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Runoff Coefficient Calculator

Calculates the proportion of total precipitation that becomes runoff.

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Runoff Coefficient

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Overview

The Runoff Coefficient (C) is a dimensionless ratio used in hydrology to describe the fraction of precipitation that contributes to surface runoff. It is calculated by dividing the total runoff (Q) by the total precipitation (P) over a specific period and area. This coefficient is crucial for understanding the hydrological response of a drainage basin, indicating how much water flows over the surface rather than infiltrating the ground or evaporating. Its value varies significantly depending on land cover, soil type, slope, and rainfall intensity.

Symbols

Variables

Q = Runoff, P = Precipitation, C = Runoff Coefficient

Runoff
mm
Precipitation
mm
Runoff Coefficient
dimensionless

Apply it well

When To Use

When to use: Apply this formula to quantify the water balance within a drainage basin, especially when assessing flood risk, designing stormwater management systems, or evaluating the impact of land-use changes on water resources. It's used when you have measurements for both total runoff and total precipitation.

Why it matters: Understanding the runoff coefficient is vital for sustainable water resource management, urban planning, and environmental protection. It helps engineers design effective drainage systems, informs agricultural practices to prevent soil erosion, and aids in predicting flood magnitudes, thereby protecting communities and infrastructure.

Avoid these traps

Common Mistakes

  • Using inconsistent units for Q and P.
  • Confusing runoff with infiltration or evapotranspiration.
  • Applying a coefficient derived from one area to a vastly different area without adjustment.

One free problem

Practice Problem

Over a specific month, a drainage basin received 120 mm of precipitation. During the same period, the total runoff measured from the basin was 48 mm. Calculate the runoff coefficient for this basin.

Runoff48 mm
Precipitation120 mm

Solve for:

Hint: Ensure both runoff and precipitation are in the same units before dividing.

The full worked solution stays in the interactive walkthrough.

References

Sources

  1. Hydrology and Floodplain Analysis (Bedient, Huber, and Sartor)
  2. Principles of Hydrology (Ward and Robinson)
  3. Wikipedia: Runoff coefficient
  4. Britannica: Runoff
  5. Britannica: Runoff coefficient
  6. Chow, V. T., Maidment, D. R., & Mays, L. W. (1988). Applied Hydrology. McGraw-Hill.
  7. Bedient, P. B., Huber, W. C., & Sartor, J. E. (2019). Hydrology and Floodplain Analysis (6th ed.). Pearson.
  8. Chorley, R.J., Schumm, S.A., Sugden, D.E. (1984) Geomorphology. Methuen & Co. Ltd.; AQA A-level Geography - Water and Carbon Cycles