Runoff is the portion of precipitation that appears in unregulated surface streams, rivers, or lakes. It represents the “excess” water in the hydrological cycle that is not evaporated, transpired, or intercepted by vegetation and exceeds the soil’s infiltration capacity. In the global water budget, runoff is the mechanism that returns surplus terrestrial water back to the oceans.
Types of Runoff
Runoff is classified based on the path it takes to reach a stream channel and the time it takes to get there.
| Type | Path and Characteristics |
| Surface Runoff | Also known as Overland Flow. Water that travels over the ground surface to the nearest channel. It occurs when the rainfall rate exceeds the infiltration rate. |
| Subsurface Runoff | Also known as Interflow. Water that infiltrates the soil but moves laterally through the upper soil layers to reach a stream without joining the main water table. |
| Base Flow | This is delayed runoff. It is groundwater that has moved through the saturated zone and slowly discharges into the riverbed. It sustains rivers during dry periods. |
Factors Influencing Runoff
The volume and peak flow of runoff from a drainage basin are determined by a combination of meteorological and physical factors.
Meteorological Factors
- Rainfall Intensity: High-intensity rain quickly saturates the soil surface, leading to rapid surface runoff.
- Rainfall Duration: Longer duration leads to soil saturation, increasing the percentage of rain that becomes runoff.
- Precipitation Type: Rain produces immediate runoff, whereas snow acts as temporary storage, releasing runoff only when temperatures rise (snowmelt).
Physical and Watershed Factors
- Land Use and Cover: Forests and grasslands increase infiltration and decrease runoff. Conversely, urban “impervious” surfaces (concrete, asphalt) significantly increase runoff.
- Soil Type: Sandy soils have high infiltration (low runoff), while clayey or rocky soils have low infiltration (high runoff).
- Topography: Steeper slopes increase the velocity of overland flow, giving water less time to infiltrate, thereby increasing runoff.
- Drainage Density: A high density of streams in a basin leads to a more rapid concentration of runoff.
The Hydrograph
A Hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river.
- Rising Limb: Reflects the increase in discharge as runoff reaches the channel.
- Peak Discharge: The highest point on the hydrograph, representing the maximum flow during a storm event.
- Recession (Falling) Limb: Represents the withdrawal of water from storage in the valley and the gradual return to base flow.
- Lag Time: The time interval between the center of the mass of rainfall and the peak discharge. Shorter lag times indicate a higher risk of Flash Floods.
Runoff and Water Management in India
- Watershed Management: Programs like the PMKSY (Pradhan Mantri Krishi Sinchayee Yojana) focus on “Runoff Harvesting” through structures like farm ponds and check dams to recharge groundwater.
- Urban Flooding: In cities like Hyderabad, Chennai, and Bengaluru, the destruction of natural drainage and increasing “concrete jungles” has turned manageable rainfall into destructive runoff (Urban Heat Island effect also contributes to localized heavy bursts).
- River Interlinking: The concept is based on transferring runoff from “surplus” basins (e.g., Brahmaputra) to “deficit” basins (e.g., Cauvery).
Technical Terms and Trivia
- Runoff Coefficient: A dimensionless number (between 0 and 1) that relates the amount of runoff to the amount of precipitation received. It is near 0 for forests and near 1 for asphalt parking lots.
- Catchment Area: The entire geographical area from which all surface runoff flows to a common point (usually a river confluence or mouth).
- Perennial vs. Ephemeral Streams: Perennial streams flow year-round because they are fed by base flow (groundwater). Ephemeral streams flow only in immediate response to precipitation runoff.
- Time of Concentration: The time required for runoff to travel from the most hydraulically distant point of a watershed to the outlet.