Provisional. 20 assumptions carried this answer and a person has accepted 0 of them. What that means is under the figures.
[REFUSED] A practice refusal is in force: “The practice drains too large an area for its size” (contributing area outside the practice's range).
Each mark is repeated beside the figures it affects and set out in full under them.
Runoff summary
This is an increase, not a reduction. The proposed condition produces 8,223 cu ft (0.189 ac-ft) more runoff than the existing condition at the 3.57 in design storm, 32.2 percent of the existing runoff volume. Runoff depth is 0.185 in more (32.2 percent of the existing depth).
Existing runoff depth
0.575 in
Proposed runoff depth
0.760 in [REFUSED]
Existing runoff volume
25,574 cu ft (0.587 ac-ft)
Proposed runoff volume
33,797 cu ft (0.776 ac-ft) [REFUSED]
Change in runoff depth, proposed less existing
+0.185 in, an increase [REFUSED]
Change in runoff volume, proposed less existing
+8,223 cu ft (0.189 ac-ft), an increase [REFUSED]
Increase, share of the existing runoff volume
32.2 percent
Existing peak
7.17 cfs
Proposed peak, no practice
18.79 cfs
Peak leaving Bioretention basin at the low corner
16.34 cfs
Change in peak, proposed leaving Bioretention basin at the low corner less existing
+9.18 cfs, an increase
Peaks by the NRCS standard dimensionless unit hydrograph (peak rate factor 484).
Bioretention basin at the low corner overflowed in this storm: 33,797 cu ft (64.3 percent of what reached it) passed over the crest untreated.
Site map
Site boundary Site boundary 12.252 ac, centroid at latitude 35.740950, longitude -79.160000. North is up. Imagery: USGS The National Map, public domain, flown September 27, 2022.
The report below qualifies these figures.
[REFUSED] A practice refusal is in force: “The practice drains too large an area for its size” (contributing area outside the practice's range). 431,244 sq ft draining to 8,034 sq ft of bed is a loading ratio of 53.7 to 1, above the 20 to 1 ceiling. Both the required volume and the available storage scale with the bed area, so the ratio at which this profile is filled to the overflow by the treatment volume alone is 18.8 to 1 and no choice of footprint changes it. Above the ceiling the surface loading rate governs rather than the volume, and clogging, scour and pretreatment are not modeled anywhere in CurveNumber. The sizing conclusion for this practice is void. Bioretention basin at the low corner reduces the increase by 18,760 cu ft (0.431 ac-ft), from 26,983 cu ft (0.619 ac-ft) without it to 8,223 cu ft (0.189 ac-ft).
These are the report's own findings, quoted from it. They are printed here because a recipient of this address reads the summary above before the report below, and a figure read without them is read without the thing that says what it is a comparison of.
Provisional: assumptions not acknowledged. 20 assumptions carried this answer and a person has accepted 0 of them. 20 carry no acceptance, so this report is provisional. Provisional means the figures on this page were computed and are not withheld, and that no licensed person had yet put their name to every assumption underneath them. This count was read at Sep 26, 2026, 7:23 PM EDT and is frozen here: acknowledgments recorded after that do not reach this copy. This copy was published on Sep 26, 2026, 7:23 PM EDT. The figures on this page were computed at Sep 26, 2026, 6:40 PM EDT, which is the run this publication is of and is not the moment it was published.
An acceptance on this report is an account holder submitting one, at the time printed with it. This product does not verify professional licensure: it holds no license number and no state of licensure, and it records no evidence that an entry was read as distinct from accepted. An acceptance is not a seal, a signature or a certification, and the absence of one is not a finding that anything is wrong.
The figures above reproduce the selected run. They are a summary and not the analysis: what they rest on, what CurveNumber refused and what nobody has checked is in the report below, and a figure quoted out of this page without it is a figure quoted without its qualifications.
Anybody who has this address can open it. No account, no sign-in and no password is needed, so treat the address itself as the control.
CNR-0D028D7D81B1Wooded tract to 1/2-acre lotsCurveNumber 0.9.0
Published at https://app.curvenumber.com/p/urVBMNyHRohmiH1SpmW2C6_2qe8xtd6a. Computational aid only. Not an engineering deliverable or a certification of compliance. Requires review by a qualified professional.
This copy of report CNR-0D028D7D81B1 is published at https://app.curvenumber.com/p/urVBMNyHRohmiH1SpmW2C6_2qe8xtd6a. Open that address to compare this printout with the published document.
Reports are shown as they print, on a light page.
CurveNumber
Runoff report
Report CNR-0D028D7D81B1
Inputs CNI-1340EB7179DF
Computed Sep 26, 2026, 6:40 PM EDT
Version CurveNumber 0.9.0
Status PROVISIONAL, read at Sep 26, 2026, 7:23 PM EDT
Wooded tract to 1/2-acre lots
Example 2: Residential subdivision, client Example developer (fictional), jurisdiction Chatham County, North Carolina. Prepared by CurveNumber Examples, CurveNumber, as entered by the account holder.
Located in Chatham County, North Carolina (boundary centroid at latitude, longitude 35.7409, -79.1600).
Site boundary Site boundary 12.252 ac, centroid at latitude 35.740950, longitude -79.160000. North is up. Imagery: USGS The National Map, public domain, flown September 27, 2022.
This result rests on
[REFUSED] A practice refusal is in force: “The practice drains too large an area for its size” (contributing area outside the practice's range).431,244 sq ft draining to 8,034 sq ft of bed is a loading ratio of 53.7 to 1, above the 20 to 1 ceiling. Both the required volume and the available storage scale with the bed area, so the ratio at which this profile is filled to the overflow by the treatment volume alone is 18.8 to 1 and no choice of footprint changes it. Above the ceiling the surface loading rate governs rather than the volume, and clogging, scour and pretreatment are not modeled anywhere in CurveNumber. The sizing conclusion for this practice is void. Bioretention basin at the low corner reduces the increase by 18,760 cu ft (0.431 ac-ft), from 26,983 cu ft (0.619 ac-ft) without it to 8,223 cu ft (0.189 ac-ft).
The marks in square brackets are repeated beside the figures they affect. Each item is set out in full directly after the headline figures.
Read this before the figures
This report was produced using CurveNumber.com, a computational aid for estimating NRCS curve numbers and direct runoff. It is not an engineering deliverable and it is not a certification of compliance with any regulation, ordinance, or design standard. The curve number method, as published in USDA NRCS Technical Release 55 and National Engineering Handbook Part 630 Chapter 10, is an empirical method with substantial uncertainty, and its applicability to any specific site is a professional judgment that this software has not made. Input data were retrieved from third party sources listed at https://curvenumber.com/data-sources and are provided by those sources without warranty. All results require review, verification, and adoption by a qualified professional before use in design, permitting, or construction. CurveNumber.com is not the engineer of record for any project.
No Data Quality Grade is stated on this report. The grade is a defined measure of how good the inputs behind a result are, this version does not compute one, and an absent grade is not a good grade. What this report carries instead is the provenance of each input individually, in the dataset and assumption sections below.
8,223 cu ft[REFUSED]
Increase in runoff volume, existing against proposed, at the 3.57 in design storm: 0.189 ac-ft, 32.2 percent of the existing runoff volume. Runoff depth rises by 0.185 in (32.2 percent of the existing depth).
This is an increase, not a reduction. The proposed condition produces more runoff than the existing condition at this storm.
Existing depth
0.575 in
Proposed depth
0.760 in [REFUSED]
Existing volume
25,574 cu ft (0.587 ac-ft)
Proposed volume
33,797 cu ft (0.776 ac-ft) [REFUSED]
Design storm 3.57 in over 24.0 hr, NRCS Type II. Peak discharge by the NRCS standard dimensionless unit hydrograph (peak rate factor 484): existing 7.17 cfs; proposed 18.79 cfs with no practice, 16.34 cfs leaving Bioretention basin at the low corner. Proposed peak leaving Bioretention basin at the low corner against existing: an increase of 9.18 cfs.
Overflow. Bioretention basin at the low corner overflowed in this storm: 33,797 cu ft (64.3 percent of what reached it) passed over the crest untreated; the stage reached 3.47 ft against a crest at 3.25 ft. The practice section gives the detail.
Summary, page 2: what was entered
Condition
Area
Acres
Soil group
CN
Cover
Existing
Impervious (measured), soil B
0.058
B
98.0
Impervious
Existing
Woods, fair condition, soil B, from land cover Deciduous forest
9.704
B
60.0
Woods, Fair
Existing
Woods, fair condition, soil B, from land cover Evergreen forest
1.994
B
60.0
Woods, Fair
Existing
Woods, fair condition, soil B, from land cover Mixed forest
Streets and roads, paved, curbs and storm sewers (excluding right-of-way)
Proposed
Preserved stream buffer woods
2.352
B
55.0
Woods, Good
Time of concentration used: existing 0.222 hr (13.3 min) (CurveNumber's estimate); proposed 0.128 hr (7.7 min) (CurveNumber's estimate for the proposed cover, on the same flow path as the existing estimate).
Practices: Bioretention basin at the low corner (bioretention), storage to the crest 12,612 cu ft, routed at a 0.0125 hr (0.75 min) time step.
Summary, page 3: what a person changed, and why
Practice storage to the crest, Bioretention basin at the low corner: set to 12,612 cu ft. By CurveNumber Examples on Sep 26, 2026, 6:39 PM EDT. Reason given: Adding the 9,000 square foot bioretention basin from the grading plan (example, fictional) at the low corner, with the tested infiltration rate.
The full record follows this summary, unchanged: every section, with each figure's source.
The full record
What this result rests on, in full
[REFUSED] A practice refusal is in force: “The practice drains too large an area for its size” (contributing area outside the practice's range). 431,244 sq ft draining to 8,034 sq ft of bed is a loading ratio of 53.7 to 1, above the 20 to 1 ceiling. Both the required volume and the available storage scale with the bed area, so the ratio at which this profile is filled to the overflow by the treatment volume alone is 18.8 to 1 and no choice of footprint changes it. Above the ceiling the surface loading rate governs rather than the volume, and clogging, scour and pretreatment are not modeled anywhere in CurveNumber. The sizing conclusion for this practice is void. Bioretention basin at the low corner reduces the increase by 18,760 cu ft (0.431 ac-ft), from 26,983 cu ft (0.619 ac-ft) without it to 8,223 cu ft (0.189 ac-ft).
Existing and proposed both total 12.252 ac; they differ by 11 sq ft (0.00 percent), which is within the 0.061 ac tolerance and is shown as information, not as a finding.
Volumes and peaks are computed over the areas entered: 12.252 ac existing and 12.252 ac proposed. The drawn boundary is 12.252 ac. Land cover and soil were read from 30 m map squares. Each square counts only for the part of it inside your boundary, so the derived areas are shares of the 12.252 ac you drew. Your boundary is about 55.1 of those squares. The areas total 12.252 ac.
Land cover folded into a larger class on its soil group (each below CurveNumber's minimum share for a class of its own; waiving the fold keeps each class at its own share): On soil group B, 0.001 ac of Developed, low intensity (0.0% of that group) was folded into Deciduous forest, so its CN 69 is computed as CN 60. On soil group B, 0.099 ac of Developed, medium intensity (0.8% of that group) was folded into Deciduous forest, so its CN 69 is computed as CN 60.
The state and county named by the datasets were compared with every geographic statement typed onto this file, and they agree.
Provisional: assumptions not accepted
20 assumptions carried this answer and a person has accepted 0 of them. 20 carry no acceptance, so this report is provisional.
Provisional means that the figures on this report were computed and are not withheld, and that the account holder has not yet submitted an acceptance of every assumption underneath them. The assumptions are listed further down with what each one is worth in the answer and what evidence would displace it.
14 of the 20 outstanding are an entry for which CurveNumber's list of default values states that no external source exists, or which it rates Low confidence. Those are accepted one at a time, cannot be accepted in bulk, and each one asks for a written reason.
An acceptance on this report is an account holder submitting one, at the time printed with it. This product does not verify professional licensure: it holds no license number and no state of licensure, and it records no evidence that an entry was read as distinct from accepted. An acceptance is not a seal, a signature or a certification, and the absence of one is not a finding that anything is wrong.
This count is as of , when this copy of the page was produced. It can change after that; the report identifier does not, because it names the computation. Two printouts with the same identifier are the same computation; if their counts differ, the later one is current.
An entry is accepted on its own when it has no published source (a judgment, or a threshold CurveNumber chose) or when its confidence is low. Every other entry (a method constant, a published table, a local manual's figure or a research value rated moderate or better) may be accepted together with others in one act, and the record says so.
Site map
Site boundary Site boundary 12.252 ac, centroid at latitude 35.740950, longitude -79.160000. North is up. Imagery: USGS The National Map, public domain, flown September 27, 2022.
The boundary is the one saved on this site when it was computed, and the practice footprint is the one drawn for the practice, if any. The aerial photograph was taken from the public service when this report was made and is kept with the report, so it shows the ground as it was photographed, not as it is today. The map is for locating the site; the areas on this report are the figures, not measurements off this picture.
Where this site is
Quantity
Value
Boundary centroid
latitude 35.740950, longitude -79.160000
Boundary extent
north 35.741900, south 35.740000, east -79.158700, west -79.161300
Enclosed area
12.252 ac
Coordinate reference system
WGS84 decimal degrees (EPSG:4326), longitude first
Boundary vertices
4
This boundary was drawn or loaded on the map. It was NOT geocoded: this product does not turn an address into an area of interest, because the Census geocoder returns a point interpolated along a street centerline and runoff computed around such a point is runoff for a piece of road. The coordinates below are the polygon a person drew, and every dataset on this report was read over that polygon and over nothing else.
The boundary every dataset on this report was read over
#
Longitude
Latitude
#
Longitude
Latitude
#
Longitude
Latitude
1
-79.161300
35.740000
2
-79.158700
35.740000
3
-79.158700
35.741900
4
-79.161300
35.741900
What is claimed about this ground, and on whose evidence
Statement
Value
Source
Basis
Checked
State
North Carolina
USDA NRCS SSURGO survey area NC037
read from a federal dataset over this boundary
CHECKED
The first two characters of a survey area symbol are the postal code of the state the survey area is administered under. The soil groups on this report were read from NC037 over the boundary above.
State
North Carolina
US Census Bureau geocoder, geographies at the boundary centroid
looked up from the boundary's own coordinate
CHECKED
Chatham County, North Carolina, read from US Census Geocoder (current boundaries) at -79.160000, 35.740950. This is the county and state whose published boundaries contain that one point, not a statement about the whole site.
County
Chatham County
US Census Bureau geocoder, geographies at the boundary centroid
looked up from the boundary's own coordinate
CHECKED
The county whose published boundary contains the centroid -79.160000, 35.740950. A site straddling a county line lies in two counties and this names the one its middle falls in. It is where the ground is and not a statement about which authority reviews the application.
Region
Ohio River Basin and Surrounding States
NOAA Atlas 14 Volume 2
read from a federal dataset over this boundary
CHECKED
This volume is published for a region rather than for one state, and the states it covers are delineated on a map this product does not hold. It names the region the site is in and makes no claim about which state.
Annual NLCD Land Cover (CONUS) 2024: A national map is sampled at this boundary's own coordinates, so it agrees with the boundary whatever the boundary is and is not evidence about where the boundary is.
Where the water went in the 3.57 in NRCS Type II storm. The reduction above is a single figure and it has a composition, and the composition is what a reviewer is actually approving. Water that infiltrated left the site and does not come back: it recharges groundwater and it is retention. Water that left through the underdrain was filtered and then discharged, at full volume, to the same place the runoff was going: it is filtration, and it reduces nothing downstream. Water that was metered out through the outlet structure was detained and then discharged: it arrives later and more slowly, at full volume, and it passed through no media at all, so it is neither of the other two. Water that was drawn out of the practice for a use left the site as neither runoff nor recharge: somebody used it, and folding it into the infiltrated volume would credit the ground with water that never reached it. Many jurisdictions credit the first and not the others. The figures below are therefore reported separately and are never added together anywhere in this report, because a single treated volume would tell a reviewer something that is not true.
Retained on site. Infiltrated into the native soil beneath the practice.retention
18,760 cu ft35.7 percent of what reached the practice
This water left the site and does not come back: it recharges groundwater, and it is the only figure a retention credit is claimed against.
RETENTION IS NOT FILTRATION AND THE TWO ARE NOT ADDED. The figure above went into the ground and stayed. The figure below went down a pipe to the same place the runoff was going, cleaner and at exactly the same volume.
Left the site, filtered. Through the underdrain, under the media bed.filtration
0 cu ft0.0 percent of what reached the practice
Treated and then discharged at full volume to the same place the runoff was going. It recharges nothing, and a jurisdiction that credits retention and not filtration credits none of it.
FILTRATION IS NOT DETENTION AND THE TWO ARE NOT ADDED. The figure above passed through a media bed and was treated. The figure below passed through an opening in a wall and was treated by nothing: it was slowed down, which reduces a peak and reduces a volume by nothing.
Left the site, detained. Metered out through the outlet structure.detention
0 cu ft0.0 percent of what reached the practice
Every cubic foot of this arrives downstream, later and more slowly than it would have without the structure but at full volume. It is quantity control, it is not a volume reduction, and nothing here passed through media, so it is neither retention nor filtration. Nothing left this practice this way in this storm.
Left the site, untreated. Over the emergency crest, without entering the structure.no credit
33,797 cu ft64.3 percent of what reached the practice
It passed the crest without entering the media and without passing through any outlet. Nobody credits it, and it is printed because a practice that overflows in a storm is a design finding a reviewer has to see rather than an omission.
Still in the practice when the routing stopped. Not a destination.
0 cu ft
Held, not yet disposed of. It is not capacity: it is what is in the practice when the next storm arrives. It is not retained either: nothing credited removes it except the outlets, so it is counted in the volume leaving the site.
THESE FIGURES ARE NOT ADDED, HERE OR ANYWHERE IN THIS REPORT. Retention, filtration, detention and a use are four different claims about where the water went. Check which of them the governing jurisdiction credits before any of these figures is carried into a compliance run.
Reaching the practice: 52,557 cu ft. Leaving the site after it, which is the underdrain plus the outlet structure plus the overflow plus what was still in the practice when the routing stopped, and is the figure the change above is taken against: 33,797 cu ft. Volume balance residual -0.000003 cu ft, to the six decimal places this document carries: the inflow plus whatever the practice held before the storm, less the FIVE outflows, less what is still held. The five are infiltration, the underdrain, the outlet structure, the overflow, and water drawn out for a use. On a practice with no demand schedule the drawn term and the antecedent term are both exactly zero and the balance is the four it always was. Zero is the expected value and it is the arithmetic closing rather than a modeling allowance. Maximum stage 3.47 ft against a crest at 3.25 ft.
Infiltration rate: measured on this site
Measured. A field infiltration test was performed and its result of 0.9 in/hr is the basis for the infiltration credit claimed below. Method: Double ring infiltrometer, ASTM D3385. Location: TP-1, at the proposed bottom elevation (example, fictional). Date: . The design rate after the safety factor is 0.45 in/hr.
Basis: field test. How CurveNumber set it: field test: 0.9 in/hr, double ring infiltrometer, ASTM D3385, TP-1, at the proposed bottom elevation (example, fictional), . Native rate 0.9 in/hr, design rate 0.45 in/hr.
The curve number method, chosen by the storm depth rule: Judged area by area on each area's own curve number: P of 3.57 in is above 2 Ia on every area that yields a material share of the runoff. The curve number governs, with a mandatory sensitivity band. Sensitivity: A five CN unit uncertainty either way on every area's curve number moves the site's area weighted runoff at this depth from 0.38 in to 0.80 in, against 0.58 in as computed, a factor of 2.1 from end to end. Proposed condition: routing through the practice, chosen by the practice rule: A practice is present, so the site's runoff is routed through it. The inflow is computed by the curve number method, chosen by the storm depth rule: Judged area by area on each area's own curve number: P of 3.57 in is above 2 Ia on every area that yields a material share of the runoff. The curve number governs, with a mandatory sensitivity band.
Method by area. Each area is judged on its own curve number and initial abstraction (TR-55 Chapter 2); impervious cover is computed at its TR-55 curve number. The site figure is the area weighted mean of the runoff depths below.
Condition
Area
Acres
CN
Ia, in
Runoff, in
Method
Existing
Impervious (measured), soil B
0.058
98.0
0.041
3.34
curve number, standard
Existing
Woods, fair condition, soil B, from land cover Deciduous forest
9.704
60.0
1.333
0.56
curve number, with a sensitivity band
Existing
Woods, fair condition, soil B, from land cover Evergreen forest
1.994
60.0
1.333
0.56
curve number, with a sensitivity band
Existing
Woods, fair condition, soil B, from land cover Mixed forest
0.495
60.0
1.333
0.56
curve number, with a sensitivity band
Proposed
1/2-acre lots (17 homes)
8.500
70.0
0.857
1.05
curve number, with a sensitivity band
Proposed
Streets with curb and gutter
1.400
98.0
0.041
3.34
curve number, standard
Proposed
Preserved stream buffer woods
2.352
55.0
1.636
0.37
curve number, with a sensitivity band
Storm
Quantity
Value
Note
Design storm depth
3.57 in
derived This depth is the NOAA Atlas 14 2-year 24-hour depth read at this site (Partial duration series): NOAA Atlas 14 (Volume 2 Version 3) 24-hr duration, 2 year ARI, mean estimate, partial duration series, Ohio River Basin retrieved . Whether that return period is the one the governing requirement asks for is a regulatory choice this product does not make.
Duration
24.0 hr
The curve number method has no rainfall intensity term. The duration is recorded because it governs the Green-Ampt path and because a reviewer will ask for it, not because it changes a curve number result.
Initial abstraction ratio, lambda
0.20
CurveNumber's default initial abstraction ratio, which applies where nothing replaces it. No saved abstraction setting on this site replaced it for this run. Its source and what it is worth are in the assumptions section below.
Rainfall temporal distribution
NRCS Type II
The storm shape the inflow hydrograph was built from. A design storm is a depth and a shape, and the shape sets the peak intensity the practice sees.
Time of concentration
0.128 hr
A user input. It is not derivable from any dataset this product reads, because flow length is in none of them, and it is not defaulted.
Routing time step
0.0125 hr
The step the level pool routing ran at. What it is worth is measured rather than asserted: see the step sensitivity in the practice section.
The distribution table has not had a second check The four NRCS 24 hour distributions in this product are the published NRCS tables at a 0.1 hour step (as issued with WinTR-55 version 2.00), read from the published files by a script and compared by machine with a second copy of the same tables. They have NOT yet had a second person's check against the NRCS program files. This matters more here than a curve number would: a distribution ordinate changes the peak intensity, and the peak intensity drives the inflow hydrograph, the stage the practice reaches and therefore the volume split reported below. Where the jurisdiction publishes its own distribution, supply it.
NRCS Type II (built in) from USDA NRCS WinTR-55 tabular rainfall distributions (NEH Part 630 Ch. 4) (WinTR-55 version 2.00) Type I, IA, II and III 24 hour mass curves at 0.1 hr; applicability per TR-55 (1986) Appendix B, Figure B-2; not transcribed; parsed by script from the WinTR-55 v2.00 tabular files and machine cross-checked against the HEC-HMS 4.13 copy of the same tables (agreement within 0.0006); checked by NOBODY; a mass curve read as one rainfall
Peak discharge
NRCS unit hydrograph peak, lag = 0.6 Tc (NEH Part 630, Chapter 16). Unit hydrograph: the NRCS standard dimensionless unit hydrograph (peak rate factor 484), because the site is not on the Delmarva coastal plain and no local rule asks for another. The site with no practice on it, under the NRCS Type II storm distribution, which was chosen for this run; its fit to this region was not assessed. Time of concentration used: existing 0.222 hr (13.3 min) (CurveNumber's estimate); proposed 0.128 hr (7.7 min) (CurveNumber's estimate for the proposed cover, on the same flow path as the existing estimate). Time step 0.0125 hr. The peak is built from runoff computed for each area separately and added (distributed). A hand check at the composite curve number gives a lower peak where impervious and pervious areas are mixed, which TR-55 itself warns a composite understates. The peak leaving the practice is the same storm routed through it; its detail is shown with the practice, and the note on the distribution table below applies to its routed peaks too.
Condition
Peak
Time to peak
Existing
7.17 cfs
12.06 hr
Proposed, with no practice
18.79 cfs
11.98 hr
Proposed, leaving Bioretention basin at the low corner
16.34 cfs
12.04 hr
Change, proposed with no practice against existing
an increase of 11.62 cfs
Change, proposed leaving Bioretention basin at the low corner against existing
an increase of 9.18 cfs
The practice was saved with a time of concentration of 0.222 hr. The routed inflow to it uses 0.128 hr, the proposed condition's time of concentration for this run, because the routing puts the whole proposed condition through the practice. The peak with no practice and the peak leaving the practice are drawn on the same Tc, so the difference between them is the practice's alone.
Distribution: NRCS Type II, chosen for this run. TR-55 Figure B-2 assigns the NRCS type by county; this product does not hold that map, so the choice has not been checked against it.
Not checked against the source The unit hydrograph shape table was transcribed and has not been checked against the source. The NRCS Type II rainfall distribution was read from the published NRCS table by script and has not yet had a second person's check.
The four NRCS 24 hour distributions in this product are the published NRCS tables at a 0.1 hour step (as issued with WinTR-55 version 2.00), read from the published files by a script and compared by machine with a second copy of the same tables. They have NOT yet had a second person's check against the NRCS program files. This matters more here than a curve number would: a distribution ordinate changes the peak intensity, and the peak intensity drives the inflow hydrograph, the stage the practice reaches and therefore the volume split reported below. Where the jurisdiction publishes its own distribution, supply it.
The datasets these areas were read from
This section is the record for this report: the editions it read and the dates they were read. Every dataset CurveNumber reads, with the edition in use, is listed at https://curvenumber.com/data-sources. A dataset's retrieved date can be earlier than the day the areas were read. CurveNumber keeps a copy of each answer a publisher's service gives it, and a later request for the same map tile, soil survey area or storm point is answered from that copy rather than from the publisher again. The read date is when this site's areas were read; the retrieved date is when CurveNumber fetched that answer from the publisher, and the edition printed with it is the one that answer carries.
Dataset
Value
What it means here
Land cover product
Annual NLCD Land Cover (CONUS)
Edition year 2024. Native resolution 30 m, 900 m² the pixel.
Map squares over this site
n = 55.1
The boundary equals 55.1 map squares. 74 squares touch it, each counted for the part inside it. The note under this table says what n tells a reader.
Map square count
At least 10
Enough map squares for CurveNumber not to warn that the land cover is coarse for this site. That is a floor and not a grade.
Impervious fraction
Read off the fractional impervious map
A measured per pixel percentage on the same grid and the same year as the classes, not a class midpoint.
Impervious citation
Annual NLCD Fractional Impervious Surface (CONUS) (Annual NLCD Fractional Impervious Surface (CONUS), 2024, 30 m, EPSG:5070 native grid) 74 pixels at 30 m retrieved
Land cover citation
Annual NLCD Land Cover (CONUS) (Annual NLCD Land Cover (CONUS), 2024, 30 m, EPSG:5070 native grid) retrieved
Soil survey areas
NC037 (NC037 v29 )
Each survey area publishes its own edition. A site crossing two of them stands on two editions, and a hydrologic soil group read from one is not the same claim as the same letter read from the other.
Map units
3
Distinct soil map unit polygons inside the boundary. The minimum site area rule treats this count as the soils equivalent of a pixel count.
Map units the survey rates a hydrologic soil group for
3 of 3
Every map unit inside this boundary carries a published hydrologic soil group. Mapped ground and rated ground are the same ground here.
Soil map units
Map unit
Share of site
Published group
Group used
How it was decided
Georgeville silt loam, 6 to 10 percent slopes (GaC), survey area NC037
57.7%
B
B
As published.
Georgeville-Badin complex, 10 to 15 percent slopes (GkD), survey area NC037
36.5%
B
B
As published.
Georgeville silt loam, 2 to 6 percent slopes (GaB), survey area NC037
5.8%
B
B
As published.
Soils citation
USDA NRCS SSURGO via Soil Data Access (SSURGO, NC037 v29 ) retrieved
Area of interest
12.25 ac
Geodesic area of the derived boundary. This is the denominator in n above.
n is the boundary's area measured in land cover map squares: the area divided by the area of one square. Every square that touches the boundary is counted only for the part of it inside, so the derived areas add up to the boundary. n says how many independent readings of the ground the land cover rests on; a site of a few squares is coarse for this dataset whatever its areas add up to.
What the datasets said about themselves.
2 land cover classes were folded into a larger class on the same soil group, because each was smaller than one 30 m land cover pixel (0.222 ac) on that soil group, below what the map resolves: On soil group B, 0.001 ac of Developed, low intensity (0.0% of that group) was folded into Deciduous forest, so its CN 69 is computed as CN 60. On soil group B, 0.099 ac of Developed, medium intensity (0.8% of that group) was folded into Deciduous forest, so its CN 69 is computed as CN 60. Waiving the fold keeps every class at its own share.
CurveNumber raised no refusal on the existing or proposed condition itself during this run.
Practice refusals: 1, 1 in force and 0 waived.“The practice drains too large an area for its size” (in force). Each is printed in full in the practice section.
The practice, and the two checks a reviewer performs
These dimensions came from the kind template, not from a section on a drawing: top length, top width, ponding depth, media depth, gravel depth. The others were entered. They are a starting point for a cell described by its area alone, and every one of them is replaced by a designed profile before anything is stamped.
Water table separation The practice bottom is 3.25 ft below the rim and the seasonal high water table is stated at 5.00 ft, a separation of 1.75 ft, less than the 2 ft minimum. Infiltration and storage below the water table are not available in the wet season; check the jurisdiction's separation requirement. Depths below the rim, taken as finished grade.
Item
Value
Note
Practice
Bioretention basin at the low corner
As recorded on this site.
Kind
bioretention
As modeled by CurveNumber.
Condition
proposed
Which side of the delta this practice is on.
Plan at grade
164.3 by 54.8 ft, 9,000 sq ft
The footprint. Not the filter bed.
Filter bed
8,034 sq ft
The bottom of the excavation, after the side slopes have taken their share. This is the area every loading ratio is taken against.
Profile
9 in ponding over 18 in media over 12 in stone
Layer depths from the bottom of the excavation up.
Side slopes
3H:1V
The ponding volume is computed as a frustum and not as a prism on the footprint: 6,385 cu ft against 6,750 cu ft, which is what a spreadsheet on the footprint at grade produces.
Overflow crest
3.25 ft
Above the bottom of the excavation. The rim is at 3.25 ft and everything above the crest is freeboard rather than storage.
Live storage
12,612 cu ft
18.8 in of water over the filter bed. Nothing above the crest counts, because it is leaving. No permanent pool is declared on this practice, so this is the storage to the crest.
Underdrain
none
A practice without an underdrain or an outlet structure drains only into the ground, so it is an infiltration practice and needs a field test.
Outlet structure
none
There is no outlet structure. The practice drains through infiltration, through the underdrain, or through neither.
Permanent pool
none
No permanent pool is declared, so every cubic foot of the storage above is live.
Drainage area to the practice
431,244 sq ft
The routing assumes the whole site drains to this practice. Every area's runoff is convolved into one hydrograph and all of it is routed, with no bypass, no flow split and no untreated remainder, because CurveNumber represents one practice and one site. The contributing area recorded on the practice is used for the capture check and the loading ratio only. Where the two disagree, the routed figures describe a site that all drains here and the capture check describes the area stated.
Check
Result
What it means
Drawdown from full
41.9 hr
Integrated from the stage-storage and stage-discharge relations, not the stored volume divided by the outflow at full stage, which gives 41.9 hr and is a diagnostic rather than a result. The limit is 48 hr and this practice is within it.
Required treatment volume
12,796 cu ft
Over the contributing impervious area at the treatment depth in CurveNumber's list of default values.
Available storage
12,612 cu ft
To the overflow crest. There is no permanent pool, so every cubic foot of it is live.
Surplus
no figure reported
The capture check was refused. The refusals below say why, and this is not a surplus of nothing: no comparison was made.
Loading ratio
53.7 to 1
Contributing area against filter bed area. Above the ceiling in CurveNumber's list of default values the surface loading rate governs rather than the volume, and clogging, scour and pretreatment are modeled nowhere in CurveNumber.
Peak attenuation
13.0 percent
Peak inflow 18.788 cfs at 11.975 hr against a peak discharge of 16.345 cfs at 12.044 hr.
Step sensitivity
0.33 percent
The same storm routed again at half the time step. This is a measurement of how far the peak has converged, not a prediction of it. The volumes move much less than the peak does.
Design finding: exceeded in the 3.57 in NRCS Type II storm
33,797 cu ft, 64.3 percent of what reached the practice, passed over the crest and left the site untreated. The stage reached 3.47 ft against the emergency overflow crest at 3.25 ft, which is 0.22 ft over it.
EXCEEDED in the 3.57 in NRCS Type II storm: 33,797 cu ft, 64.3 percent of the inflow, passed over the crest at 3.25 ft and left the practice untreated. The maximum stage was 3.47 ft. That is not by itself a failure: every practice overflows in a large enough storm, and the design question is whether it overflows in the storm it is being sized against.
Refused: contributing area outside the practice's range
431,244 sq ft draining to 8,034 sq ft of bed is a loading ratio of 53.7 to 1, above the 20 to 1 ceiling. Both the required volume and the available storage scale with the bed area, so the ratio at which this profile is filled to the overflow by the treatment volume alone is 18.8 to 1 and no choice of footprint changes it. Above the ceiling the surface loading rate governs rather than the volume, and clogging, scour and pretreatment are not modeled anywhere in CurveNumber.
To proceed: deepen the profile, add pretreatment and a second practice in series, or divide the drainage area.
The sizing conclusion is void: the figures it covers are blank above and must not be read as zeros.
What CurveNumber said about this routing.
The practice was saved with a time of concentration of 0.222 hr. The routed inflow to it uses 0.128 hr, the proposed condition's time of concentration for this run, because the routing puts the whole proposed condition through the practice. The peak with no practice and the peak leaving the practice are drawn on the same Tc, so the difference between them is the practice's alone.
The contributing area recorded for this practice is 431,244 sq ft and the site is 533,697 sq ft. The capture check and the loading ratio were taken over the first; the routing put the whole of the second through the practice, because CurveNumber has one practice and one site and no flow split between them. Where only part of a site drains to a practice, evaluate that sub area as its own site.
The practice bottom is 3.25 ft below the rim and the seasonal high water table is stated at 5.00 ft, a separation of 1.75 ft, less than the 2 ft minimum. Infiltration and storage below the water table are not available in the wet season; check the jurisdiction's separation requirement.
Both aggregation paths
Aggregation path
Existing
Proposed (after the practice)
Change, proposed less existing
What it is
Distributed (reported)
0.5750 in
0.7599 in
+0.1849 in, increase
Runoff computed on each area, then area weighted.
Composite
0.5695 in
1.0696 in
+0.5002 in, increase
Curve number area weighted, then runoff computed once. This is what a hand check produces.
Composite curve number
60.2
70.3
Area weighted.
Divergence, composite less distributed
-0.97 percent
-9.49 percent
Signed. The direction reverses between small and large storms, so the sign is reported rather than a direction asserted.
With a practice, the proposed distributed figure is what leaves the site after the practice, and the proposed composite is the land surface before it, so the two proposed figures do not compare like with like.
The two paths differ because runoff is a non-linear function of the curve number, so averaging the curve numbers first and averaging the runoff first do not give the same answer; the distributed path is reported because it applies the method to each surface as the method was fitted, and the composite path is printed beside it because it is what a reviewer recomputing by hand from a single area weighted curve number will obtain.
What was estimated, from what, and what each estimate is worth
Check these 3 first: temporal distribution, a factor of 8.37 on peak one hour rainfall fraction; time of concentration, a factor of 7.18 on time of concentration; irrigation demand, July, a factor of 4.00 on irrigation demand in the governing month. The other 4 move their own figures by a factor of 3.33 or less and are reported rather than put in front of you.
RANK 1 HEADLINE Temporal distribution derived
0.277 fraction of total depth
Can change the peak one hour rainfall fraction by up to 8.4 times.
Rests on: NOAA Atlas 14 Volume 2 Version 2.1 temporal distribution file for Volume 2 (Ohio River Basin and Surrounding States), area general, 24h duration, fetched and hashed rather than transcribed. WHICH of its 45 curves is a judgment and is CurveNumber's default value “Which of the NOAA Atlas 14 quartile cases and probability of occurrence lines a design storm shape is taken from”.
The selection spans a factor of 5.10 on the peak one hour fraction inside this file, and a factor of 8.37 against the NRCS Type II curve the product uses without it. The volume is untouched.
Displaced by: The quartile case and probability line the governing jurisdiction names, which supersedes outright. Failing that, NOAA's own guidance on which case a design should use, which is published alongside the files and which this product has not read. A local gauge record of the storms that actually produced the design flows displaces the whole question.
NOAA Atlas 14 (Volume 2 Version 2.1) first quartile case, 10 percent cumulative probability of occurrence line, 24h duration, Volume 2 (Ohio River Basin and Surrounding States), area general retrieved
RANK 2 HEADLINE Time of concentration computed
0.222 hr
Can change the time of concentration by up to 7.2 times.
Rests on: TR-55 segmental: sheet flow over woods, light underbrush at n = 0.4 for 100 ft, then shallow concentrated flow. Slope 10.79 percent from USGS 3DEP, two year 24 hour depth 3.57 in from NOAA Atlas 14 Volume 2 Version 3, flow length 1036 ft estimated as the longest chord of the drawn boundary, which is not a flow path.
The surface is worth a factor of 7.18 on Tc and decides whether the peak discharge is drawn at the TR-55 minimum; the sheet flow cap 2.05; the estimated flow length 1.09.
Displaced by: The flow path drawn on the grading plan, measured from the hydraulically most distant point to the design point, which is what the method actually asks for and what no dataset contains. A LiDAR flow accumulation routing over the graded surface displaces it too and is not implemented.
Computed here from the figures named above. There is no external source because nothing external was read for it.
RANK 3 HEADLINE Irrigation water demand defaulted
6,492 cu ft/day
Can change the irrigation demand in the governing month by up to 4.0 times.
Rests on: gridMET monthly reference evapotranspiration (archive built 03 March 2026) as a landscape water budget: reference rate times a plant factor of 0.5, divided by an irrigation efficiency of 0.75, over 531,156 sq ft. Both coefficients are CurveNumber defaults, not figures from a published page.
In July the demand is 6,492 cu ft per day at the shipped coefficients, 3245.9 at the generous end of both and 12983.5 at the lean end, so a 1,500 gallon tank of 200.5 cu ft empties in 0.7 hours, 1.5 hours or 0.4 hours.
Displaced by: A plant factor from the WUCOLS listing or from the state model ordinance that governs the site, for the species actually specified on the landscape plan, which is a document the project already has. A metered irrigation record displaces both. A distribution uniformity test on the system as installed, which is the measurement this number stands for and which irrigation auditors run as a matter of course. A drip system designed and certified to a stated efficiency displaces it directly.
CurveNumber default value (“Plant factor in a landscape water budget”), which is an assumption and not a measurement.
RANK 4 Antecedent dry period derived
10 days
Can change the antecedent dry period by up to 3.3 times.
NOAA NCEI GHCN-Daily (station record through ) station USC00311677, 90th percentile of interevent dry period length retrieved
RANK 5 Slope of the receiving area derived
10.79 percent
Can change the slope across the area by up to 3.3 times.
USGS 3DEP (1/9 arc-second DEM (about 3 m)) Slope Degrees raster function retrieved
RANK 6 Green roof evapotranspiration rate defaulted
0.132 in/day
Can change the green roof recovery time by up to 2.5 times.
CurveNumber default value (“Ratio of an extensive green roof's evapotranspiration to the grass reference”), which is an assumption and not a measurement.
RANK 7 Contributing flow path length defaulted
1,036 ft
Can change the contributing flow path length by up to 1.5 times.
CurveNumber default value (“Which geometric bound is offered for a jurisdictional credit measurement”), which is an assumption and not a measurement.
Design storm depth REGULATORY
A REGULATORY CHOICE, refused rather than defaulted. The answer exists and is written in an ordinance; nothing in a climatological dataset says which one applies here.
No return period was named, and CurveNumber does not pick one. The return period is a REGULATORY choice and not a climatological one: it is written into an ordinance, it differs between water quality, channel protection and flood control on the same site, and nothing in NOAA Atlas 14 says which applies here. NOAA Atlas 14 Volume 2 Version 3 gives these depths for the 24-hr duration at this point, each with its ninety percent interval in brackets. 1 yr: 2.95 in (2.77 to 3.16). 2 yr: 3.57 in (3.34 to 3.82). 5 yr: 4.46 in (4.18 to 4.78). 10 yr: 5.17 in (4.82 to 5.53). 25 yr: 6.12 in (5.69 to 6.55). 50 yr: 6.87 in (6.38 to 7.36). 100 yr: 7.64 in (7.07 to 8.19). 200 yr: 8.44 in (7.78 to 9.06). 500 yr: 9.53 in (8.75 to 10.2). 1000 yr: 10.4 in (9.5 to 11.2). That is a factor of 3.53 on the depth between the ends. Name the return period the jurisdiction requires, or enter the manual's own design depth under Local rules, which then takes precedence.
Receiving flow path length NOT OFFERED
DELIBERATELY LEFT BLANK. A bound computable from the site boundary would make the condition easier to satisfy, so the field stays empty, the condition comes back unchecked and the credit is denied.
Receiving flow path length: the length of pervious ground the flow crosses, which is a dimension of a practice nobody has drawn yet and not a dimension of the site. The narrowest crossing of the site boundary is a bounding dimension and using it here would credit a disconnection across ground that may hold no disconnection at all. The field is left empty, so a condition reading it comes back unchecked and the credit is not granted, which is the correct outcome.
Filter strip width NOT OFFERED
DELIBERATELY LEFT BLANK. A bound computable from the site boundary would make the condition easier to satisfy, so the field stays empty, the condition comes back unchecked and the credit is denied.
Width: the width of a filter strip measured across the face it treats, which needs the flow direction and the strip, and CurveNumber has neither. A bounding dimension of the site is not a strip width. The field is left empty and a condition reading it denies the credit.
Indoor water demand NO DATASET
NOT IN ANY DATASET THIS PRODUCT READS. There is nothing to estimate from and a plausible figure would be an invention.
An indoor water demand cannot be estimated from anything this product reads: it depends on occupancy, which no dataset holds. The irrigation part of a cistern's demand is estimated; the indoor part is not, because a guessed continuous demand would give a cistern its whole credit on a number nobody supplied. To credit it, enter a metered monthly demand schedule with a written commitment that the use continues.
Overrides of the derived and defaulted values
Records that are not overrides. The product wrote these when a condition was copied, or they are the design of a practice.
Practice storage to the crest, Bioretention basin at the low corner: 12,612 cu ft, set by CurveNumber Examples on Sep 26, 2026, 6:39 PM EDT. A design input of the practice, not a change to a derived value. Reason: Adding the 9,000 square foot bioretention basin from the grading plan (example, fictional) at the low corner, with the tested infiltration rate.
The aerial photograph, and what was read off it
No aerial photograph of this site was classified.
Nothing on this report rests on an aerial photograph. This is the ordinary case and it is stated rather than left to be inferred from the absence of a section, because a page that does not mention imagery and a page that says no photograph was classified read identically and mean different things.
The curve number fitted to an observed record, and what it is about
No curve number was fitted to an observed record for this site.
Nothing on this report rests on a gauged rainfall and discharge record. This is the ordinary case and it is stated rather than left to be inferred from the absence of a section. It is also the expected case: a curve number fitted at a stream gauge describes the whole watershed above that gauge, and this product will not print such a figure against a site that is not inside that watershed and a large enough fraction of it, which most sites are not.
Refusals a supplied input resolved
No refusal was resolved by a supplied input.
Nothing on this report rests on a figure a user handed CurveNumber to answer a refusal with. Where a refusal fired, it fired and it is printed as a refusal.
Refusals a person waived
No refusal was waived.
Every figure on this report is one CurveNumber produced. Nobody waived a refusal, and no figure here rests on a signature in place of a computation.
The assumptions that carry this answer
Hydrologic condition defaultedFair (a judgment with no published source; low confidence)
PENDINGno acceptance recorded
Worth in the answer: 5 to 12 CN units. One of the two largest silent levers in the method.
Displaced by: A site visit, current aerial imagery, or a measured ground cover fraction against the NEH Chapter 9 mapping.
Curve number suppression band, lower defaulted2 multiples of Ia (a threshold CurveNumber chose; low confidence)
PENDINGno acceptance recorded
Worth in the answer: A five CN unit uncertainty is worth a factor of 18.1 at 1.5 Ia, 4.4 at 2 Ia, 2.3 at 3 Ia and 1.4 at 9 Ia.
Displaced by: Expert panel calibration.
Curve number caution band, upper defaulted5 multiples of Ia (a threshold CurveNumber chose; low confidence)
PENDINGno acceptance recorded
Worth in the answer: Above this the method is in the regime it was fitted for.
Displaced by: Expert panel calibration.
Impervious connectivity defaulted0 (a judgment with no published source; low confidence)
PENDINGno acceptance recorded
Worth in the answer: Bounded at 10.2 CN units. Figure 2-4 adjusts the composite as CN = CNp + (Pimp/100)(98 - CNp)(1 - R/2), so the whole span from fully connected to fully unconnected is 0.5 (Pimp/100)(98 - CNp), largest at the 30 percent total impervious ceiling the figure is valid to and at the lowest pervious curve number TR-55 publishes, which is 30: 0.15 x 68 = 10.2 units. At a pervious CN of 61 the same bound is 5.55 units, and at 80 it is 2.7.
Displaced by: A delineated drainage path showing impervious area discharging to pervious ground. Automated detection from LiDAR routing is an open gap.
Layer depths that each practice kind starts from defaulted (a judgment with no published source; low confidence)
PENDINGno acceptance recorded
Worth in the answer: The template depths are the whole capture answer, because the effective storage depth over the bottom area is what the loading ratio is checked against. The bioretention template gives 20.3 in of water storage over its bottom, the infiltration trench template 19.2 in, the permeable pavement template 7.2 in and the dry swale template 21.1 in, all four on a 1,000 sq ft footprint. A designer who takes the bioretention template and drops the gravel to 6 in loses 2.4 in of that, 11.8 percent of the storage, and the required footprint grows in the same proportion. The permeable pavement figure is the one to read twice: 600 cu ft on that footprint against the bioretention template's 1,167.8 cu ft on the same ground, because a pavement section has no ponding and no media and the stone is all of it.
Displaced by: The section on the drawing. These are starting dimensions for a cell described by its area alone, and every one of them is replaced by a designed profile before anything is stamped.
Effective porosity, engineered bioretention media defaulted0.25 (a judgment with no published source; low confidence)
PENDINGno acceptance recorded
Worth in the answer: The media holds 259.4 cu ft of the reference cell's 1,167.8, which is 22.2 percent of the storage the practice is credited with. Moving 0.25 to 0.20, which is the low end of what manuals credit, removes 51.9 cu ft, or 4.4 percent of the total. The whole range in use, 0.20 to 0.40, spans 207.5 cu ft, 17.8 percent of the storage and four times that step, and it lands on the layer whose porosity nobody measures on the job.
Displaced by: A porosity or a water holding capacity measured on the mix actually specified, which the media supplier can produce and almost never is asked for.
14 further default values applied to this computation and are left out here, because a reader shown thirty defaults checks none of them. Those above are ordered by the strength of the evidence behind them, weakest first. The complete list, with what each is worth and what would replace it, is on the Reference page of CurveNumber. Across all 20 of them, 0 have been acknowledged and 20 have not; the machine readable companion to this report lists every one with its state.
Assumptions with no external source
These values carry the answer and nothing external supports them: some are judgments for which no source exists at all (a judgment with no published source), and others are product thresholds that were reasoned rather than measured (a threshold CurveNumber chose). They are assumptions, recorded as assumptions. The text below is CurveNumber's own statement of each value, verbatim.
Hydrologic condition defaultedFair (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS. Fair is the mid-range assignment and avoids systematic bias. Recorded as a judgment, not a citation, because a user told this is 'the published default' will not think to change it, and a user told it is a mid-range assumption worth 5 to 12 CN units will.
Worth in the answer: 5 to 12 CN units. One of the two largest silent levers in the method.
Displaced by: A site visit, current aerial imagery, or a measured ground cover fraction against the NEH Chapter 9 mapping.
Curve number suppression band, lower defaulted2 multiples of Ia (a threshold CurveNumber chose)
REASONED, NOT MEASURED. Proposed default awaiting calibration.
Worth in the answer: A five CN unit uncertainty is worth a factor of 18.1 at 1.5 Ia, 4.4 at 2 Ia, 2.3 at 3 Ia and 1.4 at 9 Ia.
Displaced by: Expert panel calibration.
Curve number caution band, upper defaulted5 multiples of Ia (a threshold CurveNumber chose)
REASONED, NOT MEASURED.
Worth in the answer: Above this the method is in the regime it was fitted for.
Displaced by: Expert panel calibration.
Impervious connectivity defaulted0 (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS. The conservative assumption.
Worth in the answer: Bounded at 10.2 CN units. Figure 2-4 adjusts the composite as CN = CNp + (Pimp/100)(98 - CNp)(1 - R/2), so the whole span from fully connected to fully unconnected is 0.5 (Pimp/100)(98 - CNp), largest at the 30 percent total impervious ceiling the figure is valid to and at the lowest pervious curve number TR-55 publishes, which is 30: 0.15 x 68 = 10.2 units. At a pervious CN of 61 the same bound is 5.55 units, and at 80 it is 2.7.
Displaced by: A delineated drainage path showing impervious area discharging to pervious ground. Automated detection from LiDAR routing is an open gap.
Layer depths that each practice kind starts from defaulted (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS. The templates follow. Bioretention: 9 in ponding over 18 in of media over 12 in of gravel on 3H:1V sides. Infiltration trench: 48 in of stone with vertical walls and no media and no underdrain. Permeable pavement: 18 in of stone with vertical walls, no ponding and no media. Dry swale: 9 in ponding over 18 in of media over 9 in of gravel on 3H:1V sides. They are the middle of what is commonly drawn and they are not a recommendation. A template is applied only where the user supplies no dimension, and every value it supplies is recorded as defaulted so it prints as an assumption rather than as a design.
Worth in the answer: The template depths are the whole capture answer, because the effective storage depth over the bottom area is what the loading ratio is checked against. The bioretention template gives 20.3 in of water storage over its bottom, the infiltration trench template 19.2 in, the permeable pavement template 7.2 in and the dry swale template 21.1 in, all four on a 1,000 sq ft footprint. A designer who takes the bioretention template and drops the gravel to 6 in loses 2.4 in of that, 11.8 percent of the storage, and the required footprint grows in the same proportion. The permeable pavement figure is the one to read twice: 600 cu ft on that footprint against the bioretention template's 1,167.8 cu ft on the same ground, because a pavement section has no ponding and no media and the stone is all of it.
Displaced by: The section on the drawing. These are starting dimensions for a cell described by its area alone, and every one of them is replaced by a designed profile before anything is stamped.
Effective porosity, engineered bioretention media defaulted0.25 (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS for a design storage credit. Total porosity of a sand based media is measurable and is not the quantity wanted here: what the practice can store and then give back is the drainable fraction between saturation and field capacity, which is smaller than total porosity and depends on the mix. 0.25 is the middle of the 0.20 to 0.40 range that manuals credit and is recorded as a judgment.
Worth in the answer: The media holds 259.4 cu ft of the reference cell's 1,167.8, which is 22.2 percent of the storage the practice is credited with. Moving 0.25 to 0.20, which is the low end of what manuals credit, removes 51.9 cu ft, or 4.4 percent of the total. The whole range in use, 0.20 to 0.40, spans 207.5 cu ft, 17.8 percent of the storage and four times that step, and it lands on the layer whose porosity nobody measures on the job.
Displaced by: A porosity or a water holding capacity measured on the mix actually specified, which the media supplier can produce and almost never is asked for.
Design filtration rate through the engineered media defaulted2 in/hr (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS for a single design figure. Specified media commonly tests between 2 and 8 in/hr when new and falls with age, and manuals variously require a minimum, a maximum, or both. 2 in/hr is the low end of the new condition range and is chosen because a design that works only at the new rate stops working, which is the failure this value exists to prevent.
Worth in the answer: It sets the drawdown of every practice with an underdrain, because the media and not the pipe is the constraint: on the reference cell with a 4 in underdrain the media passes 115.3 cu ft/hr against the pipe's 2,699 cu ft/hr at full stage, a factor of 23.4. Halving it to 1 in/hr takes the drawdown from 7.5 to 13.0 hours, 73 percent longer, and it is halved by nothing more exotic than three years of sediment.
Displaced by: A measured infiltration rate on the installed media, which is the acceptance test some jurisdictions already require, or a long term rate from a maintenance record.
Safety factor applied to the measured native infiltration rate defaulted2 (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS for a national default. Jurisdictional factors that supersede this one run from 2 to 4, and some scale the factor with the number of tests or with the consequence of failure. 2 is the low end and is therefore the least conservative choice consistent with common practice, which is recorded here rather than left to be discovered: a factor of 2 on a single test in a heterogeneous soil is not a safety factor, it is an average of one measurement.
Worth in the answer: The largest single lever in practice sizing. Because storage and infiltration both scale with the bottom area, the drawdown time does not depend on the footprint at all, and the safety factor sets the maximum depth of water a practice may store: at a measured 1.0 in/hr and a 48 hour limit it is 24 in of water at a factor of 2 and 12 in at a factor of 4. On half an acre of pavement at a 1 in treatment depth, 1,815 cu ft, that is a bottom area of 908 sq ft against 1,815 sq ft, exactly double, and 4.2 against 8.3 percent of the drainage area. On the reference cell it is the difference between 40.5 hours, which passes, and 81.1 hours, which CurveNumber refuses.
Displaced by: A factor named by the governing jurisdiction, which supersedes outright. Failing that, the number and spread of the field tests: a factor of 2 on a single boring is a different claim from a factor of 2 on one test per 5,000 sq ft of bottom area.
Fraction of the wetted sidewall area credited with infiltration defaulted0 fraction (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS. Sidewall infiltration is real and the default declines to credit it, for three reasons that are stated because the choice costs the designer footprint. The walls are the surface most likely to be smeared and sealed by the excavator bucket, and nobody tests them. The credit is largest when the practice is fullest, which is exactly the condition the design case checks. And the wetted wall area falls as the practice drains, so a credit taken at full stage flatters the drawdown by more than the area ratio suggests.
Worth in the answer: On the reference cell the walls below the media surface are 320.1 sq ft against a bottom of 691.6, so crediting them adds 46.3 percent to the infiltrating area and takes the drawdown from 40.5 to 30.6 hours. It decides the answer at the margin: at a safety factor of 4 the same cell goes from 81.1 hours, which is refused, to 61.2 hours, which is still refused, and at a measured 0.75 in/hr it moves a 54 hour cell to 41 hours and turns a failure into a pass.
Displaced by: A jurisdictional rule, which several publish in both directions, or a measured rate taken on the excavation wall rather than on the bottom.
Contributing area to practice area ratio above which the practice is flagged defaulted15 (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS for a national figure. The conventional range quoted for bioretention is roughly 5 to 1 up to 15 to 1 of contributing area to filter bed area, and the upper end is where the surface loading rate, not the storage volume, starts to govern: clogging, scour across the media surface and pretreatment capacity are all functions of the ratio, and none of the three is modeled anywhere in CurveNumber. The flag says so instead of implying the model covers it.
Worth in the answer: At 15 to 1 the treatment volume is 15 in of water over the bottom area against the reference cell's 20.3 in of storage, so 74 percent of the practice is spoken for by the design storm alone, with nothing left for the sediment that arrives with it. The flag is worth what pretreatment is worth, which is the difference between a cell that lasts fifteen years and one that clogs in three.
Displaced by: A jurisdictional loading ratio, which several publish, or a designed pretreatment train with its own sizing, which is what the flag is asking for.
Contributing area to practice area ratio above which sizing is refused defaulted20 (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS. 20 to 1 is set at the crossing point computed above for the default bioretention profile, which is where the conventional upper end of 15 to 1 also sits once a margin for sediment and for the frustum loss on the ponding volume is allowed. A practice loaded beyond this is outside what the practice does, and approximating it would produce a capture volume that is arithmetically correct and professionally wrong.
Worth in the answer: This is the ratio at which the arithmetic runs out rather than a preference. The required volume and the available storage both scale with the practice's bottom area, so the ratio at which they are equal is a property of the profile alone: the reference cell stores 20.3 in of water over its bottom, so at 20.3 to 1 the treatment volume fills it to the overflow and no choice of footprint changes that. Above 20 to 1 the practice as profiled cannot hold the volume however it is proportioned, and the only remedies are a deeper profile, a shallower treatment depth or a second practice.
Displaced by: A jurisdictional maximum, which supersedes. A deeper profile moves the crossing point and CurveNumber recomputes it from the practice's own effective storage depth rather than from this number, which is the default ceiling and not the physics.
Routing step to practice response time ratio at which the peak is flagged defaulted0.25 (a threshold CurveNumber chose)
REASONED, NOT MEASURED, and weaker than it looks. The response time is the practice's emptying time at the crest, and it is the right scale for the recession and the wrong one for the peak: the peak is governed by the local time constant dS/dz over dO/dz at the stage the routing actually reaches, which at an overflow weir or just above an orifice invert is orders of magnitude shorter than the emptying time and cannot be known before the routing is run. This ratio is therefore a cheap screen that catches the gross case, and the practice routing additionally routes the same storm at half the step and reports the difference in the peak, which is the check that catches what this one misses.
Worth in the answer: Measured on two 400 cu ft trenches with response times of 0.823 and 3.080 hr, routing a triangular inflow sampled identically at every step so that only the routing changes: the peak discharge runs 0.65 and 3.00 percent below the converged value at this ratio, 2.17 and 6.10 percent below at 0.5, 5.54 and 18.22 percent below at 1.0 and 20.96 and 42.72 percent below at 2.0. The error is a clipped peak and it is one sided, so it is not conservative for a downstream pipe. The volumes are far less sensitive but they are not untouched, and the difference between those two statements matters: the volume BALANCE closes exactly at any step, by construction, while the volume SPLIT shifts a little because how much infiltrates depends on how long the water stands. Across a factor of forty in the step on the first of those trenches, the volume leaving the site moved by 1.5 percent and the infiltrated share by 10 percent of itself, against 2.3 percent on the peak over the same range.
Displaced by: The step halving check the practice routing runs on the case in hand, which measures the sensitivity of this practice and this storm instead of predicting it from a ratio.
Routing step to practice response time ratio at which routing is refused defaulted1 (a threshold CurveNumber chose)
REASONED, NOT MEASURED. Set at the ratio where one step drains the whole practice from its crest, because a recession described by its two endpoints is not a routing, and because that is also where the measured peak error passes a twentieth on the better behaved of the two practices and a fifth on the other. Where the line falls inside that range is a judgment and nothing published says where it is. Refused rather than warned because the peak is what a downstream pipe is sized on.
Worth in the answer: On the same two measurements, the peak discharge is 5.54 and 18.22 percent below the converged value at this ratio and 20.96 and 42.72 percent below at twice it. The spread between the two practices is as large as the error itself, which is the reason this is a screen and not a correction.
Displaced by: Not evidence-displaceable; it is a product policy line.
Cover type, NLCD 41, 42 and 43 forest defaultedWoods, Fair (a judgment with no published source)
NO EXTERNAL SOURCE EXISTS. Classes 41, 42 and 43 all map to woods with the condition left to “Hydrologic condition”. What is lost is real and unrepresentable: evergreen canopy intercepts through the winter and deciduous litter changes with the season, and the method has no term for either.
Worth in the answer: The condition axis carries this mapping and the family does not. Woods Poor 66 against Good 55 on group B is 0.545 in against 0.195 in, a factor of 2.8; on group A it is 45 against 30, which is 0.024 in against exactly zero. Distinguishing deciduous from evergreen is worth nothing in this method, because there is one woods row.
Displaced by: Litter depth, canopy cover and evidence of grazing observed on site, read against the NEH Chapter 9 woods condition descriptions.
Inputs, for recomputation
Existing condition
Area
Area, ac
CN
Impervious, %
Provenance
Curve number basis
Impervious (measured), soil B
0.058
98
100
derived
Read from the datasets; no cover class is named; impervious share measured from Annual NLCD Fractional Impervious Surface (CONUS) (Annual NLCD Fractional Impervious Surface (CONUS), 2024, 30 m, EPSG:5070 native grid) 74 pixels at 30 m retrieved . USDA NRCS TR-55 (1986, 2nd ed.) Table 2-2a, Impervious, paved and roofed, CN 98 for hydrologic soil group B. Read from the public datasets on Sep 26, 2026, 6:36 PM EDT.
Woods, fair condition, soil B, from land cover Deciduous forest
9.704
60
0
derived
Woods, Fair, hydrologic soil group B. USDA NRCS TR-55 (1986, 2nd ed.) Table 2-2c, Woods, fair condition, CN 60 for hydrologic soil group B. Read from the public datasets on Sep 26, 2026, 6:36 PM EDT.
Woods, fair condition, soil B, from land cover Evergreen forest
1.994
60
0
derived
Woods, Fair, hydrologic soil group B. USDA NRCS TR-55 (1986, 2nd ed.) Table 2-2c, Woods, fair condition, CN 60 for hydrologic soil group B. Read from the public datasets on Sep 26, 2026, 6:36 PM EDT.
Woods, fair condition, soil B, from land cover Mixed forest
0.495
60
0
derived
Woods, Fair, hydrologic soil group B. USDA NRCS TR-55 (1986, 2nd ed.) Table 2-2c, Woods, fair condition, CN 60 for hydrologic soil group B. Read from the public datasets on Sep 26, 2026, 6:36 PM EDT.
Proposed condition
Area
Area, ac
CN
Impervious, %
Provenance
Curve number basis
1/2-acre lots (17 homes)
8.500
70
25
enteredCN derived
Residential districts, 1/2 acre lots (25 percent impervious), hydrologic soil group B. USDA NRCS TR-55 (1986, 2nd ed.) Table 2-2a, Residential districts, 1/2 acre lots (25 percent impervious), CN 70 for hydrologic soil group B. Area, cover and soil group typed by CurveNumber Examples on Sep 26, 2026, 5:51 PM EDT.
Streets with curb and gutter
1.400
98
0
enteredCN derived
Streets and roads, paved, curbs and storm sewers (excluding right-of-way), hydrologic soil group B. USDA NRCS TR-55 (1986, 2nd ed.) Table 2-2a, Streets and roads, paved, curbs and storm sewers (excluding right-of-way), CN 98 for hydrologic soil group B. Area, cover and soil group typed by CurveNumber Examples on Sep 26, 2026, 5:52 PM EDT.
Preserved stream buffer woods
2.352
55
0
enteredCN derived
Woods, Good, hydrologic soil group B. USDA NRCS TR-55 (1986, 2nd ed.) Table 2-2c, Woods, good condition, CN 55 for hydrologic soil group B. Area, cover and soil group typed by CurveNumber Examples on Sep 26, 2026, 5:52 PM EDT.
How to read the provenance marks
entereddouble borderA person typed this area: its area, and its cover and soil group where named. Who and when are printed on the row. Where the curve number is the TR-55 table value for that cover and soil group, the row also says the curve number was looked up; the area and the choice of cover are still the person's.editeddouble border, underlinedA person changed the area, cover, soil group or curve number of this area after it was derived, entered or copied. The change, who, when and the reason are printed on the row and in the overrides section.copiedheavy solid borderCopied from the other condition. The row says whether the area it was copied from was read from the datasets or typed by a person.derivedsolid borderRetrieved from a published source. The citation and its edition are printed with the value, because TR-55 1986 and TR-55 1975 disagree and a citation without an edition cannot be checked.defaulteddashed borderAn assumption, not a measurement. CurveNumber's list of default values entry is named, and what the assumption is worth in the answer is stated with it. Where no external source exists for the assumption, this report says so in those words.overriddendouble border, hatched, boldA person changed the value. Who, when, the prior value and the reason given are recorded in the overrides section and cannot be edited afterwards, because the parameter record is append only.supplieddouble border, boldA person gave CurveNumber an argument and it recomputed from it: a measured flow path length in place of the geometric bound, the flow surface the ground actually is, the month the design storm is taken in. Nothing the product had stored was overwritten, which is why this mark is not the override mark; what the argument displaced is CurveNumber's own figure, and the estimates section prints it. Who, when and the reason given are in the overrides section and the record is append only.accepteddouble border, dotted inner rule, boldA person examined a figure a machine produced and adopted it unchanged. It carries a person's mark because a person put it into this record under their own name, and it is not the supplied mark because nothing was handed to anything and nothing recomputed: the magnitude is the machine's own. Two acts carry it. The acceptance of a classification of an aerial photograph, which is a guess about an image and not a measurement of the ground, and which the imagery section explains; and the acceptance of a curve number fitted to a gauged rainfall and discharge record, which is a property of the whole watershed above that gauge and not a measurement of this site, and which the calibration section explains. The mark is also on an area of ground the soil survey does not rate, computed on the most likely soil group a person accepted (an estimate, not a survey rating), and on an impervious area sized by an accepted photograph. Who, when and the reason given are in the overrides section and the record is append only.fitteddashed border, dotted inner ruleCurveNumber fitted this magnitude to an observed record. It carries no person's mark, because no person put it here, and it is not the computed mark, because a computed value is a function of the inputs printed beside it and a fitted one is a function of the inputs AND of conventions: change the baseflow separation and not one input moves while the magnitude moves by curve number units. It is never printed without its interval, and a fitted figure becomes an input to a design only when a named person accepts it, which is a separate act carrying the accepted mark above.
The marks are separated by the word, by the border style and by the fill, in that order of reliability, so this report can be read correctly from a black and white photocopy and from a browser printing with background graphics switched off.
Reproducibility
Item
Value
Report identifier
CNR-0D028D7D81B1
Inputs fingerprint
CNI-1340EB7179DF
Report format version
16
Run identifier
run_04d75482042da989
Computed at
Report document built at
This copy rendered at
CurveNumber version
0.9.0
Times on this report
The site's local time: Eastern Time (America/New_York). Found from the site's location in the U.S. Department of Transportation's time zone boundaries (Bureau of Transportation Statistics, updated Jan 5, 2023), stored with CurveNumber.
Identifier basis
The report identifier is a SHA-256 hash of the run identifier, CurveNumber version, the storm, the method badge, the headline figures and the area table. It excludes the generation time, so re-rendering an unchanged run reproduces the same identifier. IT ALSO EXCLUDES THE ACKNOWLEDGMENT STATE, which is not a property of the run: who has accepted which assumption is a record of what people have done since, it is read afresh every time this document is rendered, and the instant it was read is printed with it in the status line above and under the count on page one. Two printouts bearing this identifier are the same computation over the same inputs. Where their acceptance counts differ, they were read at the two times printed on them and neither figure has moved. The identifier cannot be checked against a printout alone: an edited copy can carry it. To verify a printout, ask the applicant for the published link, which serves the same document from this product. The inputs fingerprint beside the identifier is a hash of the inputs alone, so two runs of the same inputs share it.
Decision support. Every figure in this report requires independent verification, and a licensed professional is responsible for the design. Values marked DERIVED carry a citation with an edition. Values marked DEFAULTED carry an assumption rather than a measurement, and for some of them no external source exists, which CurveNumber's list of default values states on its face and which this report repeats rather than hides. Values marked OVERRIDDEN were changed by a named person, and the prior value and the reason given are printed in the overrides section.
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Steps and their effect
Before the first run (steps 1 to 14). No run existed yet, so these steps were not priced. The first run is priced against the storm it was computed at.
1. Created the site Wooded tract to 1/2-acre lots.
by CurveNumber Examples, Sep 26, 2026, 5:36 PM EDT
2. Set the site boundary at 12.252 ac.
by CurveNumber Examples, Sep 26, 2026, 5:36 PM EDT
3. Derived 4 areas over a 12.252 ac boundary from the published datasets.
by CurveNumber Examples, Sep 26, 2026, 5:38 PM EDT
Steps 4, 5, 6, 7, 12, 13, 14. Estimates CurveNumber recorded for this site
by CurveNumber Examples, Sep 26, 2026, 5:38 PM EDT
Derived temporal distribution at 0.277 of the total depth
Defaulted irrigation water demand at 6,492 cu ft/day
Derived antecedent dry period at 10 days
Defaulted green roof evapotranspiration rate at 0.132 in/day
Computed time of concentration at 0.222 hr
Derived slope of the receiving area at 10.792 percent
Defaulted contributing flow path length at 1036.228 ft
Each is an input some practices and credits use. The report's estimates section says what each rests on and whether this run used it.
8. Added area 1/2-acre lots (17 homes) to the proposed condition, 8.500 ac at CN 70, residential districts, 1/2 acre lots (25 percent impervious).
by CurveNumber Examples, Sep 26, 2026, 5:51 PM EDT
9. Added area Streets with curb and gutter to the proposed condition, 1.400 ac at CN 98, streets and roads, paved, curbs and storm sewers (excluding right-of-way).
by CurveNumber Examples, Sep 26, 2026, 5:52 PM EDT
10. Added area Preserved stream buffer woods to the proposed condition, 2.352 ac at CN 55, woods good.
by CurveNumber Examples, Sep 26, 2026, 5:52 PM EDT
11. Derived 4 areas over a 12.252 ac boundary from the published datasets, replacing 4.
by CurveNumber Examples, Sep 26, 2026, 6:36 PM EDT
15. Ran the site at the 3.57 in 24 hr storm by the curve number method.
by CurveNumber Examples, Sep 26, 2026, 6:38 PM EDT
Effect on the figures: At the 3.57 in storm this run moves runoff depth from 0.575 to 1.182 in, an increase of 105.5 percent, and volume from 25,574 to 52,557 cu ft, an increase of 105.5 percent.
16. Added a bioretention practice, Bioretention basin at the low corner, to the proposed condition, 12,612 cu ft of storage to the crest.
by CurveNumber Examples, Sep 26, 2026, 6:39 PM EDT
Reason given:Adding the 9,000 square foot bioretention basin from the grading plan (example, fictional) at the low corner, with the tested infiltration rate.
Effect on the figures: At the 3.57 in design storm there was no treatment volume required of the proposed practice before this change, and it is now 12,796 cu ft. There was no storage to the crest on the proposed practice before this change, and it is now 12,612 cu ft. There was no infiltrated volume on the proposed practice before this change, and it is now 12,612 cu ft. There was no drawdown on the proposed practice before this change, and it is now 41.86 hr.
17. Ran the site at the 3.57 in 24 hr storm by the curve number method.
by CurveNumber Examples, Sep 26, 2026, 6:40 PM EDT
Effect on the figures: At the 3.57 in storm this run moves runoff depth from 0.575 to 0.760 in, an increase of 32.2 percent, and volume from 25,574 to 33,797 cu ft, an increase of 32.2 percent. The proposed side was routed through the practice, so the change includes the practice and not a change of method.
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This document was produced with CurveNumber, which reads the soils, land cover, slope and design rainfall over a drawn boundary out of the public datasets and records what every figure rests on. The product is described at curvenumber.com. Nothing in this paragraph qualifies the figures above; what qualifies them is on the document itself. Every public dataset CurveNumber reads, with the edition in use, is listed on the data sources page.