How to refill Lake Toba and save its surroundings

By Pasi Lehmusluoto

JAKARTA (JP): A careful reading of the article, "Renun River dilemma brought about by deforestation" (The Jakarta Post, Aug. 8, 2000) revealed that the diversion of the Renun River into Lake Toba, its historical direction, was not planned to raise the water level of the lake.

The diversion was planned to provide additional electricity via the PLTA Asahan hydroelectric plant to PT Inalum for its aluminum production -- at the expense of Lake Toba's environment. Lake Toba is one of the key sites for the conservation of freshwater biodiversity in Indonesia.

The government's plan to raise the lake's water level by diverting the Renun River into it is insufficient, as PLTA Asahan has planned to drain the additional water at the other end of the lake to generate electricity.

The only effective means to raise Lake Toba's water level is to reregulate the operation of PLTA Asahan dam and to bring the lake's hydrology back to its natural state.

Deforestation in the Lake Toba watershed or even in North Sumatra does not directly cause the Renun River dilemma. It is mainly caused by regional and global changes in climate, which also affect the rainfall in the area.

The National Institute of Aeronautics and Space has stated that significant climatic changes were brought about by recent large-scale deforestation in the watershed area and in North Sumatra. However, deforestation started thousands of years ago with the clearing of land for agricultural purposes by the Bataks. There is scientific evidence to back this up. There is also evidence the Lake Toba surroundings and North Sumatra were already widely deforested in 1932, more widely than today. Did those activities cause consequent climatic changes?

The Lake Toba watershed has a vegetation cover, which protects it from erosion. The ecological fact is that the more forest there is, the more water escapes into the air by evapotranspiration. The question is whether this moisture will ever return as rain to the Lake Toba area?

If the Renun River catchment area were effectively reforested, it would deliver less water to the river and to Lake Toba. Reforestation is not an answer, but it helps to maintain the forest-based natural services and livelihoods in the area.

As said in the article, the Renun River diversion may not be sufficient to increase Lake Toba's water level or to save the Renun hydroelectric and PLTA Asahan projects.

From the beginning, the efficient capacity of Lake Toba (i.e. the difference between the maximum level of the lake and its minimum level multiplied by the lake's surface area) before PLTA Asahan started operation was 2.86 km3. This allowed a water flow from the lake of only 90.7 m3 per second, and a reduction in the lake's water level by some 2.5 meters.

However, the efficient capacity of the lake has decreased since PLTA Asahan started to operate the regulating dams in 1982. The level of Lake Toba and the flow of the Asahan River leading from the lake is controlled by these dams.

Statistics, which began in 1954, show the reduced rainfall in the region has significantly decreased the amount of water flowing into the lake.

The reduced net inflow, high water evaporation rate from the lake surface and the uncertain hydrological assumptions in the PLTA Asahan planning has lead to water being released at a higher rate than the net inflow. The chances of increasing the level of the lake without a drastic increase in regional rainfall, or a drastic reduction in the amount of water released by PLTA Asahan, are limited.

The representative of the Lake Toba Heritage Foundation stated in the beginning of the article there was still enough time to save the Renun Hydroelectric Project. What about trying to raise Lake Toba's water level?

What is really needed to save Lake Toba, the world's largest crater lake, from deterioration? In the first place the threats must be known.

The major threats are the release of lake water by PLTA Asahan at a higher rate than the net inflow, thus lowering the water level, discharge of waste waters, chemicals used in agriculture, dumping of waste and garbage, oil spills, exotic fish introductions affecting existing fish populations, increasing cage fish farming and excess growth of water hyacinths.

What needs to be done? Lake Toba can be saved if it is understood that the threats are much closer to the lake than usually thought. When Lake Toba's water level is brought back to its pre-PLTA Asahan level and all the other above threats are adequately removed, there is hope to save Lake Toba for the generations to come. The pumping of treated waste waters from Parapat to the Asahan River instead of into the lake should also be considered. Even the Asahan River flow may increase. improving the diluting capacity of the river.

Finally, was the planting of pine trees in the watershed as part of a previous reforestation program the real cause of the reduction in the number of the endemic "ikan Batak" fish, a kind of fish that is only found in the lake?

-- Pasi Lehmusluoto, Ph.D. has been working on issues related to the Indonesian environment since 1974. One of his main environmental concerns has been bodies of freshwater, particularly large lakes. He has been promoting sound scientific approaches in the management of the environment.