# Stormwater Runoff Reference Dataset 2026 — Methodology

**Publisher:** Huntsville Yard Drainage Research  
**Dataset version:** 1.0.0  
**Last verified:** July 25, 2026  
**Records:** 567  
**CSV SHA-256:** `44823cba62ecf0af0a5684ca1c59dd7f7653be5c51eb3dd94d041f334afa5cfa`  
**JSON SHA-256:** `75ae5f9b78f023e4864498461b6e23d7ba48ee96a13b840ec9184dbe77c59417`

## What the dataset contains

The dataset contains 567 records:

| Record type | Records |
|---|---:|
| Rational Method runoff-coefficient records | 284 |
| Huntsville PDS-based precipitation-frequency central estimates | 190 |
| Derived short-duration average intensities | 50 |
| Runoff-coefficient frequency factors | 16 |
| Coefficient-free gross-rainfall scenarios | 15 |
| Source-provenance records | 6 |
| Exact unit conversions | 3 |
| Primary-source reconciliation records | 2 |
| City manual adoption record | 1 |
| **Total** | **567** |

## Verification tiers

- **★** — read directly against the issuing agency or original data producer.
- **◆** — reproducibly calculated from one or more cited ★ records using the formula stored with the record.

No pending or partially verified records are published in version 1.0.0.

## Source collection

On July 25, 2026, the source values used on the page were checked in these primary publications:

1. NIST Handbook 44 (2026), Appendix C.
2. Federal Highway Administration, *Urban Drainage Design*, HEC-22, 4th edition, FHWA-HIF-24-006, February 2024.
3. Texas Department of Transportation, *Hydraulic Design Manual*, Chapter 4, Section 12.
4. Oregon Department of Transportation, *Hydraulics Manual*, Hydrology Appendix F, April 2014.
5. Washington State Department of Transportation, *Hydraulics Manual M 23-03.12*, Chapter 2, April 2026.
6. North Central Texas Council of Governments, *iSWM Technical Manual—Hydrology*, April 2010, revised September 2014.
7. NOAA Atlas 14 Volume 9 and the NOAA Precipitation Frequency Data Server.
8. NOAA Atlas 15 informational materials.
9. City of Huntsville, *Stormwater Management Manual 2020*.
10. City of Huntsville Ordinance No. 20-1062.

## Cross-manual category mapping

The source category is preserved in the `parameter` field. The page-level comparison maps only categories close enough to compare and prints a dash when a manual does not publish a close category.

The mapping does not assert that unlike soil, slope, density, or land-use categories are physically identical. Five of the six manuals publish explicit flat-lawn categories; their values span 0.05 to 0.17 across the categories actually printed.

Ranges remain ranges. Single values remain single values. No midpoint or national average was manufactured.

## Core calculations

### Gross rainfall volume

`Gross gallons = precipitation depth (in) × area (ft²) × (48/77)`

The exact conversion follows from NIST's definitions:

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

Thus one square-foot-inch equals exactly `48/77` U.S. gallon. Decimal fields use finite representations of that repeating value.

### Estimated event runoff volume

`Estimated runoff gallons = gross gallons × effective event runoff fraction`

An effective event runoff fraction is not automatically interchangeable with a Rational Method peak-flow coefficient.

### Rational Method peak flow

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

The exact U.S.-customary factor is `121/120`; 1.0083333333333333 is a finite decimal representation. U.S. manuals conventionally use a conversion factor of 1.0.

### Composite coefficient

`C_weighted = Σ(Cᵢ × Aᵢ) ÷ ΣAᵢ`

### Derived average intensity

`Average intensity (in/hr) = precipitation depth (in) ÷ duration (hours)`

The result is an average over the stated duration, not an instantaneous observed rain rate.

### Gross-rainfall scenarios

The 15 scenario records use only source precipitation depth, source area, and the exact `48/77` conversion. They do not apply a Rational Method coefficient, collection factor, or event runoff fraction.

## PDS average recurrence interval versus AEP

The City of Huntsville Figure 4-1 table is explicitly PDS-based and labels its columns by average recurrence interval. NOAA Atlas 14 states that the inverse of annual exceedance probability is **not** PDS average recurrence interval. Version 1.0.0 therefore leaves the `annual_exceedance_probability_percent` field blank for PDS-based precipitation and derived-intensity records instead of filling it with `100 ÷ ARI`.

## Huntsville source reconciliation

Two worked examples in the City manual do not match Figure 4-1:

- PDF page 82 states 3.78 inches for the 2-year, 24-hour value; Figure 4-1 on PDF page 118 states 3.98 inches.
- PDF page 77 states 6.21 inches for the 25-year, 24-hour value; Figure 4-1 states 6.69 inches.

Both sides of each conflict are retained. The calculator's Huntsville presets use Figure 4-1 because it is the manual's dedicated precipitation-frequency table, and the page discloses the decision.

## Quality-control checks

- 567 record IDs were checked for uniqueness.
- Required source titles, source URLs, verification dates, and units were checked for all published numeric records.
- Coefficients were checked to remain within 0 to 1.
- Confidence-interval lower bounds were checked to be below central estimates and upper bounds above them.
- PDS depth and intensity records were checked for blank reciprocal-AEP fields.
- Gross-rainfall scenarios were checked to contain no runoff-coefficient value.
- The 3.4× flat-lawn span, 0.50–0.85 gravel span, exact fractions, displayed decimal conversions, and worked examples were recalculated programmatically.
- CSV and JSON record counts were compared before export.
- SHA-256 hashes were generated for both files.

## Refresh triggers

- Recheck NOAA PFDS at least quarterly.
- Recheck NOAA Atlas 15 immediately when preliminary CONUS estimates are released and again when published estimates become available for use.
- Recheck the City of Huntsville manual and ordinance at least every six months and whenever the City posts a revision.
- Recheck WSDOT, TxDOT, ODOT, FHWA, and NCTCOG manuals annually or when an edition changes.
- Regenerate derived records and increment the dataset version whenever an upstream value changes.
- Change the visible “Last verified” date only after the underlying source review is completed.