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1st International Conference on Rain Water Cistern Systems
Honolulu, Hawaii, USA - June 1982

Section 3: Design, Cost, And Policy

Page 171

Reliability of Roof Runoff in Selected Areas of Indonesia

J.L. Irish
UNESCO, Indonesia

Meteorological and Geophysical Service, Indonesia

D. Murdiyarso
Institut Pertanian Bogor (Agricultural University), Indonesia


Indonesia, an archipelago of about 12,000 islands spread 4 000 km east-west across the equator (Fig. 1), has a population approaching 160 million, with an estimated $350 (1970) per capita gross domestic product, of which oil production represents about 18% and agriculture about 32%. Approximately 65% of the population lives on the island of Java (including the adjacent island of Madura), which has an area of only 132 000 km2, or less than 7% of the total land area. Other densely populated areas include Bali and Lombok, and the province of Lampung at the southern tip of Sumatra. More than two-thirds of the population live in rural areas, generally in villages or small towns, and engage principally in subsistence agriculture. Rice is the main crop; others of importance are maize, cassava, soya beans, copra, tea, coffee, rubber, palm oil and sugar.

The mean annual rainfall ranges from less than 1 000 mm in a few small areas (e.g., Palu Valley, Saluwest) to over 4 000 mm in exposed coastal areas and parts of Kalimantan and Irian Jaya. Estimates of mean annual rainfall and per capita water availability for each of the main island groups are shown in Table 1.

The population is largely concentrated on islands with water resources below the national average. This situation is aggravated by the seasonal pattern of rainfall that is most marked in the southeastern islands of Indonesia: Timor, Nusa Tenggara (Flores, Sumba, Sumbawa, Lombok, among others), Bali and Java, which have a "dry" season from May/June until September/October. Rarely does a month receive no rain at all, even in the dry season, except in Timor and the lightly populated parts of Nusa Tenggara where the wet cultivation of rice is only possible during the wet season and transition periods, unless advanced irrigation is practiced. Elsewhere in Indonesia, rainfall has a less marked seasonal pattern, and cropping throughout most of the year is possible in many areas (Oldeman, Las, and Darwis 1979; Oldeman and Syarifuddin 1977). Surface water resources are consequently more reliable in those areas than in Java and the islands to the southeast.

Rain generally occurs about 120 days per year, typically, as intense bursts. Pan evaporation is relatively uniform-spatially and temporally-being about 1 500 mm/yr.

Public water supply is reticulated only in the largest towns and cities of Indonesia; elsewhere, the source of water is from streams or wells, or from stored roof runoff. During the dry season, people walk several kilometres or more in many parts of southeastern Indonesia to obtain water.

The Indonesian government is implementing a series of five-year development plans. The current plan, REPELITA III, emphasizes the goal of equitable social development as a result of the rapid growth in the value of exports (oil, timber, tin, rubber), an expanding manufacturing sector, and improvements in agricultural productivity. Transmigration from Java and Bali to the outer islands is being encouraged, and a successful family planning program hopefully will limit the population explosion. In line with the United Nations' International Drinking Water and Sanitation Decade, Indonesia is promoting the improvement of water supplies as a basic step toward better health and welfare. Many of these projects are small in scale and geared to rural villages, where the majority of the population lives, and to which special programs are directed. One such program involves UNICEF (a UN agency concerned with the welfare of women and children) and the government of Indonesia for selected regions, mainly the poorer and relatively dry areas of Yogyakarta, Madura, East Java and Lombok.

Little in the way of hydrological studies has been done to support the current program to increase the use of roof-runoff systems in villages. Instead, the approach has been to improve access to potable water and to allow villages to determine the safe yield from experience. The United Nations Educational and Cultural Organization (Unesco) decided to support a hydrological study, financed from its regional component of the International Hydrological Programme (IHP), to demonstrate technology transfer and the methods of applying technology appropriate to the needs of a developing country. This is consistent with the rational use and management of national water resources emphasis in the later phases of the IHP, and extends and is consonant to the synthesis of scientific knowledge, education and training of the International Hydrological Decade and the first phase of the IHP.

Village water supply projects are being implemented at numerous sites throughout the nation. This hydrologic network, which should cover a broad area, is based on planning that recognizes the large gradients of mean annual rainfall in the typical mountainous terrain of much of the highly populated areas.

A large quantity of daily and monthly rainfall data are available in Indonesia: the Dutch colonists were pioneers in the collection and study of such data. There are at present over 3000 sites (whose station density roughly approximates the population) that regularly report daily rainfall to the national Meteorological and Geophysical Service. By 1922, there were already 2800 stations with. five or more years of data; by 1941, this had increased to 4400 stations. The Pacific War, the disruption during the struggle for independence and the political turmoil of the 1950s and the early 1960s reduced the number of stations making regular climate or rainfall reports; however, this has been redressed in the last decade. A computer has recently been installed to store and retrieve climatological data, and monthly rainfall has been published in yearbooks for more than 80 years, However, the shortage of trained personnel in the provinces, and especially outside the cities, means that these data are hardly accessible to many potential users.

It was therefore decided to carry out a regional study based on the UNICEF-GoI programme of village improvement. The techniques developed might then be applied elsewhere in Indonesia and generally in Southeast Asia by using local data to obtain relevant statistical relationships. The area selected for study is broadly from Yogyakarta to Surabaya (parts of Central and East Java), the island of Madura, the drier parts of the island of Bali, Lombok and all of Sumbawa, an area of about 50 000 km2 with a population of about 20 million.

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