The natural lake of Yliki has a surface area of 25 square kilometers, a watershed of 2.423 square kilometers, a maximum volume capacity of 590 million m3 of water and an operational volume 580 million m3. It is situated on a lower altitude compared to Athens. For that reason, the lake water intake structures depend on the operation of various pump units of significant power capacity (both submerged and on - land). Its central pumping station is one of the largest in Europe. The aforementioned pumping facility consumes large quantities of electric energy, a fact that constitutes EYDAP the second largest customer of the Public Power Company (DEH) and the largest customer in the medium voltage range.
Yliki Lake is the only water resource, which poses a problem since it exhibits a high rate of water loss, due to the pervious areas in the lakebed. Solving this problem is difficult due to the large surface area, and the complicated mechanisms of water dissipation. In order to alleviate the negative aspects of this problem, water levels of the lake are kept within a certain range so as to minimize water losses. This delicate balance is achieved in conjunction with the maintenance of the necessary volume in the remaining reservoirs.