Freezing of Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 37

Fig. 2.11 Annual course of
the water balance in Lake
Vanajanselk , southern Finland

(Jokiniemi 2011): a Inflow and
outflow; b Lake - atmosphere
water exchange

ä

2774 km 2 . Due to the large size of the drainage basin, the in fl ow well dominates over

precipitation as the source of water.
The

flushing time or the water renewal time of a lake provides one fundamental time
scale. This scale is de

fl

ned as the ratio of volume to input or output: t R = S/(PA + I)=S/

(EA + O). In this de nition the volume and fl fluxes represent long-term averages, and to

make sense we must have dS/dt uence the overall

water budget but the renewal of the water storage slows down (Fig. 2.12 ). Freezing of lake
water just changes water molecules to the solid state but the water storage is unchanged.
And as long as the ice is

≈ 0. Freezing and melting do not in fl

fl floating, liquid water level elevation is unchanged.

Example 2.8.
(a) Over the annual cycle the net change of water storage is normally close to zero and
then (P

· ≈ ≈

- E) A=I - O. In dry regions, P 0 and O = 0, and at equilibrium AE I.