Surface Runoff and the Water Cycle

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By Water Science School June 8, 2019

When water "runs off" the land surface, that’s runoff! Due to gravity, the water you wash your car with runs down the driveway as you work, and rain runs downhill. Runoff is an important component of the water cycle.

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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 that runs off the landscape

Atlanta, Georgia: Impervious areas affect rainfall runoff, flooding, and water quality — Surfaces that are not pervious, which means they do not allow water to pass through them, can include roofs of buildings, roads, driveways, cars, sidewalks, parking lots, and more. Areas with impervious surfaces 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.

A simple way to put it is: precipitation falls on the land, flows overland (runoff), and runs into rivers, which then empty into the oceans. While much of the water in rivers comes directly from runoff from the land surface, they also gain and lose water to the ground.

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. Surface runoff, especially when it runs across surfaces like roads, can pick up and then deposit particulate matter and sediment into the river (which isn't good for water quality).

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. The vegetation or lack thereof on the land surface either aids or detracts from how much water the land can absorb. 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 the soil, contributing to groundwater. Surface runoff can also be diverted by humans for their own uses.

Image: Stormwater Runoff in Rapid City, SD — Much of the water in rivers comes directly from rainfall runoff from the landscape. 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.

Credit: Galen Hoogestraat, USGS

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 in 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

Erosion caused by unrestricted rainfall runoff — Severe erosion can occur when people manipulate the landscape without regard to how flowing rainfall runoff will erode exposed soil.

Credit: Howard Perlman, USGS

Human activities can affect runoff

As more and more people inhabit the Earth, and as more development and urbanization occur, more of the natural, vegetated 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. We see greater volumes of runoff and faster movement of runoff into streams from rainfall and snowmelt when we remove vegetation and soil, grade the land surface, and construct draining networks. As a result, the peak discharge, volume, and frequency of floods increase in nearby streams.

Forested section of flooded stream channel in Ellicott City — Here is a forested section of one of three drainage channels for a stream in Ellicott City. We can see the aftermath of flowing floodwater with debris and eroding banks.

Urban development and flooding

Urbanization can have a great effect on surface runoff and other hydrologic processes. 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, or absorption capacities) sloping uphill away from the stream. When it rains, some water is absorbed into the sponge (infiltration) while some runs off the surface of the sponge and into the stream (runoff). Let's do a thought experiment to understand the impacts of urban development on surface runoff. When a storm lasting one hour occurs, one-half of the rainfall enters the stream, 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, so instead it runs off 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.

Chart comparing streamflow and time for a rural versus urban site.

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. Essentially, in an urban environment, the stream is more susceptible to quick flashes of intense water flow because of the impervious surfaces that surround it which make the water move faster.

USGS Research on Effective Stormwater Management

Urban creek with concrete banks — Stream channels in urban environments are often very straight to encourage water to move off the landscape and downstream quickly, but it’s not natural for a stream to be so straight.

Slow and steady runoff is important for many ecosystem processes, so USGS scientists are studying how new stormwater management systems can help.

Here’s an example. Ellicott City, Maryland has a watershed that's a little under 4 square miles. It's about 65% developed and 27% forested. At least 15% of the basin surface is covered by roads, parking lots, driveways and rooftops, which makes it impervious.

Traditional stormwater management has focused on getting precipitation that falls on the ground into drainage channels and out of the watershed as quickly as possible.

Below is a forested section of one of three drainage channels for a stream in Ellicott City. We can see the aftermath of flowing floodwater with debris and eroding banks. It’s important to understand that, in addition to flood damage, all the water that's running off so quickly ends up leaving the watershed without recharging local groundwater levels or even giving the local ecosystem a chance to benefit like would happen under more natural conditions. So urban watersheds end up losing twice, both during and after a flood. Fortunately, there are some new approaches to urban stormwater management that can produce much better results.

Two diagrams depicting older stormwater management vs. newer stormwater management. — Older stormwater management vs. newer stormwater management.

Newer stormwater management practices, which are referred to by a variety of names, such as “green infrastructure, low impact development, distributed stormwater management, or best management practices” are starting to be implemented in urban and suburban areas. This type of stormwater management often includes smaller stormwater features dispersed in many locations across the landscape, to better manage stormwater closer to where runoff begins.

These features are intended to absorb some of the stormwater into the ground, which helps replenish groundwater, provide water for vegetation, and improve water quality.

Neighborhood with curb cuts that allow stormwater to easily drain into rain gardens — Curb cuts like these allow stormwater to easily drain into rain gardens and bioswales. By utilizing dense vegetation, absorbent soils, and underground storage capacity, these installations act as natural filters of the stormwater and prevent flooding of homes and streets.

Credit: Center for Neighborhood Technology

USGS scientists continue to research the effectiveness of this newer form of stormwater management to better protect our natural water resources from the negative environmental consequences of urban development.

We often partner with local cooperators like state and county agencies. We collect and analyze various datasets on streamflow, groundwater levels, water quality, stream erosion, and the types of insects and fish living in nearby streams. Some of our results show that runoff is reduced during smaller storms, but very intense rain events can still overload these types of stormwater management systems. Some contaminants from runoff are also better controlled than others. For example, we still see an increase in the saltiness of nearby streams due to road salts applied in the winter. Our research informs how we manage our stormwater.

Read more about our research on newer stormwater management practices, and watch a video that goes through these concepts: https://www.youtube.com/watch?v=lspyUfexvCY 