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  Studying Modeling Water Quality
 

Part I - Analysis and Interpretation of RUSS Data

As a water resource manager, you have been asked by a prospective lake property developer to compare the water quality of two lakes, Ice Lake in northern Minnesota and Lake Independence, located near Minneapolis. Relatively few historical data exist for either lake. Luckily, you have two RUSS units available that you quickly place in the two lakes.

Program the RUSS to collect a surface-to-bottom data profile from Ice Lake and Independence Lake.
Examine the data for temperature, dissolved oxygen, and turbidity for both lakes, shown in Tables 1 and 2 below:

Table 1
Ice Lake profile data - 6/22/98

Depth (m)
Temp (°C)
DO (mg/L)
Turbidity
0
19.9
11.4
2
1
19.9
11.4
1
2
18.4
12.2
2
3
16.9
11.5
1
4
11.5
13.4
1
5
9.1
9.8
2
6
7.2
3.6
1
7
6.1
0.9
1
8
5.7
0.6
1
9
5.5
0.5
<1
10
5.4
0.4
<1
12
5.0
0.4
<1
14
5.0
0.4
<1
16
5.0
0.4
<1

 

Table 2
Lake Independence profile data - 6/19/98

Depth (m)
Temp (C)
DO (mg/l)
Turbidity
0
21.2
9.1
15
1
21.1
8.8
22
2
21.0
8.9
15
3
21.0
8.9
5
4
20.8
8.5
3
5
17.8
4.3
1
6
17.4
3.8
3
7
16.9
2.0
0
8
15.9
0.2
0
9
13.9
0.0
0
10
12.8
0.0
5
12
12.4
0.0
4
14
12.0
0.0
4
16
12.0
0.0
3

  1. Graph the data on the graphs provided, with temperature and dissolved oxygen on the X axis and depth on the Y axis (Figures 1 and 2).

Figure 1
Ice Lake temperature and DO profile - June 22, 1998

graph

 

Figure 2
Lake Independence temperature and DO profile - June 19, 1998

graph
  1. Describe any differences that you observe in the temperature and dissolved oxygen profiles of the two lakes. Do this for both surface water and deep water. Why might they differ?



  2. How do turbidity values compare between the two lakes?




    • What causes turbidity to be higher in one lake than another? In other words, what does turbidity measure?




  3. Can you determine, with any certainty from these data, which lake has poorer water quality? (i.e., more likely to have algal blooms in the summer, likely to be less transparent, poorer fish habitat, less aesthetically pleasing, etc.). Why or why not?




  4. You decide that you need additional information to answer your client's question with any certainty, so you search for historical data and discover that Secchi readings have been taken by volunteers on the two lakes and that a few phosphorus and chlorophyll analyses have been done (Table 3).

Table 3
1998 Water Quality Data for Ice and Independence Lakes

Mean summer
epilimnion values

Ice Lake

Lake Independence

Total phosphorus (ppb)
8-10

42-115
Secchi transparency (m)
3.1-4.3

<1->3

Chlorophyll a

<1-5

5-30

  1. What do these data tell you about the relative productivity of the two lakes and about their water quality?






Part II - Using Geographic Information Systems (GIS) to assess land use patterns within watersheds

Your client is not satisfied when you tell her which lake is likely to be "cleaner." She wants to know what is causing the differences and whether there are simple control measures that could be taken to improve the condition of the poorer quality lake. You decide you need additional information about the watersheds of the lakes to answer her question. Therefore, you turn to a geographical information system (GIS) to get the information you need.

Call up the maps of Ice Lake and Lake Independence. Use this information to compare and contrast the landscape features of the two lakes. Information not available from the maps may be obtained from the Lake Summaries.

  1. Which of the two watersheds has a greater road density? How might this affect water quality?







  2. Use Arc View to fill in the following data table (Table 4) for each lake.

Table 4
Lake attributes derived from GIS maps

 

Ice Lake

Lake Independence

Maximum lake depth (m)

   
Lake area (ha)
   
Watershed area (ha)
   
Lake volume (m3)
   

  1. How do the two lakes compare in terms of area and volume?



    • Which lake has a proportionately larger watershed in relation to the size of the lake?




    • Calculate the ratio (watershed area/lake area) and fill in the blanks below.

Table 5

Ice Lake

Lake Independence

Watershed area : lake area Ratio (Aw:Ao)

__________ : 1

__________ : 1

  1. How might a larger watershed, in relation to lake area, impact water quality?







  2. Can you determine, using Arc View, the inflows and outflows to each lake? If not, consult the topographic maps to determine this, and include on a sketch of each lake, below.

Note: One relatively new GIS product is called a Digital Raster Graphic, or DRG. DRGs are simply topographic maps which have been scanned into a digital format and georeferenced. Thus, they are on-line versions of topographic maps that can be used in conjunction with other GIS layers to understand landscapes.

View the DRG for Ice Lake here

View the DRG for Lake Independence here

Note to teachers: This requires students to learn to interpret slope and elevation off topographic maps and relate to GIS-derived images.

  1. Have the lake drainages been altered in any way by human activities? In what way? How can you tell?




  2. What are the predominant land uses in each watershed? Fill in Table 5 with percentages of each land use type.


Note to teachers: If learning Arc View is one of the objectives of this lab, students could be asked to do their own Arc View work to determine areas dominated by various land uses.

Table 5
Percent land use from ARC-View analyses

Ice (855 ha)

Independence (1946 ha)

Agriculture

   

Urban

   

Wet

   

Forest

   

Total(%)

   

  1. Describe the most obvious differences in land uses between watersheds.




    • Which would you expect to contribute sediments and phosphorus lakes? Why?

Part III. Using models to predict sediment and phosphorus loadings

You have determined the differences in land uses in the two watersheds. Does this help you to explain the differences in water quality? You may think so, but your client remains skeptical and concerned about whether simple changes in management practices could help to improve the poorer quality lake. You go back to the books and decide to do some simple modeling to estimate loadings of sediment and phosphorus from each watershed, based on land use.

  1. The first step is to convert the land use percentages given in Table 6 into actual areas occupied by each land use. Place the results of this conversion into Table 7.

Table 6
Percent land use from ARC-View analyses

Ice (855 ha)

Independence (1946 ha)

Agriculture

Urban

Wetland

Forest

Total(%)

 

 

 

Table 7
Percent land use from ARC-View analyses

Ice (855 ha)

Independence (1946 ha)

Agriculture

 

Urban

 

Wetland

 

Forest

 

Total

 

  1. Table 8 contains loading estimates that you dug out of the scientific literature. Use these estimates and the land use areas from Table 7 to fill in Table 9. These numbers, summed up, are the predicted loadings of total suspended sediments and total phosphorus to each lake in a year.

Table 8
Sediment and phosphorus loading estimates

Default Loadings (kg/ha)

TSS

TP

Agriculture

1013

0.90

Urban

200

0.55

Wetland

5

0.05

Forest

86

0.11

Grass

346

0.13

Other

50

0.10

 

Table 9
Predicted sediment and phosphorus loadings

Predicted TSS (kg/yr)

Predicted TP (kg/yr)

Ice

Independence

Ice

Independence

Agriculture

Urban

Wetland

Forest

Grass

Other

Total

61808

1450490

116

1344

  1. To be able to compare loading between the two lakes, you need to know how much sediment and phosphorus are contributed per unit area or per unit volume to the lakes (fill in Table 10 using data from the lake summary dor each lake), so next you need to convert the total load in kg/yr to a load per unit area (g/m2/yr) and a load per unit volume (mg/L).

Table 10
Morphometry of lakes Ice and Independence

Ice

Independence

Lake Area (104 m2)

   

Volume (106 m3)

   

a. Complete this step for Tables 11 and 12.

Example: Ice Total Suspended Sediment (TSS) = 5966 kg/yr and its lake area is 16.6 ha.

equals 36 g/m2/yr

Based on these data, what will you tell your client about the effects of land use practices on water quality? Are there some simple Best Management Practices that could be implemented to affect water quality?

 

Table 11
Predicted annual loadings for Ice Lake

TSS

TP

Areal *

Volumetric **

 

Table 12
Predicted annual loadings for Lake Independence

TSS

TP

Areal

422 (g/m2/yr)

0.391 (g/m2/yr)

Volumetric

77 (mg/L)

0.071 (mg/L)


* Annual load per hectare of lake surface
** Annual load per m3 of lake volume

 


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date last updated: Saturday March 06 2004