DrainageCalculators

Hydrology Calculators

Calculate runoff, generate hydrographs, and analyze watershed response using industry-standard methods from NRCS, USGS, and other authoritative sources.

15+ Calculators

About Hydrologic Analysis

Hydrology calculators help you estimate runoff volumes and peak discharge rates from rainfall events. These calculations are fundamental to drainage design, determining how much water your site must handle during storms and informing the sizing of all downstream conveyance and storage systems.

Use these calculators to:

  • Calculate peak discharge using the Rational Method for small watersheds
  • Estimate runoff depth and volume using SCS Curve Number method
  • Determine time of concentration for your watershed
  • Generate design hydrographs and storm hydrographs for analysis
  • Route floods through channels and detention basins
  • Perform frequency analysis for rare and extreme storm events

Runoff Estimation

Calculate peak discharge and runoff volumes for design storms

Rational Method

Calculate peak discharge using Q = CiA for small watersheds (< 200 acres)

Peak Flow Small Watershed

SCS Curve Number

Calculate runoff depth using the NRCS curve number method

Runoff Volume NRCS

Runoff Coefficient Lookup

Look up runoff coefficients (C values) for the Rational Method

Reference C Values

Modified Rational Method

Find critical storm duration for detention storage design

Detention Critical Duration

Time of Concentration

Calculate travel time for runoff to reach the outlet

Time of Concentration

Calculate Tc using Kirpich, FAA, NRCS, and other methods

Tc Travel Time

Hydrograph Analysis

Generate and analyze storm hydrographs for detention design

SCS Unit Hydrograph

Generate storm hydrographs using the SCS dimensionless unit hydrograph

Unit Hydrograph NRCS

SCS Triangular Hydrograph

Generate simplified triangular hydrographs for quick estimates

Triangular Simplified

Hydrograph Convolution

Apply unit hydrograph convolution to rainfall excess

Convolution Advanced

Rainfall Distribution

Generate SCS Type I, IA, II, and III 24-hour rainfall distributions

SCS Types Design Storm

Flood Routing

Route hydrographs through channels and reservoirs

Level Pool Routing

Route hydrographs through detention basins using storage-indication method

Detention Storage Routing

Muskingum Routing

Route floods through channel reaches using Muskingum method

Channel Routing Flood

Statistical Hydrology

Frequency analysis and regional estimation methods

Flood Frequency Analysis

Analyze peak flows using Log-Pearson III and Gumbel distributions per Bulletin 17C

Statistics Bulletin 17C

USGS Regional Regression

Estimate peak flows at ungauged sites using regional regression equations

Ungauged Regional

Learning Resources

Understand the theory behind the calculations

Understanding the Rational Method

When to use Q = CiA and its limitations

Learn more: Understanding the Rational Method

SCS Curve Number Method Explained

How to select curve numbers for different land uses

Learn more: SCS Curve Number Method Explained

Reading Hydrographs

Creating design hydrographs for detention analysis

Learn more: Reading Hydrographs

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