The Lake Toba Watershed

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.

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