Digital Signal Processing Reference
In-Depth Information
drainage lakes) fit this category. Longer retention times occur in seepage lakes with no surface outlets.
Average retention times range from several days for some small impoundments to many years for
large seepage lakes.
Nutrients that accumulate over a number of years in lakes with long retention times can be recycled
annually with Spring and Fall mixing. Nutrients stored in lake sediments can continue to be released,
even after the source of nutrients in the watershed has been controlled. Thus, the effects of watershed
protection may not be apparent for a number of years. Nevertheless, lakes with long retention times
tend to have the best water quality as shown by the lower levels of the plant nutrient phosphorus in
Table (6-3)
.
Better water quality is a consequence of the greater depths of the lakes and their relatively
smaller watersheds.
Table (6-3): Several characteristics of lakes with different retention times
Retention Time in days
0-14
15-60
61-180
181-365
366-730
>730
Mean depth (ft)
6
8
11
11
13
23
Maximum depth (ft)
16
21
25
27
35
57
Mean total phosphorous (μg/l)*
94
85
56
48
33
25
Mean DB:LA ratio**
1166
142
42
15
8
6
(
Adapted from Lillie and Mason, 1983.
)
*Summer values;
**DB:LA = Drainage basin/lake area
Drainage basin/lake area ratio (DB:LA)
The size of the watershed (drainage basin) feeding a lake relative to the lake's size (area) is an
important factor in determining the amount of nutrients in a lake.
Table (6-3)
s
hows this relationship
for a sample of Wisconsin lakes, (USA). Lakes with relatively large drainage basins usually have
significant surface water inflow. This inflow carries more nutrients and sediments into these drainage
lakes or impoundments. By definition, seepage lakes have small drainage basins, more groundwater
flow, and fewer nutrients from runoff. Groundwater drainage lakes typically have an intermediate-
sized drainage basin. Low-ratio lakes (small drainage basin and large lake area) have high retention
times while high-ratio lakes have short retention times. Drainage basin:lake area ratios can be used to
estimate a lake's retention time.
Table (6-4)
shows the geomorphologic characteristics of Lake Edko
and the lake residence time. Based on the average annual inflow rate of 50 m
3
/sec, the residence time
for Lake Edko is estimated to be 5.3 days.
Table (6-4): Geomorphologic Characteristics of Lake Edko,
Water
surface
area
(km2)
Catchment
area
(km2)
Coast
length
(km)
Average
Length
of lake
(km)
Mean
width
(km)
Mean
depth
(m)
Maximum
depth
(m)
Lake
volume
(m3)
Annual
inflow
(m3/day)
Residence
time
(d)
228*10
5
19
2048
33
8
3
1.2
2.5
4320000
5.3
6.4.
ESTIMATION OF NUTRIENTS LOADS
Nutrients are fundamental for plant growth in all ecosystems, as much in water as on land. The natural
nutrient status of a water body, such as a lake, depends on its size, depth, geology, retention time and
the use of land in its catchment area. Some soils release more salts and nutrients than others and
therefore different water bodies in their natural state can be at different trophic, i.e. nutrient
concentration, levels. As mentioned above, an increase in available nutrients or the nutrient
Search WWH ::
Custom Search