Climate is a major factor in the absence or presence and severity of erosion by water. Areas with high annual precipitation and moderate temperatures can produce lush vegetation that will prevent erosion, examples being the rainforests of South America and the Olympic Peninsula of the United States. Low-rainfall areas such as the Sonoran Desert in northern Mexico and the southwestern United States have sparse plant cover and during an annual summer monsoon period, high-intensity thunder storms cause flash floods, high rates of sediment transport, and eventual deposition. Seasonal rainfall patterns and storm types affect erosion. The western coast of the United States has wet winters with extended storms with generally low- to moderate-rainfall rates, and dry summers. The continental United States has more uniformly distributed rainfall throughout the year, but spring and summer are periods of severe thunderstorm activity and high erosivity. The eastern coast of the United States has more precipitation in the summer than the winter, in contrast to the western coast. The weather in these different regions, coupled with land use and agricultural patterns, creates a unique profile of erosion hazard in each. One nearly universal factor in water erosion on rain-fed cropland, whether it is in Mali or Kansas, is that the season of seedbed preparation and planting, when plant and residue cover protection from erosion is the lowest, coincides with the start of the season of highest erosivity, as is the case for most deserts or rangelands. Seeding is generally done prior to annual rainy periods in order to utilize the anticipated moisture.
Winter hydrology creates erosion problems in many regions of the world. Areas with mean daily temperatures well below 0 °C accumulate snow throughout the winter and experience a spring melt and runoff period. Other areas, such as the inland Pacific Northwest United States, experience monthly mean temperatures slightly below 0 °C for only 2 or 3 months of the year, have dozens of diurnal freeze/thaw cycles each winter, and are subject to major soil-freezing and -thawing events on an average of more than two times per year. Data from the Pullman, WA, weather station indicate that in the 64 years from Water Year (WY) 1941 through 2004, there were 24 occurrences of rains of more than 25 mm during the brief periods after a major freezing event and when the soil was melting and liquefying at the surface, remained frozen deeper in the profile, and was highly susceptible to erosion. These occurrences result in exceptionally high erosion rates.
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