What is rain splash erosion and why is it important?
S plash erosion is the first stage in the water erosion process.
Splash erosion results from the bombardment of soil surfaces by rain drops, which act much like little bombs when they fall on exposed or bare soil. They detach soil particles and destroy soil structure.
Studies have shown that rain drops fall at speeds up to 20 mph and can throw soil particles as much as five feet horizontally and two feet vertically. Drops come in sizes ranging up to six millimeters in diameter. A six millimeter drop weighs 216 times more than a one millimeter drop. In addition, heavier drops travel at higher speeds than small drops. This means that large drops carry hundreds of times more energy than small drops. In general, the heavier the rainfall, the larger the drops. That helps explain why erosion is generally greatest during short-duration, high intensity thunderstorms.
Raindrop impacts break up clumps of soil. The lighter materials — such as fine sand, silt, clay and organic material — that are detached by raindrop are more readily carried away by runoff, leaving behind larger sand grains, pebbles and gravel.
These fine particles can have another effect as well. They mix with the water from the drops and, as the water sinks into the soil, they clog up the pores in the soil that normally absorbs the rain water. As a result, the soil becomes crusty and impermeable. If the area is flat, puddles begin to form.
If the area is on a slope, however, the unabsorbed water begins flowing downhill it in a thin sheet, carrying away the soil particles that have been loosened by the raindrop bombardment. This process is called sheet erosion. Rain splash and sheet erosion generally account for more than 70 percent of down-slope erosion.
A single rainstorm can wash away one millimeter (0.4 inches) of dirt. This may not sound like much, but it translates into more than five tons per acre. It takes about 20 years to create that much soil by natural processes. Water erosion causes the loss of an estimated four billion tons of soil and 130 billion tons of water from the nation's cropland each year. This translates into an on-site economic loss of more than $27 billion each year, of which $20 billion is for replacement of nutrients and $7 billion for lost water and soil depth. In addition, the price-tag for off-site impacts, including eutrophication of watercourses and lakes, destruction of wildlife habitats, siltation of dams, reservoirs, rivers and property damage by flooding are calculated to be equal to or greater than the agricultural losses.
Sheet erosion turns into rill erosion when the runoff begins cutting small, separate channels as it travels down hill. As the water collects in these small channels, it picks up speed and the dirt particles that it carries act like sandpaper, gradually increasing the size of the rills. Gullies are rills larger than one foot wide and one foot deep. Gully erosion can be dramatic and can dump large amounts of sediment into nearby streams and rivers.
As gullies grow larger, they also can cut below the groundwater level. When this happens the groundwater drains away and the water table falls. Many deep-rooted plants, which protect the soil from erosion, depend more on groundwater than on surface water. So a dropping water table can change the character and reduce the amount of ground cover in a watershed in ways that further increase soil erosion.
In arid areas, splash erosion plays a major role in sculpting the landscape. For example, hills and ridges shaped with splash erosion tend to have gently rounded tops that are distinct from the sharper profiles created by other forms of water erosion.
There are areas in New Mexico where the creosote and sage plants sit on distinctive mounds. That is because they protect the soil under them from raindrop impacts. The areas in between the plants have no such protection, so splash erosion lowers the level of the land in between. Over time this leaves each plant sitting on its own.