Surface runoff, also called overland flow or terrestrial runoff, is water that flows over the ground's surface. This is different from channel runoff, which is water that flows through streams or rivers. Surface runoff happens when rainwater, stormwater, melted snow, or other water sources fall faster than the soil can absorb them. This can occur if the soil is already full of water, or if the ground is covered by surfaces that do not let water soak in, such as rooftops or pavement. Surface runoff can happen naturally or because of human activities.
Surface runoff is an important part of the water cycle. It is the main cause of soil erosion by water. The area of land that contributes runoff to a single point, such as a river or lake, is called a drainage basin.
Runoff that flows over the ground before reaching a stream or river can carry pollution. This pollution may include natural materials, like decaying leaves, or human-made substances, such as oil, pesticides, and fertilizers. Surface runoff often worsens pollution from farming, which can lead to problems like nutrient pollution in waterways. This pollution can cause eutrophication, a process that harms water quality.
In cities, surface runoff is a major cause of flooding. Flooding can damage buildings, create wet and moldy conditions in basements, and flood streets.
Generation
Surface runoff is the water from rain, snow, sleet, or hail that flows directly into a stream or river without entering the soil. It is different from direct runoff, which is water that reaches streams right after rainfall or snowmelt, but does not include water from melting snowpack or glaciers.
Snow and glacier melting happens only in cold areas where snow or glaciers can form permanently. Snowmelt usually increases the most in spring, while glacier melting increases in summer. These events cause rivers to have much higher water levels during those times. The speed of melting depends on air temperature and how long the sun shines each day. In high mountain areas, streams often rise on sunny days and drop on cloudy days because of this.
In places without snow, runoff comes from rain. However, not all rain causes runoff because soil can absorb small amounts of water. In very old soils found in Australia and Southern Africa, special root structures with many tiny root hairs can absorb so much rainwater that runoff does not happen even during heavy rains. In these areas, even on clay soils that are not very fertile, large amounts of rain and high evaporation are needed to create surface runoff. This leads to streams that are often dry but can fill quickly during rain.
Surface runoff happens when rain falls faster than the ground can soak it up, and all the water that can be stored on the surface (like in small puddles) is already full. This type of runoff is called Hortonian overland flow, named after Robert E. Horton. It is more common in dry and semi-dry areas where rain is heavy, the soil is hard on the surface, or in cities where pavement stops water from soaking into the ground.
When the soil is already very wet and the surface storage is full, and rain continues to fall, water flows directly over the ground into streams. This is called saturation excess overland flow, saturated overland flow, or Dunne runoff.
After rain, soil holds some water, which affects how quickly it can absorb more water. If the soil was already wet before the rain, it will become saturated faster during the next rain. Once the soil is saturated, runoff begins. Surface runoff plays an important role in controlling how much water stays in the soil after medium or light rain.
Water that soaks into the soil on the upper part of a hill can move sideways through the soil and flow out near a stream or river. This process is called subsurface return flow or throughflow.
As water moves over the surface, some of it may be lost through evaporation or taken up by plants, stored in small depressions on the land, or soaked into the ground. Any remaining water eventually flows into a body of water, such as a river, lake, estuary, or ocean.
Human influence
Urbanization increases surface runoff by creating more surfaces, like pavement and buildings, that do not allow water to soak through the soil into underground water sources called aquifers. Instead, the water flows directly into streams or stormwater drains, where erosion and silt buildup can occur even without flooding. More runoff reduces the amount of water that can refill groundwater supplies, lowering the water table and worsening droughts, especially for farmers and others who rely on water wells.
When human-made pollutants are mixed into runoff, the pollution spreads to water systems like streams, rivers, lakes, and oceans, changing the chemical balance of these environments and harming ecosystems.
As humans add greenhouse gases to the atmosphere, climate changes are expected to alter precipitation patterns because the air can hold more water vapor. This will directly affect how much runoff occurs.
Urban runoff is rainwater, irrigation water, and car wash water that flows over surfaces created by urban development. Impervious surfaces, such as roads, parking lots, and sidewalks, are built during land development. During rain or storms, these surfaces—made of materials like asphalt and concrete—along with rooftops, carry polluted water to storm drains instead of letting it soak into the soil.
This reduces groundwater recharge and increases flooding because more water stays on the surface. Most city storm sewer systems release untreated stormwater into streams, rivers, and bays. This excess water can also enter homes through basement backups or seepage through walls and floors. Urban runoff is a major cause of flooding and water pollution in cities worldwide.
Industrial stormwater is water from rain, snow, sleet, or hail that falls on industrial sites, such as factories, mines, or airports. This runoff often carries pollutants from materials stored or used on these sites, and facilities must follow rules to control pollution.
To manage industrial stormwater, companies use best management practices (BMPs) to stop pollutants from entering runoff and to treat water before it leaves the site. Preventive steps include keeping workspaces clean, checking equipment regularly, storing materials safely, preventing spills, and training workers on pollution prevention.
To treat stormwater, facilities may build structures like detention and retention ponds, wetlands, filters, or oil-water separators. These systems help remove pollutants by settling out solids, filtering water, or using natural processes before releasing it.
Effects of surface runoff
Surface runoff can cause erosion of the Earth's surface. Eroded material may be carried far from where it was originally. There are four main types of soil erosion caused by water: splash erosion, sheet erosion, rill erosion, and gully erosion. Splash erosion happens when raindrops hit the soil, knocking loose soil particles that then move with the runoff. Sheet erosion occurs when runoff carries sediment across the land without forming clear channels. When the soil's surface is uneven, runoff can flow through narrower paths. These paths may develop into small, defined channels called rills. Rills can be as narrow as one centimeter or as wide as several meters. If rills grow larger over time, they can become gullies. Gully erosion can move large amounts of soil quickly.
Erosion often reduces crop productivity. Scientists study these effects in the field of soil conservation. Soil particles carried by runoff range in size from 0.001 millimeters to 1.0 millimeters. Larger particles settle quickly, while smaller ones can travel long distances in water. Erosion of silty soils, which contain tiny particles, can make water cloudy and reduce light in water, harming aquatic life.
In some areas, erosion has made land unproductive. For example, on Madagascar's central plateau, about 10% of the country’s land has no vegetation, with deep and wide gullies. Shifting cultivation, a farming method that sometimes includes burning, can lead to erosion by removing fertile topsoil and reducing soil quality.
Modern farming practices also contribute to erosion. Over one-third of the U.S. Corn Belt has lost its topsoil. Using no-till farming could reduce soil erosion by more than 70%.
Runoff can harm water, groundwater, and soil by carrying pollutants into these systems. These effects can harm human health, disrupt ecosystems, and affect the appearance of water. Some of the most harmful pollutants from runoff include petroleum, herbicides, and fertilizers. Studies since the 1960s have shown that pesticides in water can increase harm to plants.
When runoff enters streams and rivers, it pollutes the water. This can make drinking water unsafe and harm aquatic life. Contaminated water can change how aquatic animals live, leading to fish deaths or imbalances in species. Some pesticides, like DDT, can change the gender of fish, turning males into females.
In forests, runoff can carry nitrogen and phosphorus into lakes, causing excessive growth of algae. Runoff from coniferous forests can also add organic material to water, altering its quality. In tropical and subtropical regions, young or high islands often experience high erosion rates, sending large amounts of sediment, nutrients, and pollutants into coastal areas. These materials can affect global cycles and marine ecosystems.
For groundwater, the main risk is contamination of drinking water if the water source is used by people. Runoff can also affect soil by either carrying pollutants to other areas or depositing them on clean soil, causing health or environmental problems.
Agricultural chemicals, such as nitrates, phosphates, and pesticides, can be carried by runoff when used in excess or at the wrong time. This harms the environment and wastes valuable chemicals. Pine straw is sometimes used to protect soil from erosion and weeds, but harvesting it can increase erosion.
Surface runoff has significant economic effects. Pine straw is a cost-effective way to manage runoff. In Nigeria, elephant grass is used to reduce runoff and erosion. In China, surface runoff has harmed crops like vegetables, so they have implemented systems to reduce nutrient loss from soil.
Flooding happens when water cannot flow downstream quickly enough. The frequency of flooding is measured by how often it occurs. Flooding is a natural process that supports ecosystems, but human activities like river engineering can change how often it happens. Floods can help agriculture, as seen along the Nile, where seasonal flooding added nutrients to soil. However, as more people live in flood-prone areas, flooding becomes a dangerous event. In cities, surface runoff is the main cause of flooding, which can damage property, harm people, and disrupt communities. Floods are among the most destructive natural events. Using irrigation can help crops like maize keep nitrogen in the soil, improving water availability for plants.
Mitigation and treatment
Mitigation of the negative effects of runoff can be done in several ways:
- Controlling how land is used in cities to reduce hard surfaces that prevent water from soaking into the ground
- Managing erosion on farms and construction sites
- Using flood control methods, such as green infrastructure, to reduce water overflow
- Controlling the use and storage of chemicals in agriculture, landscaping, and industry
Land Use Controls: Many governments have studied ways to reduce runoff by limiting the use of hard surfaces like concrete and asphalt. Cities often create rules for developers to use designs that let water soak into the ground, such as narrow sidewalks and permeable pavement for driveways and paths. For example, Santa Monica, California, has a program that sets specific design, building, and maintenance rules for properties.
Erosion Controls: Farmers have used methods to prevent soil loss since the Middle Ages, such as farming along the natural curves of the land. In the 1950s, these methods became more advanced. By the 1960s, governments began requiring builders to use erosion and sediment controls, like straw bales, silt fences, and limiting construction during rainy seasons. Montgomery County, Maryland, started the first local program for controlling erosion in 1965, followed by a statewide program in 1970.
Flood Control Programs: Starting in the early 1900s, scientists began using math to predict how much water rivers could hold during heavy rains. Over time, strategies were developed to slow water flow and reduce the speed of water in rivers. Common methods include building holding ponds to store extra water, using structures to slow water movement, and controlling land use to reduce runoff.
Chemical Use and Handling: After laws like the U.S. Resource Conservation and Recovery Act (RCRA) and the Water Quality Act were passed, governments began stricter rules to keep toxic chemicals safe. These rules include requiring double-walled tanks for storing chemicals underground, tracking hazardous materials, limiting pesticide use, and regulating fertilizers and herbicides. In industry, waste must often be treated before being released to avoid polluting water systems.
Stormwater Management Requirements: The U.S. Clean Water Act (CWA) requires cities to get permits for their stormwater systems. This means cities must manage all runoff that enters their separate storm sewer systems ("MS4"). Rules from the EPA and states require six key actions in local programs:
• Educating the public about ways to prevent stormwater pollution
• Encouraging public participation in stormwater programs
• Finding and fixing illegal connections to stormwater systems
• Controlling erosion and soil loss on construction sites
• Installing long-term stormwater controls after construction is finished
• Preventing pollution through better chemical handling, such as managing fuels, fertilizers, and road salt, and keeping systems clean and well-maintained
Other groups, like state highways, universities, and military bases, must also follow MS4 permit rules if they manage stormwater systems similar to cities.
Measurement and mathematical modeling
Runoff is studied using mathematical models along with different methods for testing water quality. Scientists can use machines that automatically test water quality for pollutants like certain chemicals, pH levels, or turbidity. They can also test for other factors like dissolved oxygen. Another method involves taking one water sample and performing chemical or physical tests on it.
In the 1950s or earlier, models were created to calculate how much runoff there is, mainly to help predict floods. Starting in the early 1970s, computer models were developed to study how runoff carries water pollutants. These models looked at how chemicals mix into water, how water soaks into soil, and how much pollution ends up in rivers or lakes. One of the first models that studied how chemicals dissolve in runoff and move through water was made in the early 1970s for the United States Environmental Protection Agency (EPA). This model helped create plans to reduce pollution from land use and chemical handling.
Over time, stormwater experts have used Monte Carlo models to study stormwater processes because natural changes in many factors affect runoff quality and quantity. Monte Carlo analysis does not reduce uncertainty in data but shows how different combinations of factors can lead to risks for water quality. An example is the stochastic empirical loading and dilution model (SELDM), a stormwater quality model. SELDM turns complex data into useful information about the risks runoff poses to water bodies, the need for pollution control, and how effective those controls might be. SELDM helps quickly assess information that is hard to get by modeling how different water-related factors (with different patterns of occurrence) interact, leading to predictions about long-term outcomes from runoff and the effects of pollution control measures. SELDM also allows quick tests to see how changes in assumptions might affect water quality risks.
Other models, like the DSSAM Model, track surface runoff through rivers to see how reactive pollutants move. In this case, surface runoff is treated as a line that adds pollution to water bodies.