Precipitation and Streamflow
view Idaho achievement standards for this lesson

Overview:
There is a lot of variation in the amount of precipitation that falls in different parts of Idaho. Many factors like wind patterns and mountain ranges can have a big affect on the amount of rainfall and area receives. Once rain has fallen it can have various fates. Some may run off and end up in streams and lakes. Some may soak into the ground and trickle down into the ground water. Some is absorbed by plants and helps them grow before it is transpired from their leaves. In some habitats large amounts of water can be evaporated in the hot, dry air. This lesson will give students the opportunity to calculate how much rainfall goes to streamflow and how much goes to the other three fates: evaporation, transpiration, and groundwater. The relationship between rainfall and streamflow can vary regionally. By doing this exercise, students will be exposed to the relationship between precipitation and streamflow.

Objectives:
1. Students will be exposed to the Digital Atlas of Idaho.
2. Students will be able to calculate streamflow and precipitation when given the required information.
3. Students will understand the relationship between precipitation and streamflow.

Procedure:
Expose your students to the Digital Atlas of Idaho.
To get there: Click on Atlas Home, Climatology, then on General Climate.
Have your students read through the pages to get an understanding of the general climate section. Encourage them to click on the links so they can observe the pictures.

Do the following as a demonstration/discussion

1. Tell the class that the average flow of a stream in a particular drainage is 410 cubic feet per second. How could we determine the annual flow of this stream?
Multiply the CFS by the number of seconds in a year.
(410 ft3/s)*(60 sec/min)*(60 min/hr)*(24 hr/day) * (365 day/yr) = 12,929,760,000 ft3/yr

2. Tell the class that the average rainfall in this watershed was 29.315 inches. The area of this watershed is 456 square miles. How can we convert this to cubic feet of rainfall?
Area = 456 mi2 conversion: (456 mi2)*(27,878,400 ft2/mi2) = 12,712,550,400 ft2
Precip Volume = (29.32 in)*(1ft/12 in)*(12,712,550,400 ft2) = 31,060,998,144 ft3/yr

3. Ask the class what is the relationship between precipitation and streamflow.
12,929,760,000 / 31,060,998,144 = .42
This means that 42 percent of the rain that fell ended up in the stream.
What happened to the other 58 percent?
(The other 58 percent evaporated, got absorbed in plant matter, or ended up in groundwater before it got to the stream)
Ask the students if they think it is always 42 percent. What factors could change this percentage?