'Erosivity': The Ability of Precipitation To Cause Erosion

This reminded me of the high likelihood of erosion at this time of the year. The impact of raindrops on the soil surface is the beginning, and most important part, of the erosion process. The extent of erosion caused by rainfall (erosivity) depends on the size and velocity of raindrops and the amount of precipitation. Gentle, drizzly rain is not very erosive, whereas fierce thunderstorms and hurricanes are very erosive. High-intensity storms produce larger drops that fall faster than those of low-intensity storms and therefore have greater potential to destroy aggregates and dislodge particles from the soil matrix.

Although the same total amount of rain may fall, a short, high-intensity rainfall event causes much more erosion than a long, low-intensity storm. Total average precipitation does not vary much across Pennsylvania, but the intensity of rainfall does. Thunderstorms and hurricanes with accompanying high-intensity rainfall tend to hit the southeastern part of the Commonwealth more frequently, leading to a higher erosion threat in the southern rather than northern parts of Pennsylvania (Figure 1). Most erosive precipitation events usually occur in the late summer and early fall (Figure 2). Soils that are bare during this period are under extreme risk of soil erosion. That is why keeping the soil covered at this time of the year by living vegetation or a mulch cover is so important.

Figure 1. Erosivity varies across Pennsylvania. The precipitation is much more aggressive in the southeast than in the northern parts of the state. In fact, the precipitation in the southeast is almost two times more erosive than in the north.

Figure 2. Erosivity varies during the year. The most erosive period is in July-September, the hurricane and thunderstorm season in Pennsylvania.