InTeGrate Modules and Courses >A Growing Concern > Student Materials > Unit 5 > Rainfall and runoff erosivity (R factor)
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These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
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Initial Publication Date: October 16, 2014

Rainfall and runoff erosivity (R factor)

Consider the differences between an intense thunderstorm and a gentle rain. Which one do you think would have more impact on soil erosion?

You probably said the thunderstorm, but why? For one thing, thunderstorms typically produce bigger raindrops, which build up more kinetic energy as they fall and hit the ground with more force. This causes more soil to dislodge and become available for transport by water flowing over the surface of the land (called runoff). Thunderstorms also tend to have a higher intensity, which means that a lot of rain falls in a short amount of time. This rapid buildup of excess water can't seep into the ground very quickly and results in a lot of runoff during periods of high intensity rainfall. All of this contributes to soil erosion during a thunderstorm.

Of course not all rainfall events are created equal. You could have a storm that lasts for a few minutes or a gentle rain that lasts for days. What matters for the long-term sustainability of soils is the average erosion force in a normal year's rain. This is calculated by taking into account the amount of rainfall and the peak intensity of each storm and finding the long-term average. The resulting value is called the rainfall-runoff erosivity factor, abbreviated with the letter "R". R values vary a lot depending on regional weather patterns and local geography.

Resources

These materials are part of a collection of classroom-tested modules and courses developed by InTeGrate. The materials engage students in understanding the earth system as it intertwines with key societal issues. The collection is freely available and ready to be adapted by undergraduate educators across a range of courses including: general education or majors courses in Earth-focused disciplines such as geoscience or environmental science, social science, engineering, and other sciences, as well as courses for interdisciplinary programs.
Explore the Collection »