Is North Sumatra's Lake Toba really as sick as it sounds?

By Pasi Lehmusluoto

JAKARTA (JP): "Danau Toba sakit" (Toba Lake is Sick), screamed the headline of Medan Patriot Indonesia on Jan. 26, 2000. The newspaper explained that the "sickness" reduced the lake's value as a tourist attraction.

That morning began a workshop on Lake Toba in Tuktuk, Samosir, North Sumatra.

I will never forget my first visit to Lake Toba more than ten years ago. The scenery was beautiful with lush green slopes surrounding the lake and the blue water was crystal clear.

The lake's origins began with the explosion of the volcano "Batak tumor" some 75,000 years ago. It is the largest crater- lake in the world. Samosir "Island" divides the lake into two basins.

The northern basin is 529 meters deep, ninth deepest in the world. The water volume is 240 cubic kilometers, ranking 25th in the world and first in Southeast Asia.

The watershed, drained by 295 small rivers, is small compared to the lake's surface area. This limits the inflow of water.

Lake Toba itself with its specific ecological value as the world's largest crater lake has been overlooked.

Local industries, like "tumors in human body" bring misery to the people and to the Lake Toba ecosystem.

Some believe logging in the Lake Toba watershed has been the greatest contributor to the changes in regional climate and to the decline of the water level. However, the watershed is only a small part of North Sumatra, which was already largely deforested at least 70 years ago, without noticeable changes in the water level of the lake.

Deepening of the Asahan River channel between Porsea and the dam, which started operation in 1982, made increased discharge possible. Extensive water balance studies by the Institute of Hydrology in Wallingford, England, have shown that the dam of the Asahan River at Siruar released water at a greater rate than the net inflow.

The water level of Lake Toba has oscillated significantly since 1982 and lately has declined steadily because of sustained reduction in rainfall in the region and water inflow to the lake due to global changes.

Sustained declines in rainfall in the region as a consequence of global environmental changes has contributed to the reduced inflow. The inflow is now on a declining trend, but diversion of Renun River into the lake could improve the situation.

Actually, the present water level fluctuations are, not harmful to the Lake Toba ecosystem, because the changes in volume are only 1 percent to 2 percent. The decreased flow of the Asahan River has major effects along the river as the dilution of waste is reduced.

So far, the ecological changes in the lake have been minimal, but discharge of untreated waste water, dumping of garbage, littering, and frequent oil spills are a threat. The entire lake is still ecologically as sound as it was in 1929. My observations show that the oxygen conditions are good and nutrient levels low. However, local aesthetic problems exist near the populated areas and water hyacinths are spreading.

All the information and factual data demonstrate the serious impact of logging in the watershed on the declining water level of Lake Toba.

However, if I interpret the above information objectively, clearing of forests exerts an overall positive effect on Lake Toba, allowing more water to the lake by decreasing soil percolation and evapotranspiration by forests, thus increasing runoff to the lake. Evaporation remains practically constant.

It is necessary to return to the Lake Toba water balance study: the factors leading to changes in runoff are changes in land use and, in particular, changes in the amount of forest cover. Trees are generally considered to cause greater losses of rainfall than grass or cropland.

First, the greater leaf area tends to trap more water, some of which evaporates before reaching the ground. Second, the deeper root system leads to greater evapotranspiration losses. This means that an increased forest area tends to produce less runoff to the lake. Also rice paddies around Porsea and Balige retain water, much of which evaporates.

The water renewal time of Lake Toba is 110 years to 280 years depending on the inflow rate. According to UNEP, the world average is 17 years. Therefore, wastewater, even treated wastewater of the townships like Parapat, Porsea, Balige and Muara and of tourist facilities, solid waste dumping and littering, and agrochemicals entering the lake may have long- lasting and cumulative ecological consequences on the lake.

Due to the very diluted water and long water residence time, the lake is sensitive and vulnerable to small additions of nutrients and harmful pollutants.

And, because of its natural characteristics, the lake is biologically unproductive. Therefore, the annual fish catch of the lake is only some 24 kg per hectare. For comparison, the annual fish catch of Lake Tempe in Sulawesi is 650 kg to 700 kg per hectare. The introduction of exotic fish species may have changed the fish population.

Lake Toba is an excellent example in which unnecessary environmental disputes, economic losses and human suffering could have been avoided by applying an integrated scientific approach. Such an approach also could have assisted in finding realistic and true dimensions of environmental issues. Now the "forest has been missed for the trees".

Unbalanced water discharge at the regulating dam and subsequent Lake Toba level oscillations and decline are indicators of lack of good governance. The belief that Lake Toba watershed has been ecologically disturbed and the lake itself is sick because of local industries has caused social tensions.

However, based on scientific research of professor Ruttner of 1929 and my own studies of the 1990s, when the main basins of Lake Toba were studied down to their bottom for the first time, Lake Toba is ecologically sound. In reality, the lake is one of the soundest great lakes in Indonesia. If pollution by townships, tourist facilities and agriculture can be controlled, and solid waste dumping, littering and oil spills can be effectively managed, the lake will remain sound. This also involves effective control of water hyacinths.

Only a stable social environment and healthy natural environment can support sound economic development and tourism in the Lake Toba area, and elsewhere in Indonesia.

Lake Toba is not sakit. Talk that the lake is sick and dirty will certainly harm tourism.

The writer has been a specialist on Indonesian freshwater resources since 1974.