<P> Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation . However, the term can also describe the rolling of dislodged soil particles 0.5 to 1.0 mm (0.02 to 0.04 in) in diameter by wind along the soil surface . </P> <P> The amount and intensity of precipitation is the main climatic factor governing soil erosion by water . The relationship is particularly strong if heavy rainfall occurs at times when, or in locations where, the soil's surface is not well protected by vegetation . This might be during periods when agricultural activities leave the soil bare, or in semi-arid regions where vegetation is naturally sparse . Wind erosion requires strong winds, particularly during times of drought when vegetation is sparse and soil is dry (and so is more erodible). Other climatic factors such as average temperature and temperature range may also affect erosion, via their effects on vegetation and soil properties . In general, given similar vegetation and ecosystems, areas with more precipitation (especially high - intensity rainfall), more wind, or more storms are expected to have more erosion . </P> <P> In some areas of the world (e.g. the mid-western USA), rainfall intensity is the primary determinant of erosivity (for a definition of erosivity check,) with higher intensity rainfall generally resulting in more soil erosion by water . The size and velocity of rain drops is also an important factor . Larger and higher - velocity rain drops have greater kinetic energy, and thus their impact will displace soil particles by larger distances than smaller, slower - moving rain drops . </P> <P> In other regions of the world (e.g. western Europe), runoff and erosion result from relatively low intensities of stratiform rainfall falling onto previously saturated soil . In such situations, rainfall amount rather than intensity is the main factor determining the severity of soil erosion by water . </P>

How many processes of erosion occur in a river