Erosion and deposition Surface runoff can cause
erosion of the Earth's surface;
eroded material may be
deposited a considerable distance away. There are four main types of
soil erosion by water: splash erosion, sheet erosion,
rill erosion and
gully erosion. Splash erosion is the result of mechanical collision of raindrops with the soil surface: soil particles which are dislodged by the impact then move with the surface runoff. Sheet erosion is the overland transport of
sediment by runoff without a well defined channel. Soil surface roughness may cause runoff to become concentrated into narrower flow paths: as these incise, the small but well-defined channels which are formed are known as rills. These channels can be as small as one centimeter wide or as large as several meters. If runoff continues to incise and enlarge rills, they may eventually grow to become gullies. Gully erosion can transport large amounts of eroded material in a small time period. Reduced crop productivity usually results from erosion, and these effects are studied in the field of
soil conservation. The soil particles carried in runoff vary in size from about 0.001 millimeter to 1.0 millimeter in diameter. Larger particles
settle over short transport distances, whereas small particles can be carried over long distances suspended in the
water column. Erosion of silty soils that contain smaller particles generates
turbidity and diminishes light transmission, which disrupts
aquatic ecosystems. Entire sections of countries have been rendered unproductive by erosion. On the high central
plateau of
Madagascar, approximately ten percent of that country's land area, virtually the entire landscape is devoid of
vegetation, with erosive gully furrows typically in excess of 50 meters deep and one kilometer wide.
Shifting cultivation is a farming system which sometimes incorporates the
slash and burn method in some regions of the world. Erosion causes loss of the fertile topsoil and reduces its fertility and quality of the agricultural produce. Modern industrial
farming is another major cause of erosion. Over a third of the U.S.
Corn Belt has completely lost its
topsoil. Switching to
no-till practices would reduce soil erosion from U.S. agricultural fields by more than 70 percent.
Environmental effects The principal environmental issues associated with runoff are the impacts to surface water,
groundwater and
soil through transport of water pollutants to these systems. Ultimately these consequences translate into human health risk, ecosystem disturbance and aesthetic impact to water resources. Some of the contaminants that create the greatest impact to surface waters arising from runoff are
petroleum substances,
herbicides and
fertilizers. Quantitative uptake by surface runoff of
pesticides and other contaminants has been studied since the 1960s, and early on contact of pesticides with water was known to enhance
phytotoxicity. In the case of surface waters, the impacts translate to
water pollution, since the streams and rivers have received runoff carrying various chemicals or sediments. When surface waters are used as
potable water supplies, they can be compromised regarding
health risks and clean drinking water aesthetics (that is, odor, color and
turbidity effects). Contaminated surface waters risk altering the metabolic processes of the aquatic
species that they host; these alterations can lead to death, such as
fish kills, or alter the balance of populations present. Other specific impacts are on animal mating, spawning,
egg and
larvae viability, juvenile survival and plant productivity. Some research shows surface runoff of pesticides, such as
DDT, can alter the gender of fish species genetically, which transforms male into female fish. Surface runoff occurring within forests can supply lakes with high loads of mineral nitrogen and phosphorus leading to
eutrophication. Runoff waters within
coniferous forests are also enriched with
humic acids and can lead to
humification of water bodies Additionally, high standing and young islands in the tropics and subtropics can undergo high soil erosion rates and also contribute large material fluxes to the coastal ocean. Such land derived runoff of sediment nutrients, carbon, and contaminants can have large impacts on global
biogeochemical cycles and marine and coastal ecosystems. In the case of groundwater, the main issue is contamination of drinking water, if the
aquifer is abstracted for human use. Regarding
soil contamination, runoff waters can have two important pathways of concern. Firstly, runoff water can extract soil contaminants and carry them in the form of water pollution to even more sensitive aquatic habitats. Secondly, runoff can deposit contaminants on pristine soils, creating health or ecological consequences.
Agricultural issues The other context of
agricultural issues involves the transport of
agricultural chemicals (nitrates, phosphates,
pesticides, herbicides, etc.) via surface runoff. This result occurs when chemical use is excessive or poorly timed with respect to high precipitation. The resulting contaminated runoff represents not only a waste of agricultural chemicals, but also an environmental threat to downstream ecosystems. Pine straws are often used to protect soil from
soil erosion and weed growth. However, harvesting these crops may result in the increase of soil erosion.
Economic Issues Surface runoff results in a significant amount of economic effects. Pine straws are cost effective ways of dealing with surface runoff. Moreover, surface runoff can be reused through the growth of elephant grass. In
Nigeria,
elephant grass is considered to be an economical way in which surface runoff and
erosion can be reduced. Also,
China has suffered significant impact from surface runoff to most of their economical crops such as vegetables. Therefore, they are known to have implemented a system which reduced loss of nutrients (nitrogen and phosphorus) in soil.
Flooding Flooding occurs when a watercourse is unable to convey the quantity of runoff flowing downstream. The frequency with which this occurs is described by a
return period. Flooding is a natural process, which maintains ecosystem composition and processes, but it can also be altered by
land use changes such as river engineering. Floods can be both beneficial to societies or cause damage. Agriculture along the
Nile floodplain took advantage of the seasonal flooding that deposited nutrients beneficial for crops. However, as the number and susceptibility of settlements increase, flooding increasingly becomes a natural hazard. In urban areas, surface runoff is the primary cause of
urban flooding, known for its repetitive and costly impact on communities. Adverse impacts span loss of life, property damage, contamination of water supplies, loss of crops, and social dislocation and temporary homelessness. Floods are among the most devastating of natural disasters. The use of
supplemental irrigation is also recognized as a significant way in which crops such as maize can retain
nitrogen fertilizers in soil, resulting in improvement of crop water availability. ==Mitigation and treatment==