Catchment Runoff Calculator
Our hydrology & water resources calculator computes catchment runoff accurately. Enter measurements for results with formulas and error analysis.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Q = C x i x A (Rational Method)
Peak runoff Q (m3/s) equals the runoff coefficient C times rainfall intensity i (m/s) times catchment area A (m2). Runoff depth = rainfall x C. The SCS method uses Q = (P - 0.2S)^2 / (P + 0.8S) where S = (25400/CN) - 254.
Worked Examples
Example 1: Urban Parking Lot Drainage
Problem:50 mm rainfall over 30 minutes on a 2 km2 urban catchment with C = 0.80.
Solution:Intensity = 50 / 0.5 hr = 100 mm/hr\nPeak Flow = 0.80 x (100/3.6e6) x 2e6 = 44.4 m3/s\nRunoff depth = 50 x 0.80 = 40 mm\nRunoff volume = (40/1000) x 2e6 = 80,000 m3
Result:Peak flow: 44.4 m3/s | Runoff: 40 mm | Volume: 0.080 M m3
Example 2: Rural Agricultural Watershed
Problem:75 mm storm event over 2 hours on a 50 km2 rural catchment with C = 0.30.
Solution:Intensity = 75 / 2 hr = 37.5 mm/hr\nPeak Flow = 0.30 x (37.5/3.6e6) x 50e6 = 156.3 m3/s\nRunoff depth = 75 x 0.30 = 22.5 mm\nRunoff volume = (22.5/1000) x 50e6 = 1,125,000 m3
Result:Peak flow: 156.3 m3/s | Runoff: 22.5 mm | Volume: 1.125 M m3
Frequently Asked Questions
What is the Rational Method for runoff calculation?
The Rational Method is the most widely used approach for estimating peak runoff from small catchments (typically less than 80 hectares). The formula is Q = C x i x A, where Q is peak flow rate, C is the runoff coefficient (0-1), i is rainfall intensity for the design storm duration, and A is the catchment area. It assumes that the peak flow occurs when the entire catchment is contributing runoff, which happens when the storm duration equals or exceeds the time of concentration.
How do I choose the right runoff coefficient?
The runoff coefficient (C) depends on land surface characteristics. Impervious surfaces like asphalt and rooftops have C values of 0.85-0.95, meaning nearly all rainfall becomes runoff. Lawns on flat sandy soil range from 0.05-0.10, while lawns on steep clay soil are 0.25-0.35. Urban residential areas typically average 0.30-0.50 depending on density. For mixed-use catchments, calculate a weighted average C based on the proportion of each land use type within the watershed.
What is time of concentration and how does it affect runoff?
Time of concentration (tc) is the time required for water to travel from the hydraulically most distant point in a catchment to the outlet. It determines the critical rainfall intensity used in the Rational Method: shorter tc means higher intensity rainfall and higher peak flow. Factors affecting tc include catchment slope (steeper means faster), flow path length (longer means slower), surface roughness (smoother means faster), and channel characteristics. Common estimation methods include the Kirpich, Bransby-Williams, and NRCS lag equations.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy