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

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.