SHEET EROSION

detachment of soil particles by raindrop impact and their removal downslope by water flowing overland as a sheet instead of in definite channels or rills. A more or less uniform layer of fine particles is removed from the entire surface of an area, sometimes resulting in an extensive loss of rich topsoil. Sheet erosion commonly occurs on recently plowed fields or on other sites having poorly consolidated soil material with scant vegetative cover. There are two stages of sheet erosion. The first is rain splash, in which soil particles are knocked into the air by raindrop impact. A hundred tons of particles per acre may be dislodged during a single rainstorm. In the second stage, the loose particles are moved downslope, commonly by sheetflooding. Broad sheets of rapidly flowing water filled with sediment present a potentially high erosive force. Generally produced by cloudbursts, sheetfloods are of brief duration, and they commonly move only short distances. On relatively rough surfaces, sheetflooding may give way to rill wash, in which the water moves in a system of enmeshed microchannels, which eventually become larger and develop into gullies.