Overview - Hydraulic system

The water level in a lake is a balance between surface inflow, precipitation plus ground water inflow, and stream outflow, evaporation and seepage into ground water. The water level and retention time in the lake therefore varies in response with climatic conditions throughout the year.

The drainage basin (also “catchment area”) to a lake is the land area from which a lake receives its water. Lake water composition and quality is effected by the soil conditions, and is highly subjected to human activities in the drainage basin. Drainage basins with intensive agriculture typically contribute significant amounts of nitrogen, while phosphorus is the leading problem where the catchment basin is characterised by cities and towns.

The majority of the natural lakes in North Germany and Denmark are shallow, with an average depth under 5 meters. In deep lakes, as well as shallow lakes in the lee of a windbreak, there is often a summertime temperature differential between a warm surface layer and colder bottom water. The layer that separates the 2 water zones is called the thermocline. When the bottom water lacks contact with the atmosphere, the result is often oxygen deficiency during the summer. This has a significant effect on the organisms that live there, and leads to large differences in the chemical composition of the 2 disparate water zones.