Water: Middle School
Exercise II. From Rainfall to Runoff-Making the Connection
Make Your Own Rain Gauge
Before you get started with the activities in this exercise, it would be helpful to make a rain gauge for your home or classroom. To do this, take a container that will hold water (an old plastic milk container with the top cut off works well) and mark off inches on the side of the container. Make sure that you mark off the inches correctly to get the most accurate measurement of the number of inches of rainfall. Then attach the gauge in a secure and undisturbed place to measure how much it rains during a particular storm. Check the rain gauge after several storm events to see how much rain fell during each storm.
Determining Rainfall from a Rain Gauge
Rain gauges are used to measure the amount of rainfall from a storm event. The readout on a rain gauge tells how many inches of rain fell during the rainstorm.
However, runoff depends on more than just the number of inches of rain that fell. The volume of runoff in a watershed also depends on the area of the watershed. To calculate the volume of runoff, you multiply the amount of rain that fell by the area of the watershed.
Geometric Formulas for Area
- length × width = area
- π × radius² = area
The Volume of Your Rain Gauge
You can calculate the volume of your rain gauge by measuring its diameter and using a couple of simple geometric formulas. Calculate the volume of your rain gauge in the space provided below. Check the accuracy of your rain gauge with a graduated cylinder.
From Weatherperson to Watershed
Above, you learned how to find the volume of your rain gauge, but how do you find the volume of rain that fell in your watershed in a rainstorm? You start with the weather report. The weatherperson reports rainfall in inches. This number represents the amount of rain collected by the rain gauge at a monitoring station. If you multiply this number by the base surface area of your watershed, you can find the volume of rain released by the rainstorm into your watershed.
Every Place on Earth Is in a Watershed
You can figure out the boundaries of your watershed by connecting the highest points in the topography around a stream outlet. This will form a basin, and all the rain that falls anywhere within the basin will flow to the stream outlet. Calculate the area of that basin, and you will have the base surface area over which rain is falling.
What is the volume of rain expected in a 120-acre watershed in southeastern Pennsylvania if it rains 3.5 inches in a 24-hour period? Give your answer in acre-inches.