Peak Discharge Rational Method Calculator
Calculate peak discharge rational method with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Q = C x i x A / 360
The Rational Method formula Q = C x i x A / 360 estimates peak stormwater runoff from a catchment. C is the dimensionless runoff coefficient (0 to 1) reflecting how much rainfall becomes runoff based on land cover and soil type — higher values for impervious surfaces, lower for vegetated areas. i is the design rainfall intensity (mm/hr) at the time of concentration for the chosen return period. A is the catchment area (km²). The divisor 360 converts units to m³/s. The result Q gives the peak discharge used to size culverts, stormwater pipes, and detention basins.
Worked Examples
Example 1: Temperate Catchment
Problem:P=800mm/yr, C=0.3, ET=450mm/yr, dS=20mm, A=10km2
Solution:R = 800 - 240 - 450 - 20 = 90 mm/yr
Result:Recharge: 90 mm/year | Low Recharge
Example 2: Semi-Arid Region
Problem:P=350mm, C=0.15, ET=310mm, dS=5mm, A=25km2
Solution:R = 350 - 52.5 - 310 - 5 = -17.5 mm/yr
Result:Recharge: -17.5 mm/yr | Deficit
Frequently Asked Questions
What is Peak Discharge (Rational Method)?
Peak Discharge (Rational Method) is a fundamental concept in hydrology used to quantify water movement and distribution in natural and engineered systems. It provides essential data for water resource management, flood prediction, and environmental assessment. Understanding this helps engineers design infrastructure and scientists model hydrological processes. Peak Discharge Rational Method Calculator implements standard formulas from published hydrology references.
How is Peak Discharge (Rational Method) calculated?
The calculation uses Q = C x i x A / 360 relating key physical parameters. Each variable must be measured or estimated from field data or published tables. The formula assumes steady-state or quasi-steady conditions. Units must be consistent and results should be validated against field observations when available.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy