Looking at the Water Budget
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Through the Earth's annual cycle around the sun, the fluctuations of the seasons establish a climate signature that may vary from year to year in detail, but this seasonal signature will remain relatively constant for decades and even centuries. While the hydrograph provides a snapshot of climate dynamics in a region, examining the water budget over the course of a year provides further insight into the complex relationship between input and output of water through an area. The graphs below show precipitation and potential evapotranspiration for Silver Lake at 10,250 feet above sea level, and Boulder, Colorado, which is about thirty miles downstream at 5,430 feet of elevation. Click here for a map of the Boulder watershed or visit the EPA's Surf Your Watershed website to locate your watershed. |
The following terms help in understanding the seasonal dynamics of the water budget, which is one of the fundamental building blocks of climate variability and change.
- Potential Evapotranspiration (PE): All the water that could enter the air from plants and evaporation (if present.)
- Precipitation (P): All moisture from the atmosphere, rain, snow, hail and sleet.
- Surplus: Amount of water beyond what is lost naturally from the soil (when P is greater than PE)
- Deficit: Amount of water that would be lost beyond what is in the soil if it were present (when P is less than PE)
Following is a data table of monthly totals compares of Precipitation and Potential Evapotranspiration for Silver Lake and Boulder. The data is a composite of measurements. (Units are in inches.)
|
Silver
Lake
|
J
|
F
|
M
|
A
|
M
|
J
|
J
|
A
|
S
|
O
|
N
|
D
|
T
|
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Precip
|
1.84
|
2.29
|
3.15
|
3.37
|
3.16
|
2.57
|
2.73
|
2.4
|
1.65
|
1.7
|
1.68
|
1.47
|
28.01
|
|
PE
|
0
|
0
|
0
|
0
|
0.93
|
3.66
|
4.71
|
3.93
|
2.24
|
0
|
0
|
0
|
15.47
|
|
Boulder
|
J
|
F
|
M
|
A
|
M
|
J
|
J
|
A
|
S
|
O
|
N
|
D
|
T
|
|
Precip
|
0.66
|
0.78
|
1.66
|
2.45
|
3.18
|
2.06
|
1.67
|
1.45
|
1.54
|
1.34
|
1.04
|
0.7
|
18.53
|
|
PE
|
0
|
0
|
0
|
2.55
|
4.99
|
8.15
|
4.06
|
1.45
|
1.3
|
1.26
|
0.53
|
0
|
24.29
|
INQUIRY
Here are some inquiry questions to explore relative to understanding water budget dynamics.
1. During what months does Boulder have a water surplus? A water deficit? During what months does Silver Lake have a water surplus? A water deficit?
2. Which place has an overall deficit for the year? Which place has an overall surplus for the year? Explain why.
3. During what month is Boulder's water deficit the lowest? During what month is Boulder's water deficit the largest? During what month is Boulder's water surplus the smallest? During what month is Boulder's water surplus the highest? During what month does Boulder's water deficit peak? Give two reasons for the dramatic rise.
4. When does Silver Lake not have any PE? Explain why.
5. List the three locations that Boulder gets its water from.
6. Boulder is downstream from Silver Lake. What would you expect to happen if the Silver Lake area had a very large surplus? When would you experience the most problems connected with its surplus and why? What could planners do to minimize these problems?
Additional activities:
Chart a water balance chart showing potential evaptranspiration (PE) and precipitation for (P) . You can use data from the Western Regional Climate Center which has a summary of Climate Data in Colorado and numerous other resources on precipitation and climate.
Thanks to Steve Wanner of Boulder High School for helping develop this learning activity.
Photos courtesy of
the City of Boulder
