International Conference on Water Resources (ICWR 2009) 26 27 May 2009 | Bayview Hotel, Langkawi, Kedah, Malaysia

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International Conference on Water Resources (ICWR 2009) Impact of Climate Change and Its Variability on the Rainfall Pattern in Sarawak River

Basin

Charles Bong Hin Joo a, Ting Sie Yew a, Rosmina Ahmad Bustamia, Frederik Josep Putuhena a

a Department of Civil Engineering, Faculty of Engineering, Universiti Malaysia Sarawak,

94300 Kota Samarahan, Sarawak, Malaysia Email: bhjcharles@feng.unimas.my

Abstract Climate change due to global warming is expected to play a role in determining the availability of water in years to come. An understanding on the potential effects of climate change on the hydrology of a river basin is important in preparing adaptability measures that will produce benefits even if climate does not change. For the case of Sarawak River Basin, long term impacts of climate change on the local scale is still not clear. Thus, a detailed study is needed and this paper aims at presenting the findings of a preliminary study on the rainfall pattern for Sarawak River Basin done by the authors of this paper. The trends for temperature and evaporation rate for a station in the basin are also presented. Some adaptability measures in preparing for the potential effects of climate change for the basin will also be discussed. Keywords: Climate change, rainfall pattern, Sarawak River Basin 1. Introduction

One of the most urgent issues in todays hydrological research is regarding the impacts of climate change which are directly or indirectly influencing many hydrological processes. However, for the case of Sarawak River Basin, the long term impacts of climate change on the local scale is still unclear due to the lack of comprehensive studies done. This paper aims on presenting the findings of a preliminary study on the probable climate change impact on the rainfall pattern for Sarawak River Basin by using statistical method such as time series and mean areal rainfall. The basic element for this study is rainfall since it is the main intake of the hydrological system of the basin and is responsible for the water balance variability and has deep implications on the whole water cycle. The trend for temperature and evaporation rate for the basin has also been discussed in this paper since changes in temperature and evaporation will have direct impact on the average rainfall rates. Adaptability measures that could be applied in preparation for the potential of climate change for

the basin will also be proposed and discussed in this paper. 2. Sarawak River Basin

The Sarawak River Basin as shown in Fig. 1 has a total area of approximately 1435 km2 up to the barrage. It is consists of 2 main tributaries, namely Sarawak River Kanan and Sarawak River Kiri which meet near Batu Kitang; about 34 km upstream of Kuching city. The basin is largely undeveloped with impervious layer less than 6% and has a high average annual rainfall of 4000 mm to 4500 mm. The basin is influence by tidal and prone to flooding. To protect Kuching city from flood, the Sarawak State Government established the Kuching barrage in 1998 aimed at controlling the river and tidal flows under the Sarawak Regulation Scheme [1]. Though flooding is a frequent occurrence in the basin with the most severe flood happened from 25th to 28th January 1963, major floods seems to be happening more frequently during the recent years with water risen up to between 1 m to 3 m for some low lying localities in the basin. Examples are the floods from

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3rd to 5th February 2003, 23rd to 26th January 2004, 28th to 29th December 2006 and the most recent one from 10th to 13th January 2009 and 29th to 30th January 2009.

The floods in the basin usually occur during heavy rainfall which coincides with high tides. These two elements of heavy rainfall and high tides might be due to the impact of climate change, i.e. heavier rainfall due to higher temperature and higher tides due to rise in sea level which caused flooding in the basin become more and more frequent in the recent years.

Fig. 1. Sarawak River Basin [2].

3. Rainfall Patterns To see the trend or changes in the rainfall

patterns due to climate change, 2 methods have been used in this paper, namely the time series for annual rainfall and the mean annual areal rainfall using Thiessen Polygon method.

3.1. Time series for annual rainfall

The time series with moving mean method has

been used to study the precipitation trends in Aragon, Spain [3]. In this method, to iron out cyclical or irregular fluctuations, a moving average based on the average duration of the fluctuation to be eliminated from the series will be taken [4].

In using this method, stations with good continuous data (up to at least 30 years of data) were chosen. For Sarawak River Basin, 4 rainfall stations that fit the criteria are Batu Kitang (station no. 1402047), Kuching Airport (station no.

1403001), Bau (station no. 1401005) and Krokong (station no. 1301074). The time series for the total annual rainfalls for these 4 stations (see Fig. 2) has been smoothed by using a five-year moving mean and the its curve also appearing in the figure.

Fig. 2. Total annual rainfall (expressed in mm) for Batu Kitang, Kuching Airport, Bau and

Krokong smoothed through a five-year moving mean.

From Fig. 2, the series for the 4 rainfall stations

generally present continuous upward trends from the beginning of the seventies up to nowadays. The series also present a humid phase at the end of the

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seventies, middle of nineties and the current present days which is observable from the 4 stations.

To obtain the trend value, method of semi-averages for the moving average has been used in this study. For Batu Kitang, there is a trend of increment of +37.5 mm/decade (for the period 1965 2005), while as for Kuching Airport the increment is +42.5 mm/decade (for the period 1965 2005). For Bau, the increment is +5.57 mm/decade (for the period 1974 2004) and for Krokong is +85.5 mm (for the period 1975 2005).

3.2. Total Mean Annual Areal Rainfall

To determine the trend of the total mean annual areal rainfall for Sarawak River Basin, 7 rainfall stations which are most or less evenly distributed in the basin and with the most extensive data (at least 30 years of data without much missing data) were chosen and the Thiessen Polygon method was used.

The 7 chosen stations are 3rd mile (station no. 1503083), Kuching Airport (station no. 1403001), Batu Kitang (station no. 1402047), Bau (station no. 1401005), Krokong (station no. 1301074), Kpg. Git (station no. 1302078) and Padawan (station no. 1102019). The location of these stations and the bounding polygons for each of the stations is shown in Fig. 3. The area for each of the bounding polygons is determined by using overlay grid.

Fig. 3. Location of the rainfalls stations and the bounded area for each stations using Thiessen

Polygon method. The trend for total annual areal rainfall for

Sarawak River Basin is as plotted in Fig. 4. From Fig. 4, the total annual areal rainfall shows an upward trend. The value for this trend when calculated using semi-averages method is +76.29 mm/decade (for the period 1976 2004).

Fig. 4. Total mean annual areal rainfall (expressed in mm) for Sarawak River basin.

4. Temperature and Evaporation Trend

The hydrological response of climate change will be alterations of climatic variables such as rainfall and evaporation, and consequently changes in hydrological variables such as sea level, surface runoff, soil moisture and groundwater. Generally, it is accepted that with global greenhouse warming; the expected global hydrologic changes include an overall increase in the intensity of the hydrologic cycle due to higher average temperatures and increased evaporation, which in turn will mean higher global average rainfall rate [5]. However, for a smaller scale hydrologic processes or at watersheds level, this may not necessary be the case.

For the case of Sarawak River Basin, the trends for temperature rate and evaporation have also been studied by the authors of this paper to justify the general statement mentioned above, i.e. whether the increase in temperature and evaporation rate will also contribute to the increment in the average rainfall rate.

Fig. 5 shows the trend for the mean annual temperature for a station in Kuching which is located in Sarawak River Basin. The station is located at latitude 129 N and longitude 11020 E with a height of 21.7 m above mean sea level. From Fig. 5, the mean annual temperature shows an upward trend of 0.135C/decade calculated using the semi-averages method for the five-year moving average trend (for the period 1988 2008). As for the case of evaporation rate which is measured from the same station, Fig. 6 also shows an upward trend of 0.065 mm/decade calculated using semi-averages method for the five-year moving average trend (for the period 1988 2008). Thus, the general statement of higher average temperature and increased evaporation will lead to increase in rainfall holds for the case of Sarawak River Basin.

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Fig. 5. Annual mean temperature for Kuching.

Fig. 6. Annual mean daily evaporation for Kuching.

5. Adaptability Measures

Many countries in the world are preparing climate change action plans that describe measures to adapt potential effect of climate change. Reasons for preparing such plans are that climate change is likely to occur and many of the adaptability measures that would be taken are no-regret measures that will bring benefits even if climate does not change.

Judging by the preliminary study done by the authors of this paper for the rainfall pattern and the trends for temperature and evaporation rate, it seems that all these variables exhibit an upward trend. However, further more detailed study is needed to justify this trend and whether this is due to climate change and not just its variability and what is the impact to the runoff/stream flow and thus the water resources of the basin.

There are many uncertainties in predicting the peak discharge under climate change scenarios and a closer look at the design practices of hydraulic projects and infrastructure in the basin should be considered. It is suggested that statistics related to rainfall depth-duration-frequency data which is used for design of hydraulic projects to be updated regularly (at least every 20 years or so) [6] so as climate changes will be gradually incorporated into the record and rainfall statistics. Marginal changes in the size of planned hydraulic structures could also be considered where this change may be much

less expensive than adding capacity in the future [7].

Though this paper did not address the trend for low flow or drought due to the probable impact of climate change, it is advisable to adapt water conservation strategies in the planning for water resources in the basin for future use. Some of the water conservation strategies include water recycling, i.e. recycling of greywater from household [8], encourage efficient water use through education, voluntary compliance and pricing policies. With these water conservation strategies, demand could be reduced and thus increasing excess supply, creating a greater margin of safety for any possible future droughts. Besides that, a contingency planning for drought could be adopted such as short-term measures to adapt to water shortages could help mitigate floods. The cost of developing contingency plans is relatively small compared with the potential benefits. Structures such as long storage and detention basin could also be planned in the basin so as to store water during high flow and provide water source during low flow.

Climate change is likely to affect the frequency of floods and droughts. Thus, a proper and improve monitoring and forecasting systems for flood and drought will help in coping with these changes and be beneficial regardless of climate change [9].

All the adaptability measures suggested in this paper could be adopted in Sarawak River Basin in preparation for any potential effect of climate change. But, what is important is how the stakeholders involved plan and coordinate the use of the basin. Comprehensive planning may allow coordinated solutions to problems of water quality and quantity.

6. Conclusion

This paper presented the preliminary findings of the rainfall, temperature and evaporation trend for Sarawak River Basin in order to study the probable effects of climate change to these variables. Data from the past 3 or 4 decades have shown an upward trend for all these variables, however a more detailed and comprehensive studies need to be done to verify this trend. The impact of these trends to the stream flow such as low flow and flooding need also to be studied so as necessary planning and measures could be considered in preparation for future impact. Some adaptability measures that could be adopted in Sarawak River Basin such as changes in the design practice, water conservation, improved monitoring and forecasting systems and coordination among stakeholders have also been suggested in this paper. These adaptability measures suggested would also bring benefits and could be implemented even if climate does not changes or insignificant in the basin.

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Acknowledgements

The authors would like to thank the Department of Irrigation and Drainage Sarawak for providing the rainfall data and the Malaysian Meteorological Department for the temperature and evaporation rate data.

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Sungai Sarawak Regulation Scheme Final Report, 1994.

[2] Drainage and Irrigation Department (DID) Sarawak official website via http://www.did.sarawak.gov.my (assessed on 11th February 2009).

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[7] NAS (National Academy of Sciences), Policy Implications of Greenhouse Warming, Washington DC, National Academy Press, 1992.

[8] D.Y.S. Mah, C.H.J. Bong, F.J. Putuhena, S. Said, A Conceptual Modeling of Ecological Greywater Recycling System in Kuching City, Sarawak, Malaysia, Resources, Recycling and Conservation, Elsevier, Vol. 53, Issue 3 (2009) 113-121.

[9] R.T. Watson, M.C. Zinyowera, R.H. Moss, Climate Change 1995: The IPCC Second Assessment Report, Volume 2: Scientific-Technical Analyses of Impacts, Adaptations, and Mitigation of Climate Change, Cambridge University Press, Cambridge, 1996.

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