Modelling the decline in water level of Lake Toba, Indonesia

https://doi.org/10.1016/0309-1708(93)90039-IGet rights and content

Abstract

Between 1984 and 1987, Lake Toba, a large crater lake in northern Sumatra, suffered a severe and continuous decline in water level of about 2·5 m. The outflow from the lake was used to generate electrical power which provided the sole supply to an aluminium smelter. An investigation into the cause of the decline was carried out by means of a water balance model.

Rainfall on the lake and the outflow were determined after an examination of the available data. The runoff from the lake catchment was estimated from a regional analysis of runoff in relation to catchment characteristics. Evaporation from the lake was estimated by the Penman method which was calibrated for local conditions by direct measurements using the eddy correlation technique.

Water balance calculations showed that the decline in water levels was due to water for power generation being released from the lake at a greater rate than the net inflow. This occurred because net inflows had declined since the period before 1973 on which the design of the power plants was based and the operation of the plants had not been altered sufficiently to compensate for this decline.

The decline in net inflows of about 18% was probably caused by a small but sustained reduction in rainfall. The quality of rainfall data for the Lake Toba region itself was too poor to show whether or not there had been any significant change, but examination of long-term rainfall trends over a wider region indicated that a decline of the order of 10% may well have occurred. This would be sufficient to account for the greater part of the decline in net inflows. Other factors, such as change in land use, may have contributed, but to a lesser degree.

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Present address: IUCN, World Headquarters, Rue Mauverney 28, CH-1196 Gland, Switzerland.

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