Keywords: rainwater harvesting, island, water demand management

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1 RAINWATER HARVESTING IN SMALL ISOLATED ISLANDS Nowbuth, M. D. & Perrine *, J. G. Department of Civil Engineering, Faculty of Engineering, University of Mauritius * Faculty of Agriculture, University of Mauritius ABSTRACT: Rainwater harvesting is defined as a method for inducing, collecting, storing and conserving local surface runoff for agriculture in arid and semi-arid regions (Boers & Ben-Asher, 1982). The potential of water harvesting for improved crop production received great attention in the 1970s and 1980s. Although domestic rainwater harvesting (DRWH) is not the definitive answer to household water problems, but it does help significantly to cater for primary water uses, in some cases. This paper critically assesses the importance of rainwater harvesting in two small islands, Mauritius and Rodrigues. The factors favouring rainwater harvesting are highlighted, together with the other possible measures that need to be implemented at national level for better success. The paper notes that though optimal use of water resources is a must for small isolated island, the driving force behind the implementation of optimal water usage is more policy than technical. Keywords: rainwater harvesting, island, water demand management INTRODUCTION Rainwater harvesting refers to the small-scale concentration, collection, storage, and use of rainwater runoff for both domestic and agricultural use. Rainwater harvesting has been practiced more than forty thousand years ago (Pacey et al. 1986), resulting in the existence of various methods of rainwater harvesting. The roof water harvesting technique, commonly referred to as DRWH, is more commonly used to cater for primary needs, such as drinking water. Usually DRWHS employ simple and cheap technology which is accessible to everybody (Ariyabandu, 2001). Nowadays, it is more common to find rainwater harvesting mostly in developing countries, in small islands and in semi-arid regions where surface and groundwater resources are limited (UNEP, 1997). Details on various design of rainwater harvesters, the precautions to be used to ensure good quality of water and the various advantage rainwater harvesting offers to a country is well documented, with a vast amount of information available on the internet. However, this technique has not evolved with time, on the contrary, there has been a decline wherever it was used. According to Kerr and Pangare (2001), local water harvesting declined with the development of large systems in which water is transported hundreds of miles through canals

2 and pipes or pumped from great depths below the ground. However, nowadays with growing scarcity and intersectoral competition for water, along with groundwater depletion and the problems facing major surface-water control systems have raised interest in revitalizing water-harvesting systems that capture rainwater wherever it falls. This paper reviews the reasons, which influence the use of rainwater harvesting in two small isolated islands of different water resources situations. GENERAL INFORMATION ON MAURITIUS & RODRIGUES Mauritius (Figure 1) is a small isolated volcanic island situated in the southwest Indian Ocean, about 800km off the east coast of Madagascar, with Rodrigues being one of the dependencies of the island. Latest demographic survey reported a population figure of 1,178,848 (CSO,2000). The island covers a surface area of about 1860 km 2 enjoying a tropical climate, mostly because of its latitude and its relief. Rainfall varies from an annual average of 4000mm in the high elevation regions to 900mm along the coastal regions. The annual average rainfall for the whole island being about 2500mm (Padya, 1989). The island of Mauritius is characterised by a dense network of rivers, with drainage being essentially radial towards the sea. The island also has potential groundwater basins. Both surface and groundwater sources are harnessed to cater for water demand in the domestic, agricultural and industrial sectors. Presently (WRU, 2001), a total of 982Mm 3 of water is being harnessed, with a groundwater contribution reaching up to 15%. About 99% of the total population have access to piped water supply, and 94% of the total population is served by individual piped water supply. The island also faces periods of droughts, and in the year 1999, the island had experienced a very severe drought period. Groundwater sources are catering for about 58% domestic water demand. In Mauritius, the whole water sector covering the overall water hydrological cycle is subdivided between, and essentially managed by five institutions, namely; the Water Resources Unit, the Central Water Authority, the Central Electricity Board, the Irrigation Authority and the Wastewater Management Authority. The Water Resources Unit (WRU) is responsible for the assessing and harnessing of water resources, and the Central Water Authority (CWA) caters for the treatment, storage and distribution of water for domestic, commercial and industrial uses. The Central Electricity Board (CEB) is responsible for hydropower generation, while the Irrigation Authority (IA) is responsible for provision of water for irrigation purposes. The Waste Management Authority caters for the collection, treatment

3 and disposal of wastewater. Presently about 23% of the total average annual precipitation is exploited to cater for water demand. About 48% total water harnessed caters for irrigation water demand, some 31% goes for hydropower and the remaining 21% caters for domestic, commercial and industrial needs. GENERAL INFORMATION ON RODRIGUES Rodrigues, a dependency of the island of Mauritius, is located some 560km to the east of Mauritius (Figure 1). According to the latest official survey, the population of Rodrigues is 35,779 (CSO, 2000). Like Mauritius, Rodrigues is also of volcanic origin, and it covers a surface area of around 110km 2. Rodrigues receives on average an annual precipitation of about 1100mm, which is equivalent to 120Mm 3 of water. The island of Rodrigues is characterised by a highly rugged relief and very steep slopes, which resulted in the absence of naturally impounding reservoirs, and rivulets which flows very rapidly to the sea. Most of the surface runoff which arises during the wet season is lost to the sea. The river flows range from 1.4l/s to 56.9l/s in the month of July, and the total surface water flows were then estimated as being 32000m 3 /day. There has not been much hydrogeological studies which have been carried out in the island, and groundwater contribution to water demand is still very low. Run-of-river abstractions, boreholes and a few springs provide the water for all purposes, and normal water production reach up to 7400m 3 /day and falls to about 5000 m 3 /day during the dry period. During extended dry spells, production goes down to about 3000 m 3 /day. The Government is planning to provide the island with two desalination plants, each of capacity 2000m 3 /day, owing to the possible developments in the near future (SCENE-RIES, 2002). POTENTIAL OF RAINWATER HARVESTING IN MAURITIUS Harvesting of rainwater dates back long, when the water supply network was not well established. At that time, the rainwater collected from roof tops were being used to cater for domestic and secondary needs. With time, as the water supply network built up and became denser and denser, people started neglecting rainwater harvesting, specially for domestic needs. With outbreaks of water borne diseases, people became more conscious of the need for consuming treated water, hence further neglecting rainwater harvesting. Remaining rainwater-harvesting systems mostly caters for secondary uses and agricultural uses. POTENTIAL OF RAINWATER HARVESTING IN RODRIGUES

4 LUXCONSULT (2000) carried out a thorough study on the status and possible future development on the water resources in Rodrigues. Rodrigues has been under water stress conditions since some 30 years back, owing to both inadequate water resources and inadequate storage facilities during the wet season. During the wet period, the water harnessed is just enough to meet daily water needs, and even then water supply is intermittent with at times only two hours of supply. Some areas are served at times once or twice per week. During drought periods, daily water production drops dramatically. During such periods, the inhabitants receive water at times over duration of one hour every fifteen days. The island of Rodrigues has a water storage capacity of three weeks only during the drought period. Existing dams and reservoirs are old, of small capacities and most of the time heavily silted. These dams and reservoirs act more in the daily regulation of water, rather than provide additional storage for the dry periods. In Rodrigues, according to LUXCONSULT (2000) roof rainwater harvesting for domestic uses was very popular up until the 1990 s. A recent survey carried out by Perrine (2002), noted that of the sample surveyed, about 74% still harvest rainwater. With the development of piped water supply, this water harnessing technique has been neglected. Another aspect which has strongly contributed to the decline of rainwater harvesting in Rodrigues, is water quality (Michaelides, 1985). Rodriguans have continued to harvest rainwater in much the same way over time, there has not been much of an improvement in the approach to ensure a good and safe potable water. The study carried out by Jauze (1998) reported that conventional methods of collecting and storing the rainwater are not safe. Only recently a study carried out by the Mauritius Research Council (MRC), has come up with a new design to cater for better water quality. Rodrigues being a highly water deficient island, efforts has most of the time been concentrated on increasing the quantity of the water harnessed, and the quality aspect has not been given due consideration. Physicochemical and bacteriological tests undertaken on the water harvested, have confirmed presence of faecal coliform. With increasing standard of living, Rodriguans have been more aware of the need for good quality water and its relation to good health. FUTURE POLICIES REGARDING RAINWATER HARVESTING A study carried out on the status of water demand management in Rodrigues, (SCENE-RIES, 2002) noted that in the past, water-saving and water conservation projects have been

5 implemented in Mauritius, albeit with some success, but mostly on an ad-hoc basis. As noted in the above sections, Mauritius is not yet experiencing water stress conditions. However, there is a limit to the maximum volume of rainwater that a country can physically and economically exploit, and besides, Mauritius being an isolated island, the need to better manage water resources is of utmost importance. The study (SCENE-RIES, 2002) recommended that legislation should be adopted to encourage more efficient use of water in all sectors. The agricultural sector consumes a large volume of water (48%), to cater for both large scale farming and back yard farming. Rainwater harvesting can provide much water to cater for back yard farming. Presently, some crop planters do adopt rainwater harvesting for irrigation needs, but this approach needs to be further expanded and strengthened. As Rodriguans are already high conversant with the rainwater harvesting technique, more up to date design of rainwater harvesters need to be implemented if Rodriguans are to be encouraged in making better and more efficient use of rainwater harvesting (SCENE-RIES 2002). DISCUSSIONS & CONCLUSIONS Wherever water resource is scarce, the tendency is to focus on quantity rather than quality (Still & Thomas, 2001). Rodrigues being a heavily water stressed island, the focus in the past had been on increasing volume harvested. However, with time, it has been proved in Rodrigues itself, that unless due consideration is given to the quality aspect, this technique will no longer be favoured. Several studies (Yaziz, 1989; Lye, 1989; Gretchun, 1997; Rees & Thomas, 1999; Simmons & Smith, 1997; Koplan et al., 1979) have put much emphasis on the need and ways to go about to storage good quality water, if this water is to be used for primary uses. Another important point to note, when comparing the importance of rainwater harvesting in Mauritius and Rodrigues, is the level of water resource development. Even though Mauritius is a small isolated island, just like Rodrigues, rainwater harvesting is not so common, since firstly Mauritius receives relatively much more water than Rodrigues and the island is served by a dense and efficient water supply network. However, for small islands, water resource management is a must. One way to promote the rainwater harvesting technique is only by legal enforcement. New buildings should be encourage to make provisions for rainwater harvesting, to alleviate the stress on water supplies.

6 Some of the major constraints of roof rainwater harvesting technique is the high initial cost for building permanent storage, rainwater is not reliable and besides maintenance may be time consuming and costly. The living standard in Rodrigues is lower than in Mauritius, and just simply educating the inhabitants to go for improved rainwater harvesters will not be a solution. Though the need to adopt more sophisticated technique the inhabitants would need financial help. Another factor which contributes to the success of the rainwater harvesting technique, is the surface area of the catchment. Both Mauritius and Rodrigues are small island, and hence the surface area which is likely to contribute to rainwater harvesting is therefore very small, hence this technique cannot be relied upon to cater for large water demands. REFERENCES: Ariyabandu, R. De. D. (1999). Water security through rainwater harvesting. Integrated development for water supply and sanitation. Proceedings of the 25 th WEDC Conference, Addis Abeba, WEDC, Boers, Th M, and Ben-Asher, J A review of rainwater harvesting. Agric. Water Management 5: Central Statistical Office, housing and Population Census, Republic of Mauritius. Vol 1 housing and Living Conditions, Government Printing Press. Gretchun, R. (1997). Rainwater harvesting for Montana. Water Resource Management. Montana State University Press, Montana. Jauze, J. M. (1998). Rodrigues la troisième île des Mascareignes, L Harmattan, Université de La Reunion, Kerr, J. & Pangare, G. (2001). Water Harvesting and watershed management, Overcoming Water Scarcity and Quality Constraints, 2020 Focus,9,October Koplan, J. P, Dean, R. D, Swanston, W. H. & Tota, B. (1979). Contaminated roof-collected rainwater as a possible cause of outbreak of Salmonellisis J. Hygiene, Cambridge, LUXCONSULT (2000). The rationalisation, rehabilitation, upgrading and extension of the water transmission/distribution network of Rodrigues Island, Water Resources Unit. Lye, (1989). Bacterial levels in cistern water systems in North Kentucky, Water Resources Bulletin, 29,

7 Michaelides, G., Allybokus, M. & Mowlabaccus, M. F. (1985). Integration of rainwater harvesting with other supply systems in Rodrigues. Unpublished report. Pacey, A. & Cullis (1986). Rainwater harvesting, the collection of rainfall and run-off in rural areas, Intermediate Technology Publications, Southampton, Padya, B. M. (1989). Weather and climate of Mauritius, Mahatma Gandhi Institute, Mauritius, Perrine, J. G. (2002). Design of an improved rainwater harvesting system for Rodrigues, BSc(Hons) in Agriculture with specialisation in land and water management, final year thesis, University of Mauritius. Rees, D. G. L. & Thomas, T. H. (1999). Low-cost roofwater harvesting in the humid tropic. Integrated development for water supply and sanitation. Proceedings of the 25 th WEDC conference, Addis Ababa, WEDC, SCENE-RIES (2002). Water demand management; Country study report MAURITIUS, The World Conservation Union. Simmons, G. C. & Smith, (1997). Roof water probable source of Salmonella infection. New Zealand Public Health Report, 4, 5. Still, G.& Thomas, T. H. (2001). Guttering for roofwater harvesting in the tropics, DTU Working Paper 56, Warwick University, UK. UNEP (1997). Rainwater Harvesting from rooftop catchments. Source book of alternative technologies for fresh water augmentation in Latin America and Caribbean, International Environmental Technology Centre Technical Publication, Japan. WRU, Water Resources in Mauritius, Water Resources Unit, Ministry of Public Utilities. Yaziz, M. (1989). Variation in rainwater quality from roof catchments, Water Research, 23,

Focused Evaluation of Rainwater Harvesting: A Case Study in Sandakan, Sabah, Malaysia

Focused Evaluation of Rainwater Harvesting: A Case Study in Sandakan, Sabah, Malaysia A. I. Che-Ani 1, N. Shaari 1, H. Yahaya, M. Jamil 1, N. Mohd-Tawil 1, M.F.M. Zain 1 1 Department of Architecture, Faculty