluate the outcomes from the two methods employed in this study.

In the Boolean approach, the selected areas were absolute i.e. there were no trade-offs with other constraints criteria. Consequently, suitability in one criterion cannot compensate for a lack of suitability in any other image (Eastman, 1997). In terms of risk, this approach is very conservative. The final outcome of potential sites produced satisfied all the criteria: the reservoir sites selected were within the surface area required, were outside the settlement zone, located at an altitude between 25-90m, were on a gentle slope of 0 to 11o, have a strong foundation on either granite or metamorphic rock, and were not within the forest reserve areas. In Fig.2, two of the potential reservoir sites were located in the northern, one in the western, one in the southwest and one in the south of the Island. Two of the potential reservoir sites (Kg. Nyior Cabang and Bukit Tekoh) using the Boolean method corresponded well with the field-based study areas (Fig.2, Fig.4).

Using the WLC method, factors assigned to each criterion had been more flexible. This allowed the factors to compensate for each other while maintaining all of the variability in the continuous suitability data (Eastman, 1997). There were five potential reservoir sites produced by this method, these were located around the central region of the Island (Fig.3). These potential sites are more practical due to their central locations, as this will reduce costs in pipeline distribution throughout the Island. Four of the sites (Limbong, Ulu Melaka, Lower Ayer Tawar and Upper Ayer Tawar) identified using the WLC method matched with the field-based study areas (Fig.3, Fig.4).

6. Conclusion
A set of criteria was developed to locate suitable reservoir sites in Langkawi Island. The criteria chosen are comprehensive, taking into consideration all relevant constraints based on hydrology and hydraulics, topography, geology, economy, site location in relation to the points of abstraction and supply and environmental implications. The main considerations being safety, economy and the environment.

The inclusion of environmental and social considerations for locating reservoir sites has made the decision-making process more complicated by adding extra information layers. However, the advent of GIS has helped to overcome these multi-criteria considerations in the decision-making process.

Two approaches were considered for reservoir site selection: Boolean and Weighted Linear Combination. The Boolean technique is more direct in its approach to decision making and is very conservative in taking risk. Reservoir sites located by using the Boolean approach were scattered over the Island and two of them corresponded well with the field-based study areas. In the Weighted Linear Combination technique, its flexibility in assigning factors allowing them to compensate for each other thus gave more allowance and effectiveness in the suitability analysis of reservoir site selection. Four potential sites matched those of the field-based study areas water requirement in 2010. The sites were located at Limbong, Ulu Melaka, Lower Ayer Tawar and Upper Ayer Tawar.

Overall, this study has shown that the criteria chosen were sensitive and comprehensive. Furthermore, it has demonstrated the effectiveness of using remotely sensed data in providing the necessary spectral and spatial information for generating information layers for reservoir sites selection criteria. The GIS as a decision-making tool, has facilitated combining various information layers as well as implementing the necessary analysis on the data.

References

Baban S. M. J., and Wan Yusof, K. (2000). Mapping land use/cover distribution in a mountainous tropical island using remote sensing and GIS. International Journal of Remote Sensing, (in press).
Eastman, J. R. (1997). Multi-Criteria Evaluation - Boolean and Weighted Linear Combination. In Tutorial Exercises: Idrisi for Windows Version 2.0. Clark Labs for Cartographic Technology and Geographic Analysis. Idrisi Production, Clark University, pp. Adv-24 to Adv-34.
Gismalla Y. A and Bruen M. (1996). Use of GIS in reconnaissance studies for small-scale hydropower development in a developing country: A case study from Tanzania. In K. Kovar and H. P. Nachtnebel (Eds.), HydroGIS 96: Application of Geographic Information Systems in Hydrology and Water Resources Management, Proceedings of the Vienna Conference. IAHS Publ. no 235, 307-312.
Langkawi District Council. (1992). Langkawi Structure Plan 1990 - 2005. Prepared by Langkawi District Council, Section 5, 1-22.
Murphy, M. (1997). Planning and environmental studies. In A. R. Golze (Ed.), Handbook of Dam Engineering. Van Nostrand Reinhold Co., London, 1-96
Shaaban, A. J., and Sahat, R. (1996). Hydrological charcteristics and surface water availability of Pulau Langkawi. In Seminar on Hydrology on Small Islands, Langkawi, Kedah, Malaysia, 29 December, 1996. Published by the Department of Irrigation and Drainage, Malaysia, Kuala Lumpur, Malaysia, Section 1, paper 1.
Syed, M. (1992). Review of surface water resources and water supply system, detailed design and construction supervision of immediate works in Pulau Langkawi. Water Resources Study- A Final Report, Syed Muhammad, Hooi and Binnie Limited Company, Kuala Lumpur, pp 76.
Wan Yusof, K., and Baban S. M. J. (1999). A preliminary attempt to develop an erosion risk map for Langkawi Island, Malaysia using the USLE, remote sensing and GIS. Proceedings of the 20th Asian Conference on Remote Sensing, Hong Kong, Vol.1, 85-90.

Fig.1 The study area (after Baban and Wan Yusof, 2000)

Fig 2

Fig.2 Potential reservoir sites using Boolean method

Fig 3

Fig.3 Potential reservoir sites using Weighted Linear Combination (WLC) method

Fig 4

Fig.4 Field-based potential reservoir sites for the Langkawi Island
(Source: Syed, 1992)