Sudan, has had the strongest ecological impact downstream the Nile, with se- vere consequences for its delta, in particular). Water balance. Lake water balance (WB) represents the quantity of inflow (I) and outflow (0) analyzed over a given time interval ( day, month, season, year, or several years) and put into a mathematical formula. WB is positive when I > 0, which means that a certain water volume accumulates in the lake (+^V) and the water level rises;and negative, when I
The water level of the lake is an accurate measure of water volume. The WB-governed level hydrograph is given by hypsographic curve changes.The water balance and water level variations are the main expres- sion of climatic conditions, having facilitated medium-and long-term assess- ment of climate changes in certain regions of the world, following the pattern of some big lakes, especially closed-in lakes (without discharge). Water chemistry and mineralisation. Water chemical composition and
mineralisation depend on the rock composition of the lake basin and of the drainage basin, and on the climatic zone. The chemical composition accounts for the hydrochemical type, reflecting the proportion of major anions and cations. As a rule, three main anion-based hydrochemical types can be distinguished: Cl (chlorate, characteristic of saline and brackish lakes); S04 (sulphate, specific to saline and brackish lakes); HC03 (carbonate, hydrocarbonate, peculiar to fresh- water lakes). The main cations (Ca++, N+, and K+, Mg++) are used to identify hydrochemical subtypes,
e.g.sodium-chloride; calcium-bicarbonate, etc. According to their mineralization (quantity of salts in solution), lakes are
grouped into: freshwater (up to lg/l); brackish (1-24.7 g/l); saline (24.7-50 g/l) and ultrasaline (over 50 g/l).
Thermal regime and classification. The thermal regime and structure
of lakes depend on climatic zone, lake basin pattern and size, water volume, type of water balance and mineralization .Thus there may be direct thermal stratification, .with higher upper- layer (epilimnion) temperatures (temperature variations relate to the air temperature values); a sudden fall of temperature (thermal leap or thermocline) in the intermediate layer (metalimnion), as well as small, sometimes constant variation, close
to 4°C, in the deep layer
(hypolimnion).Alternatively,there may be indirect (reverse) sratification, with low epilimnion and higher hypolimnion temperatures while in shallow lakes, thermal uniformity is induced by mechanically ( by wind) or by convection (in seasons of transition). It is the so-called homothermal phase.
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