Stormwater Runoff Calculator

Key stormwater runoff statistics

1. Five of the six manuals in this comparison publish explicit flat-lawn Rational Method categories spanning 0.05 to 0.17—a 3.4× difference.The 0.05 low appears for very sandy or sandy flat lawns; the 0.17 high appears for heavy-soil flat lawns and ODOT's soil-unspecified flat lawn. Compiled July 25, 2026 from FHWA, TxDOT, ODOT, WSDOT, and NCTCOG primary tables.
2. The runoff-coefficient table in the current federal urban drainage manual is credited to a 1960 source. FHWA's Urban Drainage Design, HEC-22, 4th edition, published February 2024, titles Table 4.1 “Runoff coefficients for the Rational Method (ASCE 1960)” in report FHWA-HIF-24-006.
3. One inch of rain over one square foot equals exactly 48/77 of a U.S. gallon, which rounds to 0.6233766 gallon. NIST Handbook 44 (2026) defines one square foot as 144 square inches and one U.S. gallon as 231 cubic inches; 144 ÷ 231 = 48/77 = 0.623376623376….
4. One inch of rain over 1,000 square feet rounds to 623.38 gross U.S. gallons before runoff losses. Derived July 25, 2026 from the exact NIST area and liquid-volume conversions.
5. One inch of rain over one acre rounds to 27,154.29 gross U.S. gallons. Derived July 25, 2026 from 43,560 square feet × 144 cubic inches per square-foot-inch ÷ 231 cubic inches per gallon.
6. ODOT, WSDOT, NCTCOG iSWM, and the City of Huntsville all publish storm-frequency adjustments above their base 10-year-or-less or 10-year coefficient guidance. Their 25-, 50-, and 100-year adjustments are 1.10, 1.20, and 1.25, or equivalent 10%, 20%, and 25% increases. Verified July 25, 2026.
7. A 1.25 frequency factor makes the adjusted 100-year peak-flow result 25% higher than the unadjusted result; omitting it leaves the unadjusted result 20% below the adjusted result.This is the exact arithmetic implied by ODOT, NCTCOG iSWM, and Huntsville's published 100-year factor. Derived July 25, 2026.
8. The exact U.S.-customary Rational Method conversion factor is 121/120, which rounds to 1.0083333. One acre-inch per hour equals exactly 121/120 cubic feet per second, so Q = CiA is the conventional rounded form of Q = (121/120) × C × i × A. Derived July 25, 2026 from NIST units.
9. Washington State DOT caps an adjusted Rational Method runoff coefficient at 0.95 unless a Region Hydraulics Engineer approves a higher value. WSDOT Hydraulics Manual M 23-03.12, Chapter 2, April 2026.
10. Gravel has the widest numeric spread among hard-surface categories in this six-manual comparison: 0.50 to 0.85. WSDOT and NCTCOG iSWM publish 0.50 at the low end; ODOT publishes 0.85. Compiled July 25, 2026.
11. TxDOT limits the Rational Method to small drainage areas of up to about 200 acres with no significant flood storage and requires a minimum 10-minute duration for rainfall-intensity computation. TxDOT Hydraulic Design Manual, Chapter 4, Section 12, verified July 25, 2026.
12. NOAA's dated schedule lists preliminary NOAA Atlas 15 estimates for the contiguous United States in September 2026 and published estimates available for use and application in 2027. NOAA states that Atlas 15 Volume 1 will supersede Atlas 14 when published; Atlas 14 remains the authoritative standard today. Verified July 25, 2026.
13. The City of Huntsville manual contains two internal rainfall-depth conflicts: 3.78 versus 3.98 inches for the 2-year, 24-hour value and 6.21 versus 6.69 inches for the 25-year, 24-hour value.Both values are preserved in Dataset 1.0.0; the page uses Figure 4-1 because it is the manual's dedicated precipitation-frequency table. Verified July 25, 2026.

Stormwater runoff calculator

This calculator returns two different answers that are not interchangeable. Runoff volume is total water produced, measured in gallons or cubic feet. Peak flow is the fastest estimated arrival rate during the critical interval. Default mode is event runoff volume; switch tabs for peak flow or mixed surfaces.

How is runoff volume calculated?

Runoff volume begins with rainfall depth multiplied by contributing area. The exact U.S. gallon conversion is then applied, followed by a separately documented effective event runoff fraction. The first result is gross rainfall; the second is an estimate of the share that leaves the drainage area as runoff.

The exact conversion

NIST Handbook 44 (2026) gives the exact unit relationships used here:

• 1 square foot = 144 square inches
• 1 U.S. gallon = 231 cubic inches
• 1 acre = 43,560 square feet

144 ÷ 231 = 48/77 = 0.623376623376… U.S. gallon per square foot per inch of rain
43,560 × (48/77) = 2,090,880/77 = 27,154.285714… U.S. gallons per acre-inch

The two-step formula

Gross rainfall volume (gal) = depth (in) × area (ft²) × (48/77)
Estimated runoff volume (gal) = gross rainfall volume × effective event runoff fraction

For calculator arithmetic, 48/77 can be represented as 0.6233766233766234 in standard double precision. The effective event runoff fraction ranges from 0 to 1 and must be named, sourced, or stated as an assumption.

Why gross volume is not runoff

The widely repeated 623.38-gallon figure for one inch of rain over 1,000 square feet is a gross rainfall volume. It becomes an estimated runoff volume only after an explicit event-volume fraction is applied. This page publishes gross rainfall and estimated runoff as separate outputs and does not silently substitute a Rational Method peak-flow coefficient for an event-volume fraction.

How is peak stormwater flow calculated?

The Rational Method estimates peak discharge from a small drainage area as a function of runoff coefficient, rainfall intensity, and contributing area. U.S. agency manuals conventionally write the English-unit formula as Q = CiA; the exact dimensional factor is 121/120, or 1.0083333 when rounded to seven decimals.

Q (cfs) = (121/120) × C × i (in/hr) × A (acres)

Intensity is not depth

A 24-hour rainfall depth of 5.63 inches is not an intensity of 5.63 inches per hour. Its 24-hour average is 5.63 ÷ 24 = 0.235 inch per hour, while a separately published 15-minute design depth for the same average recurrence interval can yield a much higher 15-minute average intensity. The Rational Method uses mean rainfall intensity for a duration tied to time of concentration under the applicable manual.

Time of concentration

Time of concentration is the travel time from the hydraulically most distant point in the drainage area to the analysis point. It determines the duration used to select design intensity. TxDOT requires a minimum 10-minute duration for intensity computation. The City of Huntsville limits its Rational Method guidance to 5 minutes ≤ tc ≤ 30 minutes, drainage areas of 50 acres or less, and peak-flow estimates only.

When the Rational Method is not enough

TxDOT limits the method to about 200 acres with no significant flood storage. Huntsville uses a stricter 50-acre limit and requires comparison with other methods and approval by the Director of City Engineering for formal use. Systems with detention or retention, routed networks, floodplain storage, backwater, tidal influence, hydraulic restrictions, or regulatory design requirements need a fuller hydrologic and hydraulic analysis.

Why do runoff coefficients disagree between manuals?

The manuals do not use one universal category system. They divide soil, slope, density, and land use differently, and their source trails are uneven: FHWA's 2024 table credits ASCE 1960, Huntsville cites DeKalb County, Georgia (1976) for its coefficient table, and several other manuals do not name an originating table source.

We aligned only categories close enough to compare, retained the source wording in the downloadable records, and report the result as a verified six-manual sample—not a field experiment or national average.

Verification tiers: ★read directly against the issuing agency's primary publication. ◆ calculated from the displayed ★ values using the stated rule.

Source: FHWA HEC-22, 4th edition, Table 4.1; TxDOT Hydraulic Design Manual, Table 4-10; ODOT Hydraulics Manual Appendix F, Table 1; WSDOT Hydraulics Manual M 23-03.12, Table 2-2; NCTCOG iSWM Technical Manual—Hydrology, Table 1.6; and City of Huntsville Stormwater Management Manual 2020, Table 4-2. All six primary publications were checked July 25, 2026.

¹ ODOT publishes one lawn value for each slope class and does not split lawn values by soil.
² ODOT's displayed residential span combines light, normal, and dense residential categories across its three slope classes.
³ NCTCOG's displayed single-family span combines its 1/8-acre through 1-acre-or-larger lot categories.
⁴ Huntsville's displayed single-family span combines lot size, soil, and slope categories from its own table.
A dash means the manual does not publish a close category in the table reviewed. The spread column is this study's minimum-to-maximum comparison, not an agency-published national range. FHWA, TxDOT, and NCTCOG use lawn slope classes around 2% and 7%; ODOT and WSDOT use under 2%, 2–10%, and over 10%, so the slope labels are not identical across every column.

What does the spread do to an answer?

Take a 5,000-square-foot drainage area—0.114784 acres—and an illustrative rainfall intensity of 5.00 inches per hour.

At C = 0.05:
Q = (121/120) × 0.05 × 5.00 × 0.114784 = 0.02894 cfs = 12.99 U.S. gallons per minute

At C = 0.17:
Q = (121/120) × 0.17 × 5.00 × 0.114784 = 0.09838 cfs = 44.16 U.S. gallons per minute

Same area and intensity. Different published flat-lawn category selections. The peak-flow answer is 3.4× apart because the selected coefficient is 3.4× apart. On the volume side, one inch of rain over 5,000 square feet is 3,116.88 gross gallons. A defensible runoff-volume estimate requires a separately documented event-volume fraction.

Source: ODOT Hydraulics Manual Appendix F, Table 2; WSDOT Hydraulics Manual M 23-03.12, Section 2-6.2; NCTCOG iSWM Technical Manual—Hydrology, Table 1.4; and City of Huntsville Stormwater Management Manual 2020, Table 4-3. Verified July 25, 2026.

WSDOT caps the adjusted coefficient at 0.95 unless a Region Hydraulics Engineer approves a higher value. NCTCOG states that C_f × C must not exceed 1.0, and Huntsville states that its adjusted coefficient must not exceed 1.0. A 1.25 factor raises the unadjusted result by 25%. When the adjusted result is treated as the comparison baseline, omitting the factor leaves the unadjusted result 20% low.

Source: table captions, source notes, and adjacent text in the six primary publications, read July 25, 2026.

The same 1.00 / 1.10 / 1.20 / 1.25 factor series appears in ODOT, NCTCOG iSWM, and Huntsville; WSDOT states the equivalent percentage increases. Matching values alone do not prove that one current manual copied another, so the dataset records only the provenance each publication actually prints.

How much water does one inch of rain produce?

One inch of rain over one square foot equals exactly 48/77 of a U.S. gallon, which rounds to 0.6233766 gallon. Over 1,000 square feet the gross volume rounds to 623.38 gallons, and over one acre it rounds to 27,154.29 gallons. Every value in Table D is gross rainfall volume before an event runoff fraction is applied.

Source: derived July 25, 2026 from NIST Handbook 44 (2026), Appendix C: 1 square foot = 144 square inches, 1 acre = 43,560 square feet, and 1 U.S. gallon = 231 cubic inches. Values are gross rainfall volume with no runoff fraction applied; displayed decimals are rounded from the exact fractions.

Why can a gross-rainfall number differ from a runoff or capture number?

A lower figure can be valid when it is explicitly labeled as estimated runoff or captured water and states the factor applied. It is not the same quantity as gross rainfall volume. This page publishes the gross conversion and the event-volume factor separately so a reader can reproduce the result and see whether losses were applied.

Where does design rainfall data come from?

NOAA Atlas 14 is the authoritative U.S. precipitation-frequency standard as of July 25, 2026, and its point estimates are delivered through the National Weather Service Precipitation Frequency Data Server. The server provides values for a selected location, duration, frequency framework, units, and series type, with confidence bounds and downloadable data. NOAA Atlas 15 is under development and will supersede Atlas 14 when published.

How to record a reproducible NOAA point estimate

1. Open the NOAA Precipitation Frequency Data Server.
2. Select the location by map or coordinates.
3. Record whether the result is depth or intensity and record the units.
4. Record whether the table is partial duration series (PDS) or annual maximum series (AMS).
5. Record the duration and the frequency label exactly as NOAA displays them.
6. Save the displayed table or available data file.
7. Record the coordinate, series, data type, units, source volume, and retrieval date with the value.

Those fields prevent a depth from being reused as an intensity, an area estimate from being presented as a point estimate, or a PDS average recurrence interval from being mislabeled as the reciprocal of annual exceedance probability.

Source: NOAA Office of Water Prediction, NOAA Atlas 15 informational page, retrieved July 25, 2026. The same NOAA page contains conflicting CONUS publication-year wording: its dated timeline says 2027, while a later narrative answer says 2026. Both are disclosed here.

What does Atlas 15 change for a calculation run today?

Atlas 14 remains the authoritative standard today. Atlas 15 preliminary data are not a replacement for the current standard, and the Montana pilot was released for comparison and feedback rather than final use. What changes now is the required source record: every precipitation input should carry its source, location, series, duration, units, and retrieval date so the calculation can be rechecked when Atlas 15 is published.

What are the rainfall and coefficient figures for Huntsville, Alabama?

The City of Huntsville publishes a 2020 Stormwater Management Manual and links it from the City's official Storm Water Management Program page. Ordinance No. 20-1062 states that the 2020 Editionis referenced and adopted in Section 12-1 and incorporated into Article VI by reference. The figures below were checked directly on July 25, 2026 and are Huntsville point/reference values—not uniform values for every coordinate in the city or Madison County—so project work should use the current NOAA point estimate and the applicable reviewing authority's requirements.

Source: City of Huntsville Stormwater Management Manual 2020, Figure 4-1, PDF p. 118, reproducing PDS-based point precipitation-frequency estimates with 90% confidence intervals. Checked July 25, 2026.

The source table is labeled by average recurrence interval, not annual exceedance probability. NOAA Atlas 14 states that the inverse of annual exceedance probability is not PDS average recurrence interval, so this page does not add a 100 ÷ ARI probability column.

Source: derived July 25, 2026 from the central depth values in City of Huntsville Figure 4-1. Average intensity equals precipitation depth divided by duration in hours; these values are averages across the stated duration, not instantaneous observed rain rates. Worked check: the 10-year-ARI 15-minute depth of 1.22 inches ÷ 0.25 hour = 4.88 inches per hour.

Source: City of Huntsville Stormwater Management Manual 2020, Table 4-2, PDF p. 105. Checked July 25, 2026. The table cites DeKalb County, Georgia (1976), assumes good ground cover and conservation treatment for pasture/grass/farmland, and states that pervious-pavement values depend on the underlying strata.

Source: City of Huntsville Stormwater Management Manual 2020, Equation 4-5 and Table 4-3, PDF pp. 74–75 and 106. Checked July 25, 2026. The table cites Wright-McLaughlin Engineers (1969); the adjusted coefficient must not exceed 1.0.

What are Huntsville's local Rational Method limits?

The City manual limits its Rational Method guidance to:

1. Peak design-flow estimates only.
2. 5 minutes ≤ time of concentration ≤ 30 minutes.
3. Drainage area of 50 acres or less.

The manual also directs users to compare the result with other methods and requires approval by the Director of City Engineering for formal use.

Where does Huntsville's manual conflict with itself?

Two worked examples state rainfall depths that do not match Figure 4-1, the manual's dedicated precipitation-frequency table. The 2-year, 24-hour value appears as 3.78 inches in a worked example and 3.98 inches in Figure 4-1; the 25-year, 24-hour value appears as 6.21 and 6.69 inches. Both values remain visible, while the calculator uses Figure 4-1 for its Huntsville presets and directs project work to recheck the current NOAA point estimate.

Source: City of Huntsville Stormwater Management Manual 2020, PDF p. 82 versus p. 118 for the 2-year value, and PDF p. 77 versus p. 118 for the 25-year value. Checked July 25, 2026.

The 0.48-inch gap is 7.2% of Figure 4-1's 6.69-inch 25-year value. Any calculation that uses precipitation depth linearly carries the same proportional difference before other inputs are applied.

What does this data show—and what does it not show?

This study shows what six official manuals publish as Rational Method design guidance and how far their mapped categories spread. It does not show a field-measured national distribution of runoff behavior; no new rain-gauge, flume, soil-infiltration, or discharge measurements were collected. The sample establishes disagreement inside published guidance, not a census of every U.S. agency or physical equivalence among unlike soil, slope, density, and land-use categories.

How was this dataset assembled?

Every consequential value displayed on this page was reopened and checked against an issuing agency or original data producer during the July 25, 2026 audit. Source categories were preserved, ranges were not averaged, and derived calculations were regenerated programmatically. The downloadable dataset contains 567 unique records with no pending verification tier.

What we collected. Rational Method coefficient records from FHWA, TxDOT, ODOT, WSDOT, NCTCOG iSWM, and the City of Huntsville; published frequency factors; source-provenance notes; exact NIST unit conversions; NOAA Atlas 14 and Atlas 15 status; 190 Huntsville PDS-based central precipitation estimates; 50 derived short-duration intensities; 15 reproducible gross-rainfall scenarios; the 2020 Huntsville manual-adoption record; and the two Huntsville source conflicts.

Where from, and when.Primary publications were read on July 25, 2026: NIST Handbook 44 Appendix C; FHWA HEC-22 4th edition; TxDOT Hydraulic Design Manual Section 12; ODOT Hydraulics Manual Appendix F; WSDOT Hydraulics Manual Chapter 2; NCTCOG iSWM Technical Manual—Hydrology; NOAA Atlas 14 Volume 9 and PFDS materials; NOAA's Atlas 15 page; the City of Huntsville Stormwater Management Manual 2020; the City's official program page; and City Ordinance No. 20-1062.

How categories were mapped. Manuals use incompatible surface taxonomies. The page maps only categories close enough for a useful comparison, prints a dash when no close category exists, retains soil and slope qualifiers, and records every source category in the machine-readable file.

How derived values were calculated. Gallon conversions use the exact NIST-derived fraction 48/77, and the exact Rational Method factor is 121/120; their displayed decimals are rounded representations. Average intensities are source depth divided by duration in hours. Gross-rainfall scenarios use only precipitation depth, area, and 48/77, with no runoff or Rational Method coefficient applied.

How PDS frequency labels were handled. The Huntsville source table is PDS-based and labeled by average recurrence interval. NOAA states that inverse AEP is not PDS ARI, so the dataset leaves annual_exceedance_probability_percent blank for PDS depth and derived-intensity records instead of filling it by reciprocal.

How conflicts were handled. When primary sources disagree internally, both values are retained. The page states which value drives the calculator and why.

Quality control. The final export contains 567 unique record IDs. Numeric coefficient records were checked to remain between 0 and 1; confidence bounds were checked to bracket their central estimates; PDS records were checked for blank reciprocal-AEP fields; CSV and JSON record counts were compared; key calculations were rerun; and SHA-256 hashes are recorded in the methodology file.

Verification tiers

• ★ Direct: read directly against the issuing agency or original data producer.
• ◆ Derived: reproducibly calculated from one or more cited ★ records using the formula stored with the record.

No pending, excerpt-only, or second-read-needed records appear in Dataset 1.0.0.

What are the limitations of this calculator and dataset?

The limitations below define what the calculations and comparison can support. They are part of the result, not a detachable disclaimer.

1. This is an educational and planning calculator. It is not sealed engineering design and should not be submitted as one.
2. Runoff volume and peak flow are different quantities with different inputs. Neither substitutes for the other.
3. The coefficients in Table A are published design guidance, not field measurements made by this study.
4. Six manuals are a sample. The displayed spread is verified inside that sample; a national distribution is not established.
5. Cross-manual categories are not always identical. Soil, slope, density, and source category remain material.
6. The Huntsville precipitation values are point/reference estimates, not constants for every coordinate in Huntsville or Madison County.
7. PDS average recurrence interval and annual exceedance probability are not reciprocals. The Huntsville table is PDS-based.
8. A 90% confidence interval describes uncertainty around the precipitation-frequency estimate; it is not a prediction interval for one future storm.
9. Huntsville's coefficients are jurisdiction-specific and should not be generalized nationally.
10. The Rational Method does not model storage, routing, hydrograph shape, backwater, inlet capacity, outlet control, pipe capacity, or downstream constraints.
11. This page does not size any drainage component.
12. Dataset 1.0.0 carries all 190 Huntsville central estimates but publishes 90% confidence-bound fields for the 24-hour row only.
13. NOAA Atlas 15 is under development and may change the applicable precipitation-frequency estimates after publication.
14. A calculated value is an estimate or design result, not a measurement of runoff at the site.

What do these stormwater terms mean?

These definitions follow the usage in the primary manuals and NOAA materials cited below. The distinctions between depth and intensity, volume and peak flow, and PDS ARI and AEP are essential to reproducing a result.

Stormwater runoff

Precipitation that flows across a surface toward a drainage point instead of infiltrating, evaporating, being intercepted, or remaining in surface storage.

Contributing area

The horizontal land or surface area that drains to the analysis point.

Precipitation depth

Accumulated precipitation over a stated duration, expressed here in inches or millimeters.

Rainfall intensity

Precipitation depth divided by duration, expressed here in inches per hour. It is not interchangeable with total depth.

Rational Method runoff coefficient (C)

A dimensionless factor that relates peak runoff rate to rainfall intensity in the Rational Method. The selected value depends on the governing manual and its land-use, soil, slope, density, and frequency categories.

Effective event runoff fraction

The share of gross event rainfall used to estimate total runoff volume. It is related to site losses and connectedness but is not automatically the same as Rational Method C.

Peak discharge

The maximum estimated flow rate at an analysis point during an event, usually expressed in cubic feet per second.

Time of concentration

Travel time from the hydraulically most distant point in the drainage area to the analysis point.

Average recurrence interval (ARI)

The average time between exceedances in a partial duration series. NOAA states that inverse AEP is not PDS ARI.

Annual exceedance probability (AEP)

The probability that a given magnitude is exceeded at least once in any year. Its inverse measures the average time between years with at least one exceedance, not PDS average time between events.

Partial duration series (PDS)

A frequency-analysis series that includes independent events above a threshold rather than only the single largest event in each year.

Annual maximum series (AMS)

A series containing the largest value observed in each year for the selected duration.

Confidence interval

The published range around a precipitation-frequency estimate at a stated confidence level. It does not mean that one future storm has a 90% chance of falling inside the interval.

Impervious area

A surface that prevents or strongly retards infiltration. ODOT's Appendix F describes surfaces with coefficients above 0.80 as impervious under its table.

Connected impervious area

Impervious area with a continuous drainage path to the analysis point or drainage system.

What is included in the dataset download?

Dataset 1.0.0 contains 567 records and is published in CSV and JSON, with a separate methodology and checksum file. The files include the six-manual coefficient records, NIST conversions, published frequency factors, all 190 Huntsville central precipitation estimates, derived intensities, gross-rainfall scenarios, provenance, manual adoption, and conflict records.

CSV columns: dataset_version, verification_date, record_type, record_id, geography, jurisdiction, parameter, duration, duration_hours, recurrence_interval_years, annual_exceedance_probability_percent, value, value_min, value_max, lower_90_ci, upper_90_ci, unit, input_area_sq_ft, input_area_acres, input_precipitation_in, input_runoff_coefficient, formula_or_method, source_title, source_page, source_url, verification_tier, publish_status, notes.

How can this page be cited?

The information below is a neutral attribution reference. It does not request or condition reuse, linking, or citation.

What else do readers ask about stormwater runoff?

These answers restate the page's most reused calculations and distinctions in standalone form. Each answer uses the same formulas and source data as the calculator and tables above.

Which primary sources support this page?

Every external numerical or regulatory claim on this page traces to an issuing agency or original data producer. The URLs below were opened during the July 25, 2026 verification pass.

1. National Institute of Standards and Technology. Handbook 44—2026, Appendix C: General Tables of Units of Measurement. https://www.nist.gov/document/2026-nist-handbook-44-appendix-c — read July 25, 2026.
2. Federal Highway Administration. Urban Drainage Design, Hydraulic Engineering Circular No. 22, 4th edition. FHWA-HIF-24-006, February 2024. https://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif24006.pdf — read July 25, 2026.
3. Texas Department of Transportation. Hydraulic Design Manual, Chapter 4, Section 12: Rational Method. https://www.txdot.gov/manuals/des/hyd/chapter-4--hydrology/section-12--rational-method.html — read July 25, 2026.
4. Oregon Department of Transportation. ODOT Hydraulics Manual, Hydrology Appendix F: Rational Method, April 2014. https://www.oregon.gov/odot/hydraulics/Docs_Hydraulics_Manual/Hydraulics-07-F.pdf — read July 25, 2026.
5. Washington State Department of Transportation. Hydraulics Manual M 23-03.12, Chapter 2: Hydrology, April 2026. https://wsdot.wa.gov/publications/manuals/fulltext/m23-03/chapter2.pdf — read July 25, 2026.
6. North Central Texas Council of Governments. iSWM Technical Manual—Hydrology, April 2010, revised September 2014. https://iswm.nctcog.org/Documents/technical_manual/Hydrology_4-2020.pdf — read July 25, 2026.
7. NOAA National Weather Service, Hydrometeorological Design Studies Center. NOAA Atlas 14 Volume 9, Version 2.0. https://www.weather.gov/media/owp/hdsc_documents/Atlas14_Volume9.pdf — read July 25, 2026.
8. NOAA National Weather Service, Hydrometeorological Design Studies Center. Precipitation Frequency Data Server. https://hdsc.nws.noaa.gov/pfds/ — read July 25, 2026.
9. NOAA National Weather Service, Office of Water Prediction. NOAA Atlas 15 Informational Page. https://water.noaa.gov/about/atlas15 — read July 25, 2026.
10. U.S. Environmental Protection Agency. National Stormwater Calculator. https://www.epa.gov/water-research/national-stormwater-calculator — read July 25, 2026.
11. City of Huntsville, Alabama. Stormwater Management Manual 2020. https://www.huntsvilleal.gov/development/building-construction/permits-standards-a-to-z/stormwater-management-manual-2020/ — read July 25, 2026.
12. City of Huntsville, Alabama. Storm Water Management Program. https://www.huntsvilleal.gov/environment/water/storm-water/storm-water-management-program/ — read July 25, 2026.
13. City of Huntsville, Alabama. Ordinance No. 20-1062. https://mcclibraryfunctions.azurewebsites.us/api/ordinanceDownload/12962/1058143/pdf?forceDownload=true — read July 25, 2026.

What changed in this version?

Version 1.0.0 is the first production publication of the calculator page and joined dataset. The final audit removed unverified tiers, replaced excerpt-only jurisdictions with five fully checked national or regional manuals plus Huntsville, corrected the 100-year frequency-factor arithmetic, corrected the City manual adoption claim, separated PDS average recurrence interval from annual exceedance probability, and regenerated all download files.

Version 1.0.0 — July 25, 2026. Six-manual coefficient comparison; exact NIST unit derivations; four-manual frequency-factor comparison; NOAA Atlas 14 and Atlas 15 status; Huntsville 190-value reference layer; City manual adoption record; Huntsville conflict log; 567-record CSV and JSON exports.