How is the New York water quality volume calculated?

WQv = P × Rv × A / 12, in acre-feet, with Rv = 0.05 + 0.009 I. P is the 90th percentile rainfall in inches, I the impervious cover in percent and A the contributing area in acres. Source: NYS Stormwater Management Design Manual (July 31, 2024) § 4.2, p. 4-2, and Table 4.1.

Where do I find the 90th percentile rainfall?

Figure 4.1 of the design manual, a contour map of New York, or the 90 Percentile Rainfall layer of DEC’s Stormwater Interactive Map. Source: DM2024 § 4.2 and Figure 4.1.

Is there a minimum rainfall depth?

Under the July 31, 2024 manual the 90th percentile rainfall value is at least 1.0 in. The January 2015 manual states no minimum. Source: DM2024 § 4.2, p. 4-2.

Is there a minimum Rv in New York?

No. Neither the 2024 nor the 2015 manual sets one. Source: DM2024 and DM2015 Chapter 4.

What counts as impervious cover?

Any part of the site plan without permanent vegetative or permeable cover. The 2015 manual adds that gravel roads and gravel parking count as impervious. Source: DM2024 p. 4-3; DM2015 p. 4-3.

Why does the manual’s own example print 0.502 ac-ft?

The manual rounds Rv to 0.24 before multiplying. With Rv unrounded the same inputs give 0.510 ac-ft, which is what this calculator shows. Source: DM2024 Ch. 7 § 7.1 Step 2, p. 7-10.

New York water quality volume calculator

WQv = P·Rv·A/12 from the 90th percentile rainfall, impervious cover and contributing area, per the NYSDEC design manual.

PRELIMINARYReference tables are still being checked against their printed sources. How results are checked

0.510ac-ft

22,196 cf: P 1.2 in × Rv 0.24 × A 20.91 ac / 12.

Impervious cover I: 21.5 %
Runoff coefficient Rv: 0.24
WQv: 22,196 cf
Minimum P in this manual: 1.00 in

The link carries these inputs, not the answer.

Worked example

The default inputs above are a published example: DM2024 Ch. 7 § 7.1 Step 2, p. 7-10: P 1.20 in, 4.50 of 20.91 ac impervious. The manual rounds Rv to 0.24 and prints 0.502 ac-ft; unrounded Rv gives 0.510 ac-ft.

I = 4.50 ac impervious of 20.91 ac
Rv = 0.05 + 0.009 × I = 0.24
WQv = 1.20 × 0.24 × 20.91 / 12 = 0.510 ac-ft (22,196 cf)

The formula

WQv = P · Rv · A / 12, Rv = 0.05 + 0.009 I

NYS Stormwater Management Design Manual (July 31, 2024) § 4.2, p. 4-2, and Table 4.1

WQv: water quality volume, ac-ft
P: 90th percentile rainfall depth, DM Figure 4.1, in
Rv: volumetric runoff coefficient, 0.05 + 0.009 I
I: impervious cover of the contributing area, %
A: contributing area, ac

Which manual applies

The current edition is the NYS Stormwater Management Design Manual of July 31, 2024, which the construction general permit GP-0-25-001 (effective January 29, 2025) makes the technical standard. The January 2015 manual applies only on the permit’s transition path, for an eNOI submitted before January 29, 2027. Under the 2024 manual the 90th percentile rainfall is at least 1.0 in; the 2015 manual states no minimum. Pick the manual and the calculator applies its rule.

Neither manual sets a minimum Rv. The 0.2 floor some engineers carry over is a Maryland provision, not a New York one.

The current edition’s WQv rule is DM 2024 § 4.2. A detention-only practice such as a dry pond or an underground chamber does not treat WQv on its own under the 2024 manual (§ 4.6).

Questions about this calculation

How is the New York water quality volume calculated?: WQv = P × Rv × A / 12, in acre-feet, with Rv = 0.05 + 0.009 I. P is the 90th percentile rainfall in inches, I the impervious cover in percent and A the contributing area in acres.; NYS Stormwater Management Design Manual (July 31, 2024) § 4.2, p. 4-2, and Table 4.1
Where do I find the 90th percentile rainfall?: Figure 4.1 of the design manual, a contour map of New York, or the 90 Percentile Rainfall layer of DEC’s Stormwater Interactive Map.; DM2024 § 4.2 and Figure 4.1
Is there a minimum rainfall depth?: Under the July 31, 2024 manual the 90th percentile rainfall value is at least 1.0 in. The January 2015 manual states no minimum.; DM2024 § 4.2, p. 4-2
Is there a minimum Rv in New York?: No. Neither the 2024 nor the 2015 manual sets one.; DM2024 and DM2015 Chapter 4
What counts as impervious cover?: Any part of the site plan without permanent vegetative or permeable cover. The 2015 manual adds that gravel roads and gravel parking count as impervious.; DM2024 p. 4-3; DM2015 p. 4-3
Why does the manual’s own example print 0.502 ac-ft?: The manual rounds Rv to 0.24 before multiplying. With Rv unrounded the same inputs give 0.510 ac-ft, which is what this calculator shows.; DM2024 Ch. 7 § 7.1 Step 2, p. 7-10

The other calculators

Curve number and runoff. Composite CN from cover type and hydrologic soil group, then TR-55 runoff depth for a 24-hr rainfall.
Time of concentration. TR-55 Tc from a sheet flow, a shallow concentrated flow and a channel flow segment, in hours and minutes.
Rational method peak flow. Peak runoff Q = Cf·C·i·A from the runoff coefficient, rainfall intensity and drainage area, with the frequency factor for the storm.
Detention volume estimate. Storage needed to hold post-development runoff to an allowable release rate, by the modified rational critical-duration method.

How results are checked Pricing for the full app

The full app runs the whole site: Atlas 14 rainfall frozen for the project, pre and post hydrographs, and the pond routed through its outlets. Then the NYSDEC checks you select, and a PDF report with the SWMM file a reviewer can rerun.

Start trial

14-day trial, every feature, no card.