Runoff is nothing more than water "running off" the land surface. Just as the water you wash your car with runs off down the driveway as you work, the rain that Mother Nature covers the landscape with runs off downhill, too (due to gravity). Runoff is an important component of the natural water cycle.
Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.
• Water Science School HOME • Surface Water topics • The Water Cycle •
Water cycle components » Atmosphere · Condensation · Evaporation · Evapotranspiration · Freshwater lakes and rivers · Groundwater flow · Groundwater storage · Ice and snow · Infiltration · Oceans · Precipitation · Snowmelt · Springs · Streamflow · Sublimation · Surface runoff
Surface runoff is precipitation runoff over the landscape

In our section about water storage in the oceans we describe how the oceans act as a large storehouse of water that evaporates to become atmospheric moisture. The oceans are kept full by precipitation and also by runoff and discharge from rivers and the ground. Many people probably have an overly-simplified idea that precipitation falls on the land, flows overland (runoff), and runs into rivers, which then empty into the oceans. That is "overly simplified" because rivers also gain and lose water to the ground. Still, it is true that much of the water in rivers comes directly from runoff from the land surface, which is defined as surface runoff.
When rain hits saturated or impervious ground it begins to flow overland downhill. It is easy to see if it flows down your driveway to the curb and into a storm sewer, but it is harder to notice it flowing overland in a natural setting. During a heavy rain you might notice small rivulets of water flowing downhill. Water will flow along channels as it moves into larger creeks, streams, and rivers. This picture gives a graphic example of how surface runoff (here flowing off a road) enters a small creek. The runoff in this case is flowing over bare soil and is depositing sediment into the river (not good for water quality). The runoff entering this creek is beginning its journey back to the ocean.
Credit: Galen Hoogestraat, USGS
As with all aspects of the water cycle, the interaction between precipitation and surface runoff varies according to time and geography. Similar storms occurring in the Amazon jungle and in the desert Southwest of the United States will produce different surface-runoff effects. Surface runoff is affected by both meteorological factors and the physical geology and topography of the land. Only about a third of the precipitation that falls over land runs off into streams and rivers and is returned to the oceans. The other two-thirds is evaporated, transpired, or soaks (infiltrates) into groundwater. Surface runoff can also be diverted by humans for their own uses.
The small creek shown in the picture above will merge with another creek, eventually flowing into a larger river. Thus, this creek is a tributary to a river somewhere downstream, and the water in that river will eventually flow into an ocean. The concept is not that much different from the small capillaries in your body carrying blood to larger arteries, eventually finding its way to your heart, analogous to the ocean.
Meteorological factors affecting runoff:
- Type of precipitation (rain, snow, sleet, etc.)
- Rainfall intensity
- Rainfall amount
- Rainfall duration
- Distribution of rainfall over the drainage basin
- Direction of storm movement
- Precipitation that occurred earlier and resulting soil moisture
- Other meteorological and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity, and season
Physical characteristics affecting runoff:
- Land use
- Vegetation
- Soil type
- Drainage area
- Basin shape
- Elevation
- Topography, especially the slope of the land
- Drainage network patterns
- Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or delay runoff from continuing downstream
Human activities can affect runoff
Credit: Howard Perlman, USGS
As more and more people inhabit the Earth, and as more development and urbanization occur, more of the natural landscape is replaced by impervious surfaces, such as roads, houses, parking lots, and buildings that reduce infiltration of water into the ground and accelerate runoff to ditches and streams. In addition to increasing imperviousness, removal of vegetation and soil, grading the land surface, and constructing drainage networks increase runoff volumes and shorten runoff time into streams from rainfall and snowmelt. As a result, the peak discharge, volume, and frequency of floods increase in nearby streams.
Urban development and flooding
Urbanization can have a great effect on hydrologic processes, such as surface-runoff patterns. Imagine it this way: in a natural environment, think of the land in the watershed alongside a stream as a sponge (more precisely, as layers of sponges of different porosities) sloping uphill away from the stream. When it rains some water is absorbed into the sponge (infiltration) and some runs off the surface of the sponge into the stream (runoff). Assume a storm lasting one hour occurs and one-half of the rainfall enters the stream and the rest is absorbed by the sponges. Now, gravity is still at play here, so the water in the sponges will start moving in a general downward direction, with most of it seeping out and into the streambanks during the next day or two.
Next, imagine that roads and buildings have replaced most of the watershed surface. When that one inch of rainfall occurs, it can't infiltrate these impervious surfaces and will runoff directly into the stream, and very quickly, too! The result is a very quick and short-lived urban flood, rather than a gradual rise and fall in the river. Still, a flood lasting even 10 short minutes is enough to ruin your basement.
This concept is illustrated by this hydrograph of a rural (Newaukum Creek - blue line) and an urban (Mercer Creek - green line) creek in Washington State. If you measured the area under both curves (the total volume of water that flowed by the measurement location for the time period shown on the X axis) in the chart, they might be the same. But in the urban stream, the water at the measurement site rose at a much higher rate and reached a much higher stage (height) than the rural stream did. The tall, steep curve of Mercer Creek showed that much higher streamflows occurred in the urban stream. The urban stream stage fell back towards baseflow much quicker, too, indicating that it wasn't receiving much seepage from groundwater. "Base flow" is the sustained flow of a stream in the absence of direct runoff. It includes natural and human-induced streamflows. Natural base flow is sustained largely by groundwater discharges.
The rural stream rose much slower and reached a lower peak, meaning it may not have flooded at all. It took longer to fall back to baseflow as groundwater slowly seeped into the streambanks over the next week.
More topics and other components of the water cycle:
The Fundamentals of the Water Cycle
The Water Cycle for Adults and Advanced Students
Precipitation and the Water Cycle
A Comprehensive Study of the Natural Water Cycle
Streamflow and the Water Cycle
Snowmelt Runoff and the Water Cycle
Sublimation and the Water Cycle
Surface Runoff and the Water Cycle
Infiltration and the Water Cycle
Springs and the Water Cycle
The Atmosphere and the Water Cycle
Condensation and the Water Cycle
Ice, Snow, and Glaciers and the Water Cycle
- Overview
Runoff is nothing more than water "running off" the land surface. Just as the water you wash your car with runs off down the driveway as you work, the rain that Mother Nature covers the landscape with runs off downhill, too (due to gravity). Runoff is an important component of the natural water cycle.
Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.
• Water Science School HOME • Surface Water topics • The Water Cycle •
Water cycle components » Atmosphere · Condensation · Evaporation · Evapotranspiration · Freshwater lakes and rivers · Groundwater flow · Groundwater storage · Ice and snow · Infiltration · Oceans · Precipitation · Snowmelt · Springs · Streamflow · Sublimation · Surface runoff
Surface runoff is precipitation runoff over the landscape
Sources/Usage: Public Domain. Visit Media to see details.Impervious areas cause excessive runoff. In our section about water storage in the oceans we describe how the oceans act as a large storehouse of water that evaporates to become atmospheric moisture. The oceans are kept full by precipitation and also by runoff and discharge from rivers and the ground. Many people probably have an overly-simplified idea that precipitation falls on the land, flows overland (runoff), and runs into rivers, which then empty into the oceans. That is "overly simplified" because rivers also gain and lose water to the ground. Still, it is true that much of the water in rivers comes directly from runoff from the land surface, which is defined as surface runoff.
When rain hits saturated or impervious ground it begins to flow overland downhill. It is easy to see if it flows down your driveway to the curb and into a storm sewer, but it is harder to notice it flowing overland in a natural setting. During a heavy rain you might notice small rivulets of water flowing downhill. Water will flow along channels as it moves into larger creeks, streams, and rivers. This picture gives a graphic example of how surface runoff (here flowing off a road) enters a small creek. The runoff in this case is flowing over bare soil and is depositing sediment into the river (not good for water quality). The runoff entering this creek is beginning its journey back to the ocean.
Much of the water in rivers comes directly from rainfall runoff from the landscape.
Credit: Galen Hoogestraat, USGSAs with all aspects of the water cycle, the interaction between precipitation and surface runoff varies according to time and geography. Similar storms occurring in the Amazon jungle and in the desert Southwest of the United States will produce different surface-runoff effects. Surface runoff is affected by both meteorological factors and the physical geology and topography of the land. Only about a third of the precipitation that falls over land runs off into streams and rivers and is returned to the oceans. The other two-thirds is evaporated, transpired, or soaks (infiltrates) into groundwater. Surface runoff can also be diverted by humans for their own uses.
The small creek shown in the picture above will merge with another creek, eventually flowing into a larger river. Thus, this creek is a tributary to a river somewhere downstream, and the water in that river will eventually flow into an ocean. The concept is not that much different from the small capillaries in your body carrying blood to larger arteries, eventually finding its way to your heart, analogous to the ocean.
Meteorological factors affecting runoff:
- Type of precipitation (rain, snow, sleet, etc.)
- Rainfall intensity
- Rainfall amount
- Rainfall duration
- Distribution of rainfall over the drainage basin
- Direction of storm movement
- Precipitation that occurred earlier and resulting soil moisture
- Other meteorological and climatic conditions that affect evapotranspiration, such as temperature, wind, relative humidity, and season
Physical characteristics affecting runoff:
- Land use
- Vegetation
- Soil type
- Drainage area
- Basin shape
- Elevation
- Topography, especially the slope of the land
- Drainage network patterns
- Ponds, lakes, reservoirs, sinks, etc. in the basin, which prevent or delay runoff from continuing downstream
Human activities can affect runoff
Severe erosion can occur when people manipulate the landscape without regard to how flowing rainfall runoff will erode exposed soil.
Credit: Howard Perlman, USGSAs more and more people inhabit the Earth, and as more development and urbanization occur, more of the natural landscape is replaced by impervious surfaces, such as roads, houses, parking lots, and buildings that reduce infiltration of water into the ground and accelerate runoff to ditches and streams. In addition to increasing imperviousness, removal of vegetation and soil, grading the land surface, and constructing drainage networks increase runoff volumes and shorten runoff time into streams from rainfall and snowmelt. As a result, the peak discharge, volume, and frequency of floods increase in nearby streams.
Urban development and flooding
Urbanization can have a great effect on hydrologic processes, such as surface-runoff patterns. Imagine it this way: in a natural environment, think of the land in the watershed alongside a stream as a sponge (more precisely, as layers of sponges of different porosities) sloping uphill away from the stream. When it rains some water is absorbed into the sponge (infiltration) and some runs off the surface of the sponge into the stream (runoff). Assume a storm lasting one hour occurs and one-half of the rainfall enters the stream and the rest is absorbed by the sponges. Now, gravity is still at play here, so the water in the sponges will start moving in a general downward direction, with most of it seeping out and into the streambanks during the next day or two.
Next, imagine that roads and buildings have replaced most of the watershed surface. When that one inch of rainfall occurs, it can't infiltrate these impervious surfaces and will runoff directly into the stream, and very quickly, too! The result is a very quick and short-lived urban flood, rather than a gradual rise and fall in the river. Still, a flood lasting even 10 short minutes is enough to ruin your basement.
This concept is illustrated by this hydrograph of a rural (Newaukum Creek - blue line) and an urban (Mercer Creek - green line) creek in Washington State. If you measured the area under both curves (the total volume of water that flowed by the measurement location for the time period shown on the X axis) in the chart, they might be the same. But in the urban stream, the water at the measurement site rose at a much higher rate and reached a much higher stage (height) than the rural stream did. The tall, steep curve of Mercer Creek showed that much higher streamflows occurred in the urban stream. The urban stream stage fell back towards baseflow much quicker, too, indicating that it wasn't receiving much seepage from groundwater. "Base flow" is the sustained flow of a stream in the absence of direct runoff. It includes natural and human-induced streamflows. Natural base flow is sustained largely by groundwater discharges.
The rural stream rose much slower and reached a lower peak, meaning it may not have flooded at all. It took longer to fall back to baseflow as groundwater slowly seeped into the streambanks over the next week.
- Science
More topics and other components of the water cycle:
The Fundamentals of the Water Cycle
Earth's water is always in movement, and the natural water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years.Filter Total Items: 16The Water Cycle for Adults and Advanced Students
Earth's water is always in movement, and the natural water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years. Note: This section of the Water Science School discusses the...Precipitation and the Water Cycle
The air is full of water, even if you can't see it. Higher in the sky where it is colder than at the land surface, invisible water vapor condenses into tiny liquid water droplets—clouds. When the cloud droplets combine to form heavier cloud drops which can no longer "float" in the surrounding air, it can start to rain, snow, and hail... all forms of precipitation, the superhighway moving water...A Comprehensive Study of the Natural Water Cycle
Earth's water is always in movement, and the natural water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years. Note: This section of the Water Science School discusses the...Streamflow and the Water Cycle
What is streamflow? How do streams get their water? To learn about streamflow and its role in the water cycle, continue reading. Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.Snowmelt Runoff and the Water Cycle
Perhaps you've never seen snow. Or, perhaps you built a snowman this very afternoon and perhaps you saw your snowman begin to melt. Regardless of your experience with snow and associated snowmelt, runoff from snowmelt is a major component of the global movement of water, possibly even if you live where it never snows. Note: This section of the Water Science School discusses the Earth's "natural"...Sublimation and the Water Cycle
Solid, liquid, and gas - the three states of water. We see water freeze and turn to ice and we see water evaporate and turn to gas but... have you ever seen ice evaporate directly to gas? This process is called sublimation and you can read all about it below. Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.Surface Runoff and the Water Cycle
Runoff is nothing more than water "running off" the land surface. Just as the water you wash your car with runs off down the driveway as you work, the rain that Mother Nature covers the landscape with runs off downhill, too (due to gravity). Runoff is an important component of the natural water cycle. Note: This section of the Water Science School discusses the Earth's "natural" water cycle...Infiltration and the Water Cycle
You can't see it, but a large portion of the world's freshwater lies underground. It may all start as precipitation, but through infiltration and seepage, water soaks into the ground in vast amounts. Water in the ground keeps all plant life alive and serves peoples' needs, too. Note: This section of the Water Science School discusses the Earth's "natural" water cycle without human interference.Springs and the Water Cycle
A spring is a place where water moving underground finds an opening to the land surface and emerges, sometimes as just a trickle, maybe only after a rain, and sometimes in a continuous flow. Spring water can also emerge from heated rock underground, giving rise to hot springs, which people have found to make a delightful way of soaking away their problems. Note: This section of the Water Science...The Atmosphere and the Water Cycle
The atmosphere is the superhighway in the sky that moves water everywhere over the Earth. Water at the Earth's surface evaporates into water vapor which rises up into the sky to become part of a cloud which will float off with the winds, eventually releasing water back to Earth as precipitation. Note: This section of the Water Science School discusses the Earth's "natural" water cycle without...Condensation and the Water Cycle
The air is full of water, as water vapor, even if you can't see it. Condensation is the process of water vapor turning back into liquid water, with the best example being those big, fluffy clouds floating over your head. And when the water droplets in clouds combine, they become heavy enough to form raindrops to rain down onto your head. Note: This section of the Water Science School discusses the...Ice, Snow, and Glaciers and the Water Cycle
Ice and glaciers are part of the water cycle, even though the water in them moves very slowly. Ice caps influence the weather, too. The color white reflects sunlight (heat) more than darker colors, and as ice is so white, sunlight is reflected back out to the sky, which helps to create weather patterns. Read on to learn how glaciers and ice caps are part of the water cycle. Note: This section of...