Surface runoff is the flow of excess water over the land surface rather than it soaking into the ground. When precipitation, like rain or snowmelt, hits the land, the water typically has two options: infiltrate into the soil or flow across it. Runoff occurs when the water cannot infiltrate quickly enough, causing it to move downhill and eventually collect in streams, rivers, and other bodies of water.
This process is a natural hydrologic function, but human activity and intense weather can transform it into a significant environmental issue. The volume and speed of this surface flow carry major implications for flooding, water quality, and the stability of the land itself.
How Surface Runoff Is Generated
Runoff generation is directly tied to the relationship between the rate of precipitation and the soil’s ability to absorb water, known as its infiltration capacity. The infiltration capacity is the maximum rate at which the soil can take in water, and it is influenced by the existing moisture level.
Infiltration-Excess Flow
If the rate of rainfall or snowmelt exceeds the soil’s infiltration capacity, the excess water begins to flow across the surface. This process, called infiltration-excess overland flow, is the most common mechanism in urban areas where impervious surfaces like concrete and asphalt have a near-zero infiltration rate.
Saturation-Excess Flow
Saturation-excess overland flow occurs when the soil becomes completely saturated from below and cannot hold any more water. This often happens in low-lying areas or after prolonged rainfall, where the water table rises to the surface.
Variables That Influence Runoff Flow
The quantity and velocity of surface runoff are determined by a combination of natural landscape features and human modifications.
Precipitation and Soil Conditions
One primary factor is the intensity and duration of the precipitation event; a short, heavy downpour is much more likely to create runoff than a gentle rain. The existing moisture level in the soil, known as antecedent soil moisture, also plays a large role, as wet ground reaches its saturation point much faster.
Topography and Soil Type
The physical characteristics of the land are equally important, starting with the slope or topography. Steeper slopes allow gravity to pull water downhill faster, reducing the time available for infiltration and increasing the water’s erosive power. Additionally, the soil type significantly affects infiltration; porous sandy soils absorb water quickly, while dense clay soils have a low permeability and tend to generate more runoff.
Human Influence
Land use and land cover represent the greatest human influence on runoff. Urbanization drastically increases runoff volumes by replacing natural, permeable ground with impervious surfaces such as rooftops, roads, and parking lots. These hard surfaces prevent water from soaking in and channel it rapidly into drainage systems, significantly increasing the total runoff volume.
The Environmental Consequences of Runoff
Uncontrolled surface runoff has negative effects, primarily by acting as a carrier for pollution and causing physical land degradation.
Erosion and Sedimentation
As water flows over the ground, it picks up and transports loose soil particles, leading to significant erosion. This strips away fertile topsoil and deposits sediment into waterways, which reduces water clarity and can smother aquatic habitats.
Non-Point Source Pollution
Runoff is the main pathway for non-point source pollution, which originates from diffuse sources. Contaminants are carried from both urban and agricultural areas.
In urban environments, runoff includes:
- Motor oil
- Grease
- Heavy metals
- Pet waste
In agricultural areas, runoff carries excess fertilizers and pesticides into nearby streams.
Eutrophication and Flooding
The influx of nutrients, particularly nitrogen and phosphorus, can cause eutrophication in water bodies. This over-enrichment leads to excessive algal blooms, which consume dissolved oxygen when they die and decompose. The resulting lack of oxygen can create “dead zones” that harm aquatic life. Furthermore, the sheer volume of water generated by high-intensity runoff is a primary cause of urban flooding, leading to property damage and sometimes sewage overflows.
Managing and Controlling Runoff
Managing runoff focuses on reducing the volume and slowing the speed of the water flow before it enters natural waterways.
Green Infrastructure
A key strategy involves implementing green infrastructure, which uses natural systems to replicate the landscape’s pre-development hydrology. Examples include rain gardens and bioretention areas that use layers of soil and vegetation to capture and filter runoff, promoting infiltration back into the ground. Permeable pavement is another solution used on parking lots and sidewalks, allowing water to pass through the surface and seep into the soil.
Ponds and Basins
Larger-scale engineering solutions include detention and retention ponds. Detention ponds temporarily hold excess water, slowly releasing it over time to control downstream flood peaks. Retention ponds, also known as wet ponds, maintain a permanent pool of water which helps settle out pollutants and sediments before the water is released.