Lake Sempach

Lake Sempach lies on the Lucerne Plateau and draws water from a relatively small catchment, without large Alpine tributaries. Conditions nearby therefore matter: rainfall, stream discharge and the use of surrounding land. Around three quarters of the catchment is used for intensive agriculture. This creates two distinct ways of reading the lake: its level reflects the current water balance, while substances entering from the surrounding landscape can affect water quality over much longer periods.

Water leaves the lake through the Sure, listed as Suhre by the Federal Office for the Environment. A weir allows part of a flood's incoming water to be held temporarily in the lake. Once the peak has passed, it is released more evenly into the river to help protect areas downstream. A rising lake level and an initially restricted outflow can therefore be consistent: the lake is storing water instead of immediately passing every inflow peak to communities along the Sure.

Wet spells and dry weeks alter the balance between inflow, evaporation and outflow. Managed retention adds another influence. To interpret the curve, look at the preceding period and discharge into the Sure rather than just today's sunshine or the latest shower. A tributary's flood peak and the subsequent release need not occur at the same time. Seasonal comparisons describe the lake's behaviour in the measurement record; they are not fixed operating instructions for the weir.

The canton gives a water residence time of about 15 years. This describes the lake's slow renewal and helps explain the limited dilution of incoming substances. It is not a waiting time before the level can change: the surface can respond to a changed water balance long before the lake's contents have been renewed. For current conditions, read the level curve and timestamp; for water leaving the lake, read Sure discharge. An elevation in metres above sea level and a flow in cubic metres per second answer different questions.

Temperature tells only part of Lake Sempach's story. Oxygen shortages in deep water led to compressed-air support for winter circulation from the mid-1980s; air is also supplied to deep water in summer. According to the canton, natural oxygen conditions have not yet been restored. A warm surface reading therefore proves neither healthy deep water nor satisfactory bathing hygiene. Location and depth matter for temperature, while ecological condition and bathing quality require their own investigations and water samples.