How Do You Calculate Rainfall Intensity?

Rainfall intensity is calculated as millimeters per hour over a given duration. Since the heaviest rainfalls are usually quite short in length, the average time frame used for the calculation is 5 minutes.

There are many different occupations that use rainfall intensity calculations, one of which is water resource engineering. The data is used to plan, design and operate water resource projects. Some examples of these kinds of projects include storm sewers, culverts and other types of urban drainage systems. While there are different methods of obtaining the figure, such as the Gumbel technique or the Log Pearson Type III distribution, the basic steps remain the same.

  1. Determine the time frame
  2. Decide on the time frame in which the rainwater will be collected. While 5 minutes is the norm, the time can be any reasonable length.
  3. Collect the water
  4. Use a standard rain gauge to collect the rain water. A rain gauge consists of a funnel that empties into a graduated cylinder. This fits into a larger cylinder, which collects any run-off. The cylinder is marked in millimeters. Set the empty rain gauge outside in an open area, and start marking time. When the predetermined amount of time has elapsed, retrieve the rain gauge and bring it inside or under cover.
  5. Calculate the data
  6. Observe from the rain gauge how many millimeters of rain were collected. If it was during a 5-minute period of time, multiply that amount by 20. This will give an answer of millimeters of rainfall per hour.