Physical features of lakes that determine habitat characteristics
•Water turnover/residence time and the lake watershed—Chapter 9
•Mean depth and volume—Chapter 7
•Thermal stratification and physical mixing—Chapter 11
•wind./currents/wave action—Chapter 12
•Sediment deposition—Chapter 12
•Light extinction—Chapter 10
Calculating water turnover/residence time in lakes
Beauvais Lake and
Its watershed
Assume runoff
coefficient of
0.15 m
Drainage area
=7.0 km2
Lakearea
=0.9 km2
How much water
would you
expect flows into
this lake /yr?
Annual evaporationMinus
Annual precipation
=85 mm
How much water
flows out of the
lake?
Lake Area = 0.9 km2
Lake Volume = 3.8 x 106 m3
Mean depth= 4.3 m
Max depth = 10.7
Water residence time=
Mean renewal rate=
V
, τw = mean water residence time
τw =
Qo
How long would it take to refill the basin with runoff if
it were emptied?
Qo = mean discharge out of the lake
The inverse is water renewal rate or hydraulic throughput
Qi
h=
V
Qi = mean discharge from watershed into the lake
Qo = Qi + [P − E ]A
Lake Area = 0.9 km2
Lake Volume = 3.8 x 106 m3
Mean depth= 4.3 m
Max depth = 10.7
Water residence time=
Mean flushing rate=
Water residence time
Mean flushing rate
V
3.8 • 10 6 m 3
τw =
=
= 3.9 yr
5
3
−1
Qo 9.7 • 10 m • yr
Qi 1.05 • 10 6 m 3 • yr −1
−1
h=
=
yr
=
0
.
28
•
V
3.8 • 10 6 m 3
Calculating water renewal rate/residence time in lakes
Beauvais Lake and
Its watershed
Assume runoff
coefficient of
0.15 m
Drainage area
=7.0 km2
Lakearea
=0.9 km2
How much water
would you
expect flows into
this lake /yr?=
Outflow=1.05 x 106m3/yr-(0.085m * 9 x 105m2) = 9.7 x 105m3/yr
1.05 x 106m3/yr
Annual evaporationWater residence time = volume / outflow =3.8 x 106m3/9.7 x 105m3/yr
Minus
=3.9 yr
Annual precipation
=85 mm
Water renewal rate = inflow / volume =1.05 x 105m3/yr / 3.8 x 106m3
How much water
=0.39/ yr
flows out of the
lake?
How much of the
water flowing into
this lake from its
watershed could
you allocate
before the lake
would gradually
begin to
disappear?
Answer
Over 90%
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
Practice Question. Using the area tool in Adobe reader, calculate the area,
volume and mean depth of Lake Ontario from this map. Compare your
answers to values that you obtain from Google
Practice question
Bear lake has a reasonably
funnel-shaped basin, not filled
in with sediment. What do you
expect its depth ratio (mean
depth/max depth) to be?
Now calculate the mean depth
and see how close your
estimate is.
Practice Question: Estimate what you think the depth ratio should be for Turkey
Lake, Ontario and then check your estimate against an actual calculation.
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
The seasonal pattern of thermal
stratification in a deep temperate zone lake
Depth-time graph of isotherms
During spring turnover the entire
Water column is 4oC—why 4oC
Same thing happens again in the fall
Vertical thermal profiles
In very large lakes horizontal
thermal shear zones occurs
at river mouths
A thermal bar
Important habitat feature for
many fish species in spring.
Waves- the gravitational response to wind disturbance
The bigger the
wind fetch the
bigger the wave
oscillation
The velocity in
these oscillations
attenuates sharply
with depth
Wave energy and
slope together
determine the
depositional zone
boundary
Fig. 12.3
At depths > depostional
Boundary depth
fine mud accumulates
Fig. 12.7 in text
An undisturbed sediment
core containing varves from
the deposition zone of a
deep lake
The varves can be used to
calculate dates along the
core profile