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The Winooski River in Vermont drains an area of over 1,000 square miles. This corresponds to a large amount of water being moved up to dozens of miles, along with dissolved chemical species. Here, we will explore the flux of water, suspended, and dissolved load through the Winooski River Basin.
The Winooski River in Vermont drains an area of over 1,000 square miles. This corresponds to a large amount of water being moved up to dozens of miles, along with dissolved chemical species. Here, we will explore the flux of water, suspended, and dissolved load through the Winooski River Basin.
Graphs and Calculations
Graphs and Calculations
Normal Precipitation vs. Elevation
Normal Precipitation vs. Elevation
Figure 1: The normal precipitation, or 30 year average for 31 different locations across Vermont were plotted against elevation. There is a fairly strong, positive correlation between elevation and normal precipitation. The outlier was taken from the top of Mt. Mansfield at an elevation of 3,950 feet, which is significantly higher than the data from most towns.
Figure 2: Locations in Vermont where each average was taken.
Figure 3: Table of all sample locations, normal precipitation, and elevation.
Average Precipitation in Vermont
Average Precipitation in Vermont
The average precipitation for Vermont was calculated by taking the average of the normal precipitation values. It was calculated to be 42.15 inches of rain annually.
Average Precipitation in the Winooski River Basin
Average Precipitation in the Winooski River Basin
To calculate the average amount of precipitation in the Winooski River basin, the normal values for sample locations within the drainage basin were averaged. These points included Burlington, Huntington Center, Montpelier, and Waterbury. The average precipitation was 37.86 inches, slightly lower than the average precipitation taken from all of the points. The average elevation for the points within the basin is 737 feet, while the state average is 904 feet. However, when the Mt. Mansfield elevation is not considered, the state average drops down to about 800 feet, which is not far off from the average elevation of the Winooski River basin.
Figure 4: The extent of the Winooski River drainage basin.
Volume of Precipitation falling in the Winooski River Basin
Volume of Precipitation falling in the Winooski River Basin
The volume of water that falls within the basin annually was calculated using the average precipitation within the basin, 37.86 inches, and the total area of the basin, 1,044 square miles.
The average precipitation must be converted from inches to kilometers. This can be done by multiplying 37.86 inches by the conversion factor of 2.54 centimeters and dividing the result by 100,000 to obtain a value of 0.00096 km.
Next, the area of the drainage basin must be converted from miles squared to kilometers squared. One square mile is equal to approximately 2.59 kilometers squared, so this value must be multiplied by 1,044. The result is 2,704 kilometers squared.
Finally, in order to obtain the volume of water, the area must be multiplied by the amount of rain. Multiplying 2,704 km squared by 0.00096 km yields 2.60 cubic kilometers. This value represents the approximate average volume of rain that falls within the Winooski River basin within a year.
Total Runoff Value within a given year at Essex, VT Station
Total Runoff Value within a given year at Essex, VT Station
The average annual discharge for the Winooski River at Essex, 1,817 cubic feet per second, can be used to calculate the annual runoff in cubic kilometers. The calculation is shown in Figure 5. The discharge is converted from cubic feet to cubic kilometers and from seconds to years to produce a total discharge of 1.62 cubic kilometers per year. However, above we had calculated that the total volume of water that falls within the basin, or the precipitation value, is 2.60 cubic kilometers. Therefore, by subtracting the discharge, 1.62 cubic km, from the total volume, 2.60 cubic km, we are left with a value of 0.98 cubic km, which represents the amount of water that doens't make it into the stream and is considered runoff. The runoff represents nearly 38% of the total water that falls within the basin.
Figure 5: Calculating average discharge for the Winooski River at Essex. This value can be subtracted from the total volume of water that falls within the basin to determine the runoff volume.
Figure 6: Calculating the mean annual suspended load and mean annual dissolved load for the Winooski River at Essex.
Mean Annual Suspended Load and Mean Annual Dissolved Load
Mean Annual Suspended Load and Mean Annual Dissolved Load
Using data from the gage on the Winooski River at Essex, the mean annual suspended load and mean annual dissolved load were calculated to be 81 mg/L and 81.75 mg/L respectively. Figure 6 walks through the calculations that these averages were used for. These values were then converted to tons per cubic kilometer. Then, they were multiplied by the discharge in order to cancel out the cubic kilometers, resulting in values in units of tons per year. Finally, the values were divided by the area of the basin in order to find the result in units of tons per square kilometer per year. The mean annual suspended load is 4.84 x 10e-17 tons per square km per year and the mean annual dissolved load is 4.90 x 10e-17 tons per square km per year.
Analysis
Analysis
Looking at the rainfall and runoff values within the Winooski River Basin, we see some differences. It is important to consider that while a large volume of water may fall in the area, not all of the water will actually make it into the streams. The discharge, 1.60 cubic kilometers, is significantly smaller than the precipitation value, 2.60 cubic kilomters. This is because much of the water is lost before it actually ends up in a stream. It is likely that most of this loss results from evapotranspiration. Much of the basin is forested and trees pull up a significant amount of water through their roots, and in turn a lot of the water will leave the leaves through evapotranspiration. The water can be used by humans as well, but it is more likely that most of the water leaves via evapotranspiration.
Considereing the flux ov mass out of the Winooski Basin, we can compare the dissolved load to the suspended load moving out of the basin. In many cases, the suspended load is responsible for moving a majority of the material in a river, however, here we see that the dissolved load is slightly higher on average. A high dissolved load may suggest that the underlying bedrock is being weathered at a higher rate. Looking at a study that took place in Cuba (Bierman et al 2020), we can see that the rate of mass removal in Cuba is higher than that of Vermont. The results from the Cuba paper give an estimate of about 780 mg/L for the total dissolved solids, and the highest TDS value in our data is 116 ml/L. It is likely that the suspended load in Cuba is greater as well. The main reasoning why Cuba has higher TDS values and removal rates than Vermont is due to the climate and the underlying bedrock. Cuba is in a warm, tropical, mountainous area where mass removal rates are high due to a variety of underlying bedrock types (Bierman et al 2020). Warm, humid regions often have more rapid weathering rates because there is a much higher amount of water that passes through the environment and helps speed up the weathering process. Additionally, Cuba is warm all year round, while Vermont has a colder, drier winter where less weathering occurs. The underlying bedrock in Cuba is important to consider as well. Tectonic uplift in the area helps expose fresh bedrock, which can weather quickly and release lots of dissolved material into the rivers.
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