The size of a watershed area

  1. Draw a watershed of your local stream or small river:
  • Choose a stream together with students and locate it on a local map. The stream should be small.
  • Cover the map with tracing paper and draw the stream with all its tributaries.
  • Try to determine the high ridgeline all around the stream to determine its river basin area.
  1. Calculate the size of the river basin (determine what is the size of 1 dm² on the map in real space):
  • Draw a mesh of lines with 10 cm intervals among parallel lines on the top of our river basin and estimate the size of the watershed area using simple calculation.
  1. Figure out how much water drains from the watershed area:
  • First, find out what is average yearly precipitation and average yearly evaporation in your area. (For annual precipitation, you can take data from the map in the introduction. For annual evaporation, you can use data from the last part of the introduction.) The information is usually given in m² of the surface.
  • From this data, calculate how much water falls as precipitation in your watershed and how much water evaporates back into the atmosphere. The difference is water that flows as yearly outflow from your watershed.
  • If this quantity is divided with time, you will get the average flow of your stream, what we usually calculate in cubic meters per second (m³/s).