Surface runoff
Surface runoff is a term used  to describe the flow of water, from rain, snowmelt, or other sources, over the land and is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source. If a nonpoint source contains man-made contaminants, the runoff is called nonpoint source pollution. A land area which produces runoff draining to common point is called a watershed. When runoff flows along the ground, it can pick up soil contaminants such as petroleum, pesticides (in particular herbicides and insecticides), or fertilizers that become discharge or nonpoint source pollution.

# Generation
Surface runoff can be generated either by rainfall or by the melting of snow or glaciers.
Snow and glacier melt occur only in areas cold enough for these to form permanently. Typically snowmelt will peak in the spring and glacier melt in the summer, leading to pronounced flow maxima in rivers affected by them. The determining factor of the rate of melting of snow or glaciers is both air temperature and the duration of sunlight. In high mountain regions, streams frequently rise on sunny days and fall on cloudy ones for this reason.
In areas where there is no snow, runoff will come from rainfall. However, not all rainfall will produce runoff because storage from soils can absorb light showers. On the extremely ancient soils of Australia and Southern Africa, proteoid roots with their extremely dense networks of root hairs can absorb so much rainwater as to prevent runoff even when substantial amounts of rain fall. In these regions, even on relatively less infertile cracking clay soils, high amounts of rainfall and low potential evaporation are needed to generate any surface runoff, leading to specialised adaptations to extremely variable (usually ephemeral) streams.

## Infiltration excess overland flow
This occurs when the rate of rainfall on a surface exceeds the rate at which water can infiltrate the ground, and any depression storage has already been filled. This is called infiltration excess overland flow, Hortonian overland flow (after Robert E. Horton), or unsaturated overland flow. This more commonly occurs in arid and semi-arid regions, where rainfall intensities are high and the soil infiltration capacity is reduced because of surface sealing, or in paved areas.

## Saturation excess overland flow
When the soil is saturated and the depression storage filled, and rain continues to fall, the rainfall will immediately produce surface runoff. (Note in the photo to the left the microdepressions are full of water as seen in the lower left of the image.) The level of antecedent soil moisture is one factor affecting the time until soil becomes saturated. This runoff is saturation excess overland flow or saturated overland flow.

## Subsurface return flow
After water infiltrates the soil on an up-slope portion of a hill, the water may flow laterally through the soil, and exfiltrate (flow out of the soil) closer to a channel. This is called subsurface return flow or interflow.
As it flows, the amount of runoff may be reduced in a number of possible ways: a small portion of it may evaporate; water may become temporarily stored in microtopographic depressions; and a portion of it may become run-on, which is the infiltration of runoff as it flows overland. What surface runoff that remains eventually flows into a receiving water body such as a river, lake, estuary or ocean.

# Human impact on surface runoff
Urbanization increases surface runoff, by creating more impervious surfaces such as pavement and buildings, that do not allow percolation of the water down through the soil to the aquifer. It is instead forced directly into streams or storm water runoff drains, where erosion and siltation can be major problems, even when flooding is not. Increased runoff reduces groundwater recharge, thus lowering the water table and making droughts worse, especially for farmers and others who depend on water wells.
When anthropogenic contaminants are dissolved or suspended in runoff, the human impact is expanded to create water pollution. This pollutant load can reach various receiving waters such as streams, rivers, lakes, estuaries and oceans with resultant water chemistry changes to these water systems and their related ecosystems.
A contrarian could note there is considerable surface runoff in natural systems from animal wastes being entrained in runoff or from natural sediment loading in the absence of human alteration of the land. While these statements are true, they fail to convey that the most pernicious consequences to human health and ecosystems are from runoff issues related human intervention; however, in underdeveloped countries the proportion of runoff attributable to natural factors has greater dominance, principally due to the lack of isolation of water supplies from potential animal waste carrying runoff.

# Effects of surface runoff

## Erosion
Surface runoff is one of the causes of erosion of the earth's surface. Reduced crop productivity usually results from erosion, and these effects are studied in the field of soil conservation. There are four principal types of erosion: splash erosion, gully erosion, sheet erosion and stream bed erosion. Splash erosion is the result of mechanical collision of a raindrop with the soil surface, resulting in some of the soil particles becoming suspended in surface runoff solution. Gully erosion occurs when the runoff stream is so strong that it cuts a recognizable swath in the soil and creates a small rivulet of well defined channel, which could be as small as one centimeter wide or as large as several meters. Sheet erosion is the overland transport of runoff which does not have any well defined channel. Both types of runoff can transport significant quantities of sediment or other water pollutants. In the case of gully erosion, massive amounts of material can be transported in a small time increment. Stream bed erosion is not the subject of this article, but is the attrition of stream banks or bottoms by rapidly flowing rivers or creeks.
The soil particles carried in runoff have variable size typically from about .001 millimeter to 1.0 millimeter in diameter. Larger particles tend to precipitate out, or settle, with small transport distances, whereas minute particles can travel long distances suspended in the water column. For this reason it is the smaller particle silty soils that tend to generate turbidity and diminish light transmission, a condition that can disrupt aquatic ecosystems.
One of the main sources of modern erosive soil loss worldwide stems from slash and burn treatment of tropical forests. When the total ground surface is stripped of vegetation and then seared of all living organisms, the upper soils are vulnerable to both wind and water erosion. In a number of regions of the earth, entire sectors of a country have been rendered unproductive; for example, on the Madagascar high central plateau, comprising approximately ten percent of that country's land area, virtually the entire landscape is sterile of vegetation, with gully erosive 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 cause loss of the fertile top soil and reduces the its fertility and quality of the agricultural produce.

## Environmental impacts
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 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.
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 relatively pristine soils, creating health or ecological consequences.

## 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. 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.

## Agricultural issues
A common context of run-off deals with agriculture. When farmland is tilled and bare soil is revealed, rainwater carries billions of tons of topsoil into waterways each year, causing loss of valuable topsoil and adding sediment to produce turbidity in surface waters.
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. The alternative to conventional farming is organic farming which eliminates or greatly reduces chemical usage.

# Measurement and mathematical modeling
Runoff is analyzed by using mathematical models in combination with various water quality sampling methods. Measurements can be made using continuous automated water quality analysis instruments targeted on pollutants such as specific organic or inorganic chemicals, pH, turbidity etc. or targeted on secondary indicators such as dissolved oxygen. Measurements can also be made in batch form by extracting a single water sample and conducting any number of chemical or physical tests on that sample.
In the 1950s or earlier hydrology transport models appeared to calculate quantities of runoff, primarily for flood forecasting. Beginning in the early 1970s computer models were developed to analyze the transport of runoff carrying water pollutants, which considered dissolution rates of various chemicals, infiltration into soils and ultimate pollutant load delivered to receiving waters.
One of the earliest models addressing chemical dissolution in runoff and resulting transport was developed in the early 1970s under contract to the EPA.  This computer model formed the basis of much of the mitigation study that led to strategies for land use and chemical handling controls.
Other computer models have been developed (such as the DSSAM Model) that allow surface runoff to be tracked through a river course as reactive water pollutants. In this case the surface runoff may be considered to be a line source of water pollution to the receiving waters.

# Mitigation and treatment
Mitigation of adverse impacts of runoff can take several forms:
- Land use development controls aimed at minimizing impervious surfaces in urban areas
- Erosion controls for farms and construction sites
- Flood control programs
- Chemical use and handling controls in agriculture, landscape maintenance, industrial use, etc.
Regarding Land use controls, the U.S. Environmental Protection Agency and others have encouraged research on methods of minimizing total surface runoff by avoiding unnecessary hardscape. Many municipalities have produced guidelines and codes for land developers that encourage minimum width sidewalks, use of pavers set in earth for driveways and walkways and other design techniques to allow maximum water infiltration in urban settings. An example land use control program can be viewed at seen in the city of Santa Monica, California.
Erosion controls have appeared since medieval times when farmers realized the importance of contour farming to protect soil resources. Beginning in the 1950s these agricultural methods became increasingly more sophisticated. After passage of the National Environmental Policy Act (NEPA) in the United States, much more effort was focused upon mitigation of construction runoff by such tactics as: use of straw bales and barriers to slow runoff on slopes, programming construction for months that have less rainfall and minimizing extent and duration of exposed graded areas.
Flood control programs as early as the first half of the twentieth century became quantitative in predicting peak flows of riverine systems. Progressively strategies have been developed to minimize peak flows and also to reduce channel velocities. Some of the techniques commonly applied are: provision of holding ponds to buffer riverine peak flows, use of energy dissipators in channels to reduce stream velocity and land use controls (above) to minimize runoff.
Chemical use and handling has become a focal point mainly since passage of NEPA in the U.S.  States and cities have become more vigilant in controlling the containment and storage of toxic chemicals, thus preventing releases and leakage. Methods commonly applied are: requirements for double containment of underground storage tanks, registration of hazardous materials usage, reduction in numbers of allowed pesticides and more stringent regulation of fertilizers and herbicides in landscape maintenance. In many industrial cases, pretreatment of wastes is required, to minimize escape of pollutants into sanitary or stormwater sewers.
The U.S. Clean Water Act (CWA) requires that local governments in urbanized areas (as defined by the Census Bureau) obtain stormwater discharge permits for their drainage systems. Essentially this means that the locality must operate a stormwater management program for all surface runoff that enters the municipal separate storm sewer system ("MS4"). EPA and state regulations and related publications outline six basic components that each local program must contain:
- Public education (informing individuals, households, businesses about ways to avoid stormwater pollution)
- Public involvement (support public participation in implementation of local programs)
- Illicit discharge detection & elimination (removing sanitary sewer or other non-stormwater connections to the MS4)
- Construction site runoff controls (i.e. erosion & sediment controls)
- Post-construction (i.e. permanent) stormwater management controls
- Pollution prevention and "good housekeeping" measures (e.g. system maintenance).
Other property owners which operate storm drain systems similar to municipalities, such as state highway systems, universities, military bases and prisons, are also subject to the MS4 permit requirements.