Runoff in hydrology refers to the amount of water that flows into surface streams. It encompasses not just the water that moves directly over the land and through channels to join a stream but also includes interflow—the water that seeps into the soil and, under the influence of gravity, makes its way toward a stream situated above the main groundwater channel.
Additionally, runoff incorporates groundwater released into streams. When a stream is entirely fed by groundwater, it’s referred to as base flow or fair-weather runoff. This phenomenon takes place at the point where the stream channel intersects with the water table. The total runoff can be calculated by taking all precipitation and subtracting losses—like evaporation from soil and plant surfaces, temporary storage in ponds, and other similar factors.
Runoff:
Surface runoff happens when the soil can’t absorb any more water, causing excess rain, meltwater, or other sources to flow across the land. It plays a vital role in the entire water cycle. A land area that produces runoff that drains to a common point is called a drainage basin.
Types of Runoff:
Total stream flow:
Total streamflow during a rainfall event consists of the baseflow in the basin before the runoff triggered by the storm. Typically, total streamflow hydrographs are thought of as being made up of:
- Direct runoff includes contributions from surface runoff and quick interflow. When we discuss unit hydrograph analysis, we specifically focus on direct runoff.
- Baseflow consists of contributions from both delayed interflow and groundwater runoff.
Surface runoff:
Surface runoff encompasses all of the water that flows over the land’s surface, including any precipitation that directly impacts stream channels. This runoff is the primary factor that contributes to the peak discharge levels.
Interflow:
Interflow refers to the segment of streamflow that comes from water infiltrating the ground and traveling laterally beneath the surface until it eventually reaches a channel. This process occurs more gradually than surface runoff. The main components of interflow include:
- Quick interflow plays a significant role in generating direct runoff and
- Delayed interflow, which contributes to baseflow.
Groundwater runoff:
Groundwater runoff refers to the water that seeps into rivers and streams from underground sources. Unlike surface runoff, which happens quickly, this underground flow moves at a much more gradual pace.
Factors Affecting runoff:
Runoff rate and volume from an area are mainly influenced by the following factors
- Climatic factors.
- Physiographical Factors.
- Anthropogenic Factors
Climatic factors:
- Precipitation
- Type of precipitation (rain, snow [initially less runoff, but ! melting season], sleet, etc.)
- The rate (amount) and intensity
- Duration of rainfall
- Direction of storm movement
- Rainfall Patterns Across the Drainage Basin
- Previous weather (e.g. Moisture in the soil due to prior precipitation.)
- Time of year/season
- During summer, evapotranspiration rates peak, and plants engage in photosynthesis at their highest levels.
- Factors impacting evapotranspiration include temperature, wind, and relative humidity.
Physiographical Factors:
- Watershed area – the total runoff volume and peak accumulation
- The configuration of the watershed affects the travel time of water to reach the outlet.
- Elevation
- Slope of the area
- The steeper the slopes, the less likely the soil is to absorb water, leading to quicker run-off when the ground is already saturated.
- Significant impact on erosion and transport dynamics.
- The relationship between the length of the slope and the length of the valley affects the lag time to the valley and the outlet.
- Bedrock permeability plays a crucial role in how quickly run-off happens. When impermeable rocks are present at the surface or lie beneath the soil, the amount of water that can be absorbed is limited, leading to rapid run-off.
- Soil permeability refers to how well soils can absorb moisture. Soils that contain a high amount of clay tend to take in water, but they do so at a very slow rate. As a result, their permeability is considered to be low.
- The depth of the soil directly influences its capacity to absorb water; the thicker the soil, the greater the amount it can hold.
- Soils composed of larger particles, such as those formed from weathered sandstones, boast greater infiltration capacities.
- The infiltration capacity relies on various factors, including the soil’s porosity, which influences its ability to store water and impacts how easily water can penetrate deeper layers.
- Starting factors (for instance, the level of saturation in the soil and aquifers)
Anthropogenic Factors:
- Land use (e.g. agriculture, urban development, forestry operations)
- The impact on retention ability, the hydrologic balance of the watershed, and the amount of direct runoff.
- Measures:
- Increasing afforestation
- Expanding meadow spaces by reducing cultivated land.
- Limit of impervious surface
- Please prioritize the road construction that has already taken place in forest and field areas.
- Vegetation type and cover
- Interception reducing the initial surface flow
- Evapotranspiration
- Infiltration (the root systems)
- Velocity of overland flow
- To optimize retention capacity, it’s best to focus on vegetation covers such as forests, meadows, closely-seeded grains, and row crops.
- Agriculture
- Irrigation and drainage ditches enhance the flow of water transfer.
- contour tillage
Tillage on wetland compresses the subsoil, creating a “plow pan,” decreasing water holding and infiltration and increasing run-off/erosion.
Causes of Runoff:
Natural Factors:
Precipitation: Rainfall’s intensity and duration.
Soil Type: Porous soils enhance water infiltration, which helps to decrease runoff.
Vegetation Cover: Plants help reduce the speed of water movement and to improve absorption.
Topography: Steeper slopes lead to quicker runoff.
Human-Induced Factors:
Urbanization: Roads and buildings, being impervious to surfaces, contribute to an increase in runoff.
Deforestation: The decline in vegetation leads to a decrease in water absorption.
Agricultural Practices: Inefficient land management practices may result in increased runoff.
Effects of Runoff:
Positive Impacts:
- Refilling of rivers, lakes, and reservoirs.
- Nutrient delivery to ecosystems.
Negative Impacts:
- Soil Erosion: Loss of nutrient-rich topsoil.
- Flooding: Heavy runoff can overwhelm certain areas.
- Water Pollution: Runoff has the potential to transport harmful pollutants into our water sources.
- Habitat Destruction: Changes in water flow can harm ecosystems.
