Crater Lake, Oregon -589 m deep and possibly the clearest lake in the world,
Transparency up to 90 m.
Thermocline very deep for its size
No rooted plants.
Mud doesn’t accumulate on the bottom till > 90 m depth
Why is this
lake so
different
from most
lakes?
Some of the most spectacular tectonic lakes are formed in volcanic craters.
Physical features of lakes that determine habitat characteristics
•inflow from the watershed/Catchment
•Water residence time
•Morphometry, Mean depth and volume
•Thermal stratification and physical mixing
•wind./currents/wave action
•Sediment deposition
•Light extinction
How much water flows into lake Beauvais lake in a year from
its watershed?
Assume runoff
coefficient of
0.15 m
Drainage area
=7.9 km2
Lake area
=0.9 km2
How much water
would you
expect flows into
this lake /yr?
Evaporation from
lake surface
exceeds
precipitation by
0.085 mm/yr
How much water
flows out of the
lake?
Assume runoff Drainage area
=7.9 km2
coefficient of
Lakearea
0.15 m
=0.9 km2
P─ E on lake
surface
= ─ 0.085 m/yr
How much water would you expect flows into this lake /yr?
Qi = r * DA = 0.15 m/yr * 7 x 106 m2 = 1.05 x 106m3/yr
What is the net evaporation in a year?
(P-E)*A = ─ 0.085 m * lake area = ─ 0.085m/yr * 9 x 105 m2 = -7.65 x 104 m3/yr
How much water flows out of the lake in a year?
Qo = Qi + (P-E)*A = 1.05 x 106m3/yr + (─ 7.65 x 104 m3/yr) = 9.75 x 105 m3/yr
Definition of water residence time and flushing rate
Chapter 4
Water residence time  w (time units)
How long would it take for the entire volume to drain out of the lake
if no new water wer e entering it.
V
L3
V
w 
, units 3  t , renewal time 
Qo
L /t
Qi
Qo  mean discharge out of the lake
Qi  mean discharge from watershed into the lake
Qo  Qi  P  E A
The approximat e inverse is flushing or renewal rate,
How many times a year can the inflow fill the lake
Qi
L3 / t 1
h  , units 3 
V
L
t
Lake Area = 0.9 km2
Mean depth= 4.3 m
Lake Volume = 3.8 x 106 m3
Water residence time=
Mean renewal rate=
Lake Area = 0.9 km2
Mean depth= 4.3 m
Lake Volume = 3.8 x 106 m3
Water residence time=
Mean renewal rate=
Water residence time
Mean flushing rate
V
3.8 106 m3
w 

 3.9 yr
5 3
1
Qo 9.7 10 m  yr
Qi 1.05 106 m3  yr 1
1
h 

0
.
28

yr
V
3.8 106 m3
How much of the
water flowing into
this lake from its
watershed could
you allocate for
irrigation before
the lake would
gradually begin to
disappear?
Answer
Over 92%
Lake management—the water inflow budget
or what happens when you over allocate?
The Aral Sea in the former Soviet Union—mismanaging the river water inflow
Allocation to desert irrigation > inflow minus evaporation
Fig. 5.19
. Effects
Ecosystem collapse, loss of biodiversity, worsening of water-salt balance in the
agricultural areas, pollution of rivers and drinking water, changing of the regional
climate – all these are new environmental developments in Central Asia.
Calculating volume and mean depth
Mean depth = Volume/surface area
The hypsographic
curve
Area under the
curve = volume
Fig. 7.1 in text
Lakes partition themselves into temperature zones
Thermal stratification in lakes
•In deep lakes only the surface
layers are well mixed and quite
warm, whereas the deeper parts
remain cold.
•The thermocline occurs deeper
in large lakes because wind
energy is transmitted to greater
depths
•Wind energy increases with
fetch
•In small lakes convection also
plays a role in determining
thermocline depth
Fig. 11.8 in text