Kirpich Equation Calculator
Calculate time of concentration for watersheds using the Kirpich equation (1940)
Watershed Parameters
Calculation Results
Select parameters and calculate to see results.
Kirpich Equation
Tc = 0.0078 × L0.77 × S-0.385
Where:
Tc = Time of concentration (minutes)
L = Length of longest flow path (ft)
S = Slope (H/L, ft/ft)
📊 Kirpich Equation Applications
| Application | Typical L Range | Typical Tc Range | Best For |
|---|---|---|---|
| Small agricultural watersheds | 100-2,000 ft | 5-60 min | Field drainage, small catchments |
| Urban drainage | 500-5,000 ft | 10-120 min | Stormwater systems, small urban basins |
| Forest watersheds | 1,000-10,000 ft | 30-300 min | Mountainous or forested areas |
| Road drainage | 50-1,000 ft | 2-30 min | Highway and road drainage design |
Note: Kirpich equation is most accurate for small watersheds (up to 200 acres) with well-defined channels.
📚 Kirpich Equation Methodology
Original Kirpich (1940)
Developed from data on seven rural watersheds in Tennessee (3-200 acres). Tc = 0.0078 × L0.77 × S-0.385
Modified Kirpich (SCS)
Adjusted coefficients for different surface types. Tc = C × (L0.77 × S-0.385) where C varies by surface.
Limitations
Not suitable for very flat (S < 0.002) or steep (S > 0.1) slopes. Overestimates Tc for urban areas.
Best Practices
Measure L along the longest flow path. Use average slope for non-uniform terrain. Compare with other methods when possible.
Modified Kirpich Coefficients (C)
| Surface Type | Coefficient (C) | Application |
|---|---|---|
| Forest with heavy litter | 0.0078 | Original Kirpich value |
| Grass/agricultural | 0.0052 | Pastures, fields |
| Paved/concrete | 0.0028 | Urban areas, roads |
| Bare soil | 0.0043 | Construction sites |
Note: The Kirpich equation provides an estimate of time of concentration for small watersheds. Actual field conditions may vary. For critical applications, verify with multiple methods or field measurements.