Department of Geography and Environmental Studies
GG101 Introduction to Physical Geography Lab 7: Watershed Hydrology
Tasks and Questions
Climate Trends in southern Ontario and the Water Balance
In the Lab 7 background reading there are three figures that show temperature and precipitation trends. Figure 1 shows mean annual temperature trends for the Great Lakes climate region of Canada, Figure 2 shows the precipitation trend from Toronto and Figure 3 shows the precipitation trend from Wiarton.
Question 1 (2 marks)
Briefly describe the temperature and precipitation trends observed since 1960.
Impact of Climate on the Water Balance of a Watershed
Climate will strongly influence the water balance as precipitation represents the major input to a watershed and temperature strongly influences evapotranspiration, a major loss from a watershed. Mean annual temperature and total annual precipitation data may not capture processes that occur over shorter intervals of time, for example, seasonal or monthly variations in inputs and outputs from a watershed may be critically important. Yet the annual data provide us with some insight.
Water Balance Data
The water balance is given as:
Q = P – E ± ªS where Q is the Discharge or Runoff
P is Precipitation
E is Evapotranspiration
ªS is change in Storage
There are two major outputs: (i) Discharge or Runoff, and (ii) Evapotranspiration. The major input is Precipitation. At the watershed scale, we give the input (Precipitation) in units of mm. The output variable Evapotranspiration is also given in mm. The water output from the system can be given as a Discharge, which is the volume of water that is flowing from the system over a period of time in units such as m3/s or millions of m3 per year, or it can be given as a Runoff value in mm which is the amount of water that moves from the basin into the channel expressed per unit area of the watershed.
Table 1 shows data from a series of watersheds across southern Ontario. The table has columns on the following: (i) watershed Area in km2, (ii) Mean Annual Flow (MAF) in m3/s, (iii) Mean Annual Temperature (MAT) in oC, (iv) Total Annual Precipitation (TAP) in mm, (v) Total
Annual Flow (TAF) in millions of m3/yr, (vi) Specific Discharge (SQ) in m3/s per km2, (vii)
Runoff (RUN) in mm, (viii) Evapotranspiration (ET) in mm, and (ix) Runoff as a % of TAP (RUN%). The Area, MAF, MAT, and TAP data were extracted from the Ontario Watershed and Flow Assessment Tool (OFAT) and the others are calculated.
Total Annual Flow (TAF) is calculated by taking the mean annual flow in m3/s and multiplying by the number of seconds in one year to express the flow as an annual volume.
Specific Discharge (SQ) is calculated by taking MAF and dividing by the watershed Area.
Runoff (RUN) is calculated by taking the TAF and dividing by the Area and then multiplying by 1000, to yield the runoff from each basin in mm per unit area.
Evapotranspiration (ET) is calculated from the water balance, Runoff = Precipitation – ET ±ÄS so ET is calculated by subtracting Runoff (RUN) from the TAP (assumes ÄS is zero).
RUN% is calculated by expressing Runoff as a percentage of TAP.
ET% is calculated by expressing Evapotranspiration as a percentage of TAP
TABLE 1 Area MAF MAT TAP TAF SQ RUN ET RUN% ET%
Location km2 m3/s oC mm millions3 m3/s 2 mm mm % %
m /yr per km
Stokes River near Ferndale
Sydenham River near Owen Sound
Don River at Todmorden
Duffins Creek above Pickering
Etobicoke Creek below QEW
Mimico Creek at Islington
Wilmot Creek near Newcastle
Venison Creek near Walsingham
Nith River at New Hamburg
Twenty Mile Creek at Balls Falls
Big Otter Creek near Calton
Bayfield River near Varna
Open the Lab 7 folder from MyLS and download all the files to your workstation. There is a Google Earth file called GG101_Lab_7_Watersheds.kmz. In that file there is a marker that depicts the stream locations and the watershed areas above those locations.
Open the file GG101_Lab_7_Watersheds.kmz in Google Earth.
Examine the distribution of the stations and the watersheds. Some watersheds are from predominantly rural areas and others contain a mix of land uses.
Table 1 shows a complete set of data for the Stokes River (near Ferndale). The data from Table 1 are in a spreadsheet called GG101_Lab_7_Watershed_Data.xlsx.
Open the file GG101_Lab_7_Watershed_Data.xlsx in Excel or another spreadsheet program.
Task 1 (4 marks)
Calculate TAF, SQ, RUN and ET for the streams listed in Table 1. Enter your values into Table 1.
Task 2 (6 marks)
Produce a series of graphs that show the following relations:
- Watershed Area (x-axis) vs Mean Annual Flow (y-axis)
- Total Annual Precipitation (x-axis) vs Specific Discharge (y-axis)
- Total Annual Precipitation (x-axis) vs Runoff (y-axis)
- Total Annual Precipitation (x-axis) vs Evapotranspiration (y-axis)
Your graphs should be complete, with labeled axes with units, a title and the data points clearly shown, you may fit a trendline.
For each graph, briefly describe the relation. Make a series of simple declarative statements, for example: “When examining the relation between Watershed Area and Mean Annual Flow we see that as the Watershed Area increases the Mean Annual Flow___________. The relation between the two variables is __________ and __________.”
Role of Temperature as Influence on Runoff
There is some range in the mean annual temperature and total annual precipitation of the locations in
Table 1. While the climate is similar over the region, we do see that locations in Bruce and Grey County (Stokes River, Sydenham River) are cooler and wetter than locations in the developed areas of the GTA (Don River, Etobicoke Creek, Mimico Creek). The variance in temperature and precipitation permits us to examine how these climate variables influence runoff.
Task 3 (2 marks)
Calculate the Runoff % variable in the spreadsheet and enter your values into Table 1. The Runoff % is calculated by taking Runoff and dividing by TAP and expressing the result as a percentage.
Calculate the Evapotranspiration % variable in the spreadsheet and enter your values into Table 1.
The Evapotranspiration % is calculated by taking Evapotranspiration and dividing by TAP and expressing the result as a percentage.
Task 4 (4.5 marks)
- Produce a graph that has Mean Annual Temperature (x-axis) plotted against Total Annual Precipitation (y-axis).
- Produce a graph that has Mean Annual Temperature (x-axis) plotted against Runoff % (y-axis).
- Produce a graph that has Mean Annual Temperature (x-axis) plotted against ET % (y-axis).
These graphs should be complete, with labelled axes with units, a title and the data points clearly shown, you may fit a trendline.
For each graph, briefly describe the relations as you did in Task 2.
Question 2 (4 marks)
Review all of the relations examined. Explain why areas that have higher mean annual temperatures have lower runoff %’s compared to areas that have lower mean annual temperatures?
Question 3 (5 marks)
Given the temperature and precipitation trends shown in Figures 1, 2, and 3 describe what the impact will be on runoff to, and the flow volumes of, streams in southern Ontario should those trends continue in the future. In your response refer to the relations that you have uncovered in examining the water balance for streams in southern Ontario.