The Water Balance of Lithuanian Closed Glacial Lakes

Summary. The researches of the water balance of closed lakes are very important if we study a problem of inclination in humidity areas. It is especially important while speaking about the water balance of closed glacial lakes. The researches in these lakes facilitate the solution of the question: what is the determinant factor of the lakes inclination? A long-term water balance equation of a closed lake includes five main elements: precipitation reaching the lake surface (P), evaporation of the water surface and transpiration of water plants (E), surface (I_pav) and ground (I_pož) water input and ground runoff (O_pož). While analyzing a short-term water balance we have to introduce a sixth element (A), which takes into account the water changes in a lake during the time of investigation. Seeking to find out why a lake is a closed one it is necessary to determine the dependence of each water balance component on the main indices of lake depression and basin. For this purpose we divided the elements of water balance into two groups describing the vertical (P, E) and horizontal (I_pav, I_pož, O_pož) water circulation.

A dense network of meteorological observations in Lithuania would prevent from missing distinct azonal anomalies of precipitation. Anomalies of evaporation have neither been recorded in any water balance station operating under natural conditions similar to those in Lithuania. Thus, a climatically predetermined closed character of lakes (I > P) in a long-term period is impossible in Lithuania. We may assume that the ratio between P and E in humid zones determines the change of lake parameters over time, whereas the ratio between I and O – the spatial distribution of lakes. The research which is presented in this paper is based on the short-term water balance investigations. These investigation were fulfilled in the closed lakes in various hydrological region of Lithuania from 1976 to 1996.

Short-term water balance investigations in concrete closed lakes revealed that the ratio between the water balance elements in them may vary considerably. Probably to the highest extent it depends on the genesis of lake depressions. As for instance, the water balance of closed and periodically drained glacial lakes is determined by: 1) soils of the basin and depression, which define the ratio between the ground water input and output, 2) lake depth (deeper lakes are more abundantly supplied with the ground water), 3) situation of lakes with regard to morainic chains and channel lake valleys (the lakes situated in the central parts of morainic tracts are weakly supplied with the ground water, whereas lakes occupying fragments of channel lake valleys are hydraulically linked), 4) water exchange between ground horizons cut across by depressions. The ratio between ground and surface water input and a ground water output depends on the humidity of periods. A humidity of periods changes depending on the season and long-term climatic cycles. Therefore it is necessary to repeat the short-term water balance researches in the periods with different humidity.

Keywords: closed lake, water balance, endorheic basin, Lithuanian lakes.