When we talk about the hydrological cycle, we must distinguish between several events that determine the flow of water. One of these events is runoff . It is known as surface runoff and describes the flow of water—rain, snow, or other fluids—over the land. This is a key component of the water cycle.
In this article, we are going to tell you what surface runoff is, its importance and where it is generated from.
What is surface runoff
When we talk about surface runoff, we're referring to runoff that occurs on the surface before reaching a channel, such as a river or lake. This runoff includes both rainwater and any other fluids resulting from polluting discharges. Surface runoff that occurs before reaching a channel is called a nonpoint source. If this nonpoint source contains artificial pollutants, it's known as nonpoint source pollution.
The entire area responsible for draining this runoff into groundwater, which then forms the basis of numerous water resources, is known as the watershed. We know that human activity contributes to the runoff of many chemical fertilizers and other industrial wastes into surface waters. Therefore, it is necessary to analyze whether runoff flowing across the land can pick up contaminants present in the soil. These contaminants can include oil, pesticides, herbicides, insecticides, and fertilizers, among others. Understanding the presence of these contaminants in surface runoff is crucial to understanding its impact on the environment and health, so further reading on flooding and its effects is recommended .
Origin and generation of runoff
As mentioned earlier, runoff can originate from precipitation or from the melting of snow and ice in glaciers. When snow melts from glaciers during the thaw season, it typically occurs only in relatively cold areas. This snow runoff usually reaches its peak in spring when temperatures rise . Glaciers melt completely in summer, resulting in maximum runoff flow and impacting river levels. All this water ultimately increases river flow and soil erosion. The determining factor in the rate of snow or glacial melt is air temperature and the duration of incident solar radiation. To learn more about these processes, it is helpful to consult information on hydrogeology.
The term "V-shaped" refers to glacial valleys, where a large volume of water accumulates in a short period. Only during the thaw does the river flow strong enough to erode and transform the terrain. On the other hand, a U-shaped valley is a conventional valley. This formation occurs because the river's flow is constant throughout the year. These valleys also experience periods of lower water flow during the summer months.
High mountain regions have streams that rise on sunny days and decrease on cloudy days due to less solar radiation. In areas where there is no snow the runoff comes from precipitation. It should be mentioned that not all rainfall generates runoff. Precipitation runoff is generated when the soil cannot store large amounts of water due to heavy downpours.
For example, there are very old soils in Australia and southern Africa where proteoid roots are found. These roots are so dense and have so many root hairs that they can absorb large quantities of rainwater. Therefore, runoff is unlikely, as these roots can absorb substantial amounts of rainfall. Furthermore, a low evaporation rate is necessary for surface runoff to last longer.
Overland water flow with excess infiltration
An important aspect to consider in surface runoff is the infiltration process. This is the process by which water seeps into the subsurface layer of the earth. Here, the water is stored in aquifers and serves as a water resource. Almost every watershed has small underground water reserves. This infiltration occurs less frequently in arid and semi-arid regions where rainfall intensities are higher and the infiltration rate is lower due to surface impermeability.
There is also a greater amount of surface runoff and a lower infiltration rate in paved soils. Another aspect is oversaturated groundwater flow. This is a condition that leads to greater surface runoff. In this case, the soil is saturated with water, and the watershed has stored as much as possible. In these cases, greater surface runoff will occur due to the large amount of precipitation. The soil moisture level is also a factor that affects how long it takes for the soil to become saturated. The greater the soil moisture, the faster it will become saturated. Therefore, it will be able to store less water and will end up creating greater surface runoff. If you want to learn more about how these phenomena form, it is recommended to read about aquifers.
Human impact on surface runoff
It's important to note that human activity has a significant impact on the amount of runoff. We are constantly creating impermeable surfaces, such as pavements and buildings. These impermeable surfaces prevent water from infiltrating the ground into the aquifer . Instead of seeping into the soil, the water is forced directly into streams or drainage systems, where erosion and sedimentation increase. These conditions are what cause flooding in the city.
Increased surface runoff reduces groundwater recharge and lowers the water table. These conditions worsen drought, especially for agriculture and those who depend on wells. Furthermore, the presence of anthropogenic pollutants dissolved in surface runoff affects human health. This pollutant load can reach rivers, lakes, and oceans, contaminating them.
I hope that with this information you can learn more about surface runoff.