Hydrology and water balance of Lake Peipsi

Lake Peipsi is a large (3558 km(2))but shallow (up to 15.3 m deep) tripartite waterbody hydrologically investigated already since the 19th century. Surface discharge by rivers accounts for more than 80% of its water balance. The residental time of water is about two years in the whole lake but several times less in its shallower southern parts receiving the biggest rivers. The annual water regime is characterized by the highest water in spring, the average amplitude of yearly level fluctuations being 1.15 m. There are known long-term hydrological cycles of 80-90, about 22, 9-11, and even fewer years. Several temporary wind-dependent circular currents exist in the subsurface layers. Alternating transitional currents occur in the narrowest part of the lake. Five different periods are distinguishable in the annual thermic cycle. The duration of the stable ice cover is up to five months (December-April) in the shallower parts but a shorter time in the centre of the lake. The maximum surface temperature in July usually reaches 21-22 degrees C in the open regions but considerably higher (up to 27-28 degrees C in some years) on shallows. The unstable summer stratification is often disturbed by waves and currents. Biological summer, with surface temperatures over 10 degrees C, lasts on an average 134 days.