Runoff occurs when an abundance of surface water created by rainfall or snow melt is unable to be absorbed by the ground and as a result flows downhill to lower regions. This water enters the water cycle by replenishing the streams, rivers, and lakes as the water moves toward the lowest point, the ocean. While beneficial to the watersheds, runoff can pick up pollutants or strip land and deposit the contaminants into a body of water, affecting the aquatic ecosystems and the life that lives there. In order to promote good runoff and diminish bad runoff, it is important to know the factors that influence runoff.
The largest factor that influences runoff is the source of the runoff water. In areas that receive more precipitation, there will be more instances of ground saturation – when water is absorbed into the ground to a point where it cannot absorb any more – and localized surface pooling. In areas that receive less precipitation, runoff occurs from the ground being unable to absorb the water. More scenarios are influenced by the amount of rainfall that occurs. When rain falls lightly and in a way the ground type can handle and move the water, there will be less runoff concerns. More rain leads to more runoff, and hard rains will lead to flooding, as the runoff collects into a water body that is unable to move it. The same is true with the rate of snow melts, and the amount that melts.
Another factor is the shape of the land and the ability to move the water. In steeper terrains there will be less absorption and an increase in surface flow activity. In areas that are flat or bowled, water is more likely to pool and collect, which may lead to flooding. The ability of the land to move the water is a large factor in controlling runoff. It is much easier to alter the course or effect of runoff if the shape of the land can be used to move it through rivers or capture it in ponds and lakes. In the case of polluted runoff, stormwater management in environmental management practices focuses solely on controlling the way the runoff is moved and contained.
Human factors also contribute to the way runoff is influenced. One factor is deforestation and strip mining. In these cases the surface vegetation is removed and destroys a barrier against surface pooling of water. Precipitation in these areas is likely to collect and move faster, often stripping large amounts of soil as it flows. This soil becomes sediment in the streams and rivers it enters, which is one of the biggest water pollutants. If the vegetation had remained, then it would hold the land and create pathways for water to be absorbed by the plants or the ground. Best Management Practices in landscaping or selective forestry cutting is essential to creating changes in the land that minimize the chance for altered runoff damage.
The other human factor that is by far the largest influence in runoff flow is the urbanization of land as people move from the big cities in what has become known as ‘urban sprawl’. The urbanization takes land that was once capable of absorbing and holding ground water and creating an impenetrable surface barrier. Parking lots, roads, and structures create areas where water has no option but to pool. Flash flooding in low lying urban areas is the result of having an abundance of runoff that is unable to drain. Pollutants are much more likely to enter bodies of water as the water runs over the cement, pavement, and rooftops. Though most of these pollutants are non-point sources – pollution sources that are not from the end of a pipe or smoke smack – water is likely to collect oils, solvents, and heat from the surfaces, moving them into the watershed. The development of porous concrete and asphalt for parking lots and roads helps water to infiltrate the ground and have a better chance at absorbing into the ground.
While there are other factors that influence runoff, both human and environmental, they occur on smaller scales and are not as key to the threat large amounts pose. By being aware of the flow of water, aiding ground absorption, and maintaining important vegetation barriers, it is possible to influence and create positive runoff for the benefit of everyone.
USGS, Water Science for Schools. (2011). Earth’s Water: Runoff. Retrieved from