Professional Stormwater Runoff Calculator

Stormwater Calculator Pro — SCS-CN & SWMM

Professional Hydrological Calculations

Professional Stormwater
Runoff Calculator

Calculate runoff using SCS-CN and SWMM methods with custom CN support, interactive visualizations, and all equations shown.

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Select Calculation Method

Rainfall Parameters

Rainfall Depth (P) — mm

0 mm300 mm

Soil Hydrologic Group

🟢

Sandy

🟡

Loamy

🟠

Clayey

🔴

Heavy Clay

Land Use / Cover

Currently Selected

Row Crops — Straight Row

CN (Soil B)

Watershed Area

hectares

Calculation Results

Ready

Runoff Depth (Q)

—

Runoff Volume

—

m³

Runoff Coefficient

—

Q/P

Curve Number

—

Potential Retention (S)

—

Initial Abstraction

—

Water Balance Breakdown

Runoff: —

Losses: —

Runoff vs Rainfall Relationship

Step-by-Step Calculation

Click "Calculate Runoff" to see detailed steps

Scenario Comparison

Compare runoff across all soil groups

Mathematical Foundation

Equations Used in Calculations

All formulas implemented in the calculator with full variable definitions, sources, and unit specifications.

SCS-CN Method Equations

Empirical runoff estimation based on watershed characteristics

1 Direct Runoff Equation

Primary

QDirect runoff depth (mm)

PTotal rainfall depth (mm)

I_aInitial abstraction (mm)

SPotential max retention (mm)

📎 Source: USDA NRCS TR-55, Eq. 2-1

2 Simplified Runoff (Ia = 0.2S)

Standard

0.2Empirical constant (standard NRCS)

Used when detailed infiltration data is unavailable

📎 Note: I_a = 0.05S suggested for urban areas

3 CN to Potential Retention

Conversion

CNCurve Number (0 to 100)

25400 = 100 × 25.4 (unit conversion to mm)

📎 Source: NEH Part 630, Ch 10

4 Initial Abstraction

Losses

Includes interception, depression storage, and initial infiltration before runoff begins

📎 Source: USDA SCS (1972)

5 Runoff Coefficient

Ratio

CDimensionless ratio (0 to 1)

📎 Used for quick runoff estimation comparisons

6 Runoff Volume

Conversion

VTotal runoff volume (cubic meters)

AWatershed area (hectares)

Factor 10: 1 ha × 1 mm = 10,000 m² × 0.001 m = 10 m³

📎 Standard metric volume conversion

SWMM Method Equations

Per EPA SWMM User's Manual v5.1, Volume I (Hydrology)

Source: U.S. Environmental Protection Agency (2015). "Storm Water Management Model Reference Manual Volume I — Hydrology," EPA/600/R-15/162A. All equations below are presented as specified in the official documentation.

Subcatchment Runoff Process

1 Continuity Equation

Fundamental

∂S/∂tRate of storage change (m/s)

INet inflow rate (m/s)

OOutflow rate per unit width (m²/s)

📎 Source: SWMM Manual Ch.1, Sec.1.3

2 Net Inflow Rate

Balance

PPrecipitation rate (m/s)

EEvaporation rate (m/s)

fInfiltration rate (m/s)

📎 Source: SWMM Manual Ch.1, Sec.1.3

3 Manning's Overland Flow

Hydraulic

QOutflow per unit width (m²/s)

nManning's roughness coeff.

dCharacteristic water depth (m)

α_n1.0 (metric) or 1.486 (English)

📎 For wide sheet flow, R ≈ d

4 Combined Storage-Outflow

ODE

Nonlinear ODE solved numerically using Runge-Kutta method for each time step

📎 SWMM Manual Ch.1, Sec.1.3, Eq. 1-6

5 Depression Storage

Losses

d_excessAvailable for runoff/infiltration

d_depDepression storage depth (mm)

Must be filled before runoff begins. SWMM treats impervious and pervious separately.

📎 SWMM Manual Ch.1, Sec.1.4

Runoff Components

6a Impervious Runoff

Direct

No infiltration occurs on impervious surfaces in standard SWMM configuration.

📎 SWMM Manual Ch.1, Sec.1.4

6b Pervious Runoff

Infiltration

FCumulative infiltration depth (mm)

Runoff only occurs when rainfall exceeds depression storage + infiltration.

📎 SWMM Manual Ch.1, Sec.1.4

7 Total Runoff (Area-Weighted)

Average

%ImpPercent impervious area

Weighted average combines runoff from both sub-areas.

📎 SWMM Manual Ch.1, Sec.1.4

Infiltration Methods (SWMM Manual Ch.4)

8a Horton Infiltration

Exponential

f₀Max infiltration capacity (mm/hr)

f_cSteady-state capacity (mm/hr)

kDecay coefficient (1/hr)

📎 SWMM Manual Ch.4, Sec.4.1

8b Green-Ampt

Physics-based

ψWetting front suction (mm)

ΔθMoisture content change

K_sSaturated conductivity (mm/hr)

📎 SWMM Manual Ch.4, Sec.4.2

Flow Routing (SWMM Manual Ch.5)

9 Saint-Venant (Dynamic)

Full Equations

Models backwater, surcharging, and pressurized flow. Most accurate but computationally intensive.

📎 SWMM Manual Ch.5, Sec.5.1

10 Kinematic Wave

Approximation

Neglects pressure/inertia terms. Assumes S_f = S₀. Good for steep slopes, simple networks.

📎 SWMM Manual Ch.5, Sec.5.2

11 Time of Concentration (Kirpich)

Traditional

T_cTime of concentration (min)

LFlow path length (m)

Note: SWMM routing is continuous; Tc is for comparison with traditional methods.

📎 Kirpich (1940)

12 Rational Method Peak Flow

Estimation

Q_pPeak flow rate (m³/s)

IRainfall intensity (mm/hr)

360Unit conversion factor

📎 Standard hydrology comparison reference

Water Balance Equation

The fundamental hydrologic equation: All rainfall is partitioned into runoff, infiltration, interception, and surface storage.

Rainfall

Input

Runoff

Surface flow

Infiltration

Into soil

I_a

Interception

Vegetation

Storage

Puddles

References & Sources

SWMM Method References

[1]Rossman, L.A. (2015). "Storm Water Management Model User's Manual Version 5.1", EPA/600/R-15/162. U.S. EPA.
[2]U.S. EPA (2015). "Storm Water Management Model Reference Manual Volume I — Hydrology", EPA/600/R-15/162A.
[3]U.S. EPA (2015). "Storm Water Management Model Reference Manual Volume II — Hydraulics", EPA/600/R-15/162B.

SCS-CN Method References

[4]USDA Soil Conservation Service (1986). "Urban Hydrology for Small Watersheds", TR-55.
[5]USDA NRCS (2004). "National Engineering Handbook, Section 4: Hydrology", Chapter 10.
[6]Ponce & Hawkins (1996). "Runoff Curve Number: Has It Reached Maturity?" ASCE JHE, 1(1).

General Hydrology

[7]Chow, Maidment, Mays (1988). "Applied Hydrology". McGraw-Hill.
[8]McCuen (2016). "Hydrologic Analysis and Design", 4th Ed. Pearson.

Manning & Kirpich

[9]Manning (1891). "On the Flow of Water in Open Channels." TICE Ireland, 20.
[10]Kirpich (1940). "Time of Concentration." Civil Engineering, 10(6).

Note: All equations and CN values are based on the above references. Always verify results with local regulations and engineering standards.

Ahmed EsmailExplore, Learn, Succeed

Ahmed EsmailExplore, Learn, Succeed

Select Calculation Method

Rainfall Parameters

Soil Hydrologic Group

Land Use / Cover

Add Custom Land Use / CN

Watershed Area

Calculation Results

Water Balance Breakdown

Runoff vs Rainfall Relationship

Step-by-Step Calculation

Scenario Comparison

Subcatchment Parameters

Rainfall & Depression Storage

Hydraulic Parameters (Manning's n)

Infiltration (per SWMM Manual)

SWMM Simulation Results

Water Balance Breakdown

Runoff Hydrograph

Step-by-Step (per EPA SWMM v5.1 Manual)

SCS-CN Result

SWMM Result

Method Comparison Analysis

Equations Used in Calculations

SCS-CN Method Equations

1 Direct Runoff Equation

2 Simplified Runoff (Ia = 0.2S)

3 CN to Potential Retention

4 Initial Abstraction

5 Runoff Coefficient

6 Runoff Volume

SWMM Method Equations

Subcatchment Runoff Process

1 Continuity Equation

2 Net Inflow Rate

3 Manning's Overland Flow

4 Combined Storage-Outflow

5 Depression Storage

Runoff Components

6a Impervious Runoff

6b Pervious Runoff

7 Total Runoff (Area-Weighted)

Infiltration Methods (SWMM Manual Ch.4)

8a Horton Infiltration

8b Green-Ampt

Flow Routing (SWMM Manual Ch.5)

9 Saint-Venant (Dynamic)

10 Kinematic Wave

11 Time of Concentration (Kirpich)

12 Rational Method Peak Flow

Water Balance Equation

References & Sources

SWMM Method References

SCS-CN Method References

General Hydrology

Manning & Kirpich