|
This copy of report CNR-CF5859FD59A3 is published at https://app.curvenumber.com/p/JiRxJNXc-VgJ069M-b2XmQf0-zseuf5P. 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-CF5859FD59A3 Inputs CNI-F2FBC0738EF8 Computed Sep 26, 2026, 4:07 PM PDT Version CurveNumber 0.9.0 Status PROVISIONAL, read at Sep 26, 2026, 4:23 PM PDT
Pasture to townhomes, Hillsboro, OregonExample 3b: Pacific Northwest rainfall, client Example builder (fictional), jurisdiction Washington County, Oregon. Prepared by CurveNumber Examples, CurveNumber, as entered by the account holder. Located in Washington County, Oregon (boundary centroid at latitude, longitude 45.5210, -123.0185). Site boundary Site boundary 12.874 ac, centroid at latitude 45.521000, longitude -123.018500. North is up. Imagery: USGS The National Map, public domain, flown July 14, 2022. This result rests on
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. 22,169 cu ft [OVERRIDDEN] [SOIL CHOSEN] Increase in runoff volume, existing against proposed, at the 3.72 in design storm: 0.509 ac-ft, 23.0 percent of the existing runoff volume. Runoff depth rises by 0.474 in (23.0 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 2.065 in [OVERRIDDEN] [SOIL CHOSEN] Proposed depth 2.539 in [OVERRIDDEN] Existing volume 96,489 cu ft (2.215 ac-ft) [OVERRIDDEN] [SOIL CHOSEN] Proposed volume 118,658 cu ft (2.724 ac-ft) [OVERRIDDEN] Design storm 3.72 in over 24.0 hr, NRCS Type IA. Peak discharge by the NRCS standard dimensionless unit hydrograph (peak rate factor 484): existing 6.15 cfs; proposed 8.05 cfs. Summary, page 2: what was entered
Time of concentration used: existing 0.228 hr (13.7 min) (CurveNumber's estimate); proposed 0.228 hr (13.7 min) (CurveNumber's estimate for the proposed cover, on the same flow path as the existing estimate). No practice is on this run. Summary, page 3: what a person changed, and why
The full record follows this summary, unchanged: every section, with each figure's source. The full recordWhat this result rests on, in full
Existing and proposed both total 12.874 ac; they differ by 6 sq ft (0.00 percent), which is within the 0.064 ac tolerance and is shown as information, not as a finding. Volumes and peaks are computed over the areas entered: 12.874 ac existing and 12.874 ac proposed. The drawn boundary is 12.874 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 12.874 ac you drew. Your boundary is about 57.9 of those squares. The areas total 12.874 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 D, 0.099 ac of Developed, low intensity (4.8% of that group) was folded into Pasture/hay; both are CN 84, so the site figure does not change. 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 11 assumptions carried this answer and a person has accepted 0 of them. 11 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. 9 of the 11 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 mapThe 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
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
What is claimed about this ground, and on whose evidence
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. The curve number method, chosen by the storm depth rule: Judged area by area on each area's own curve number: P of 3.72 in is above 5 Ia on every area that yields a material share of the runoff. The curve number governs. 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 1.68 in to 2.44 in, against 2.06 in as computed, a factor of 1.4 from end to end. 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.
Storm
Peak dischargeNRCS 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 IA storm distribution, which was chosen for this run; its fit to this region was not assessed. Time of concentration used: existing 0.228 hr (13.7 min) (CurveNumber's estimate); proposed 0.228 hr (13.7 min) (CurveNumber's estimate for the proposed cover, on the same flow path as the existing estimate). Time step 0.025 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. A practice's effect on the peak is shown with the practice, and the note on the distribution table below applies to its routed peaks too.
Distribution: NRCS Type IA, 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 IA 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 fromThis 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.
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. 1 land cover class was 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 D, 0.099 ac of Developed, low intensity (4.8% of that group) was folded into Pasture/hay; both are CN 84, so the site figure does not change. Waiving the fold keeps every class at its own share. Where each source can be read.
Refusals CurveNumber raised on this runCurveNumber raised no refusal on either condition or on a practice during this run. That is a statement about this run and not a warranty: a refusal is raised where a named condition is met, and the conditions CurveNumber tests are the ones in its list of refusals. Both aggregation paths
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 worthCheck these 3 first: slope, a factor of 12.23 on slope across the area; time of concentration, a factor of 7.72 on time of concentration; antecedent dry period, a factor of 4.67 on antecedent dry period. The other 4 move their own figures by a factor of 4.00 or less and are reported rather than put in front of you. RANK 1 HEADLINE Slope of the receiving area derived 4.1 percent Can change the slope across the area by up to 12.2 times. Rests on: USGS 3DEP (1 m DEM, acquired 2024) Slope Degrees raster function retrieved . Median of 2208 sampled pixels, in percent, which is the tangent of the angle and not the angle. The site spans P10 1.37 to P90 16.75 percent, a factor of 12.23, around a median of 4.10. A maximum slope condition is decided conservatively by the P90 and a minimum slope condition by the P10, and no single number is conservative for both, so the median is reported with both beside it. Displaced by: A graded surface from the plan, which is what the credit is actually checked against: the slope that matters is the one the receiving area will have after construction, and a DEM measures the one it has now. USGS 3DEP (1 m DEM, acquired 2024) Slope Degrees raster function retrieved RANK 2 HEADLINE Time of concentration computed 0.228 hr Can change the time of concentration by up to 7.7 times. Rests on: TR-55 segmental: sheet flow over short grass prairie at n = 0.15 for 100 ft, then shallow concentrated flow. Slope 4.10 percent from USGS 3DEP, two year 24 hour depth 2.25 in from NOAA Atlas 2 (1973) Volume X (Oregon), 1973; HDSC and Bureau of Reclamation GIS grids, 2001, stored with CurveNumber (downloaded ), flow length 1060 ft estimated as the longest chord of the drawn boundary, which is not a flow path. The surface is worth a factor of 7.72 on Tc and decides whether the peak discharge is drawn at the TR-55 minimum; the sheet flow cap 1.83; the estimated flow length 1.14. 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 Antecedent dry period derived 14 days Can change the antecedent dry period by up to 4.7 times. Rests on: NOAA NCEI GHCN-Daily (station record through ) station USC00352997, 841 interevent dry periods over to , plus the choice of the 90th percentile, which is a judgment and not a measurement. P50 is 3 days and P90 is 14 days at this station, a factor of 4.67, and on a green roof whose media empties in between the two it is the difference between the whole credit and half of it. Displaced by: A continuous simulation over the local record, which answers the question this percentile is a proxy for: how often the roof is full when a storm arrives, rather than how dry a typical gap is. A jurisdiction that names an antecedent dry period supersedes outright. Failing both, a local gauge record in place of the station this one reads, which is 4.8 km away and is a different point. NOAA NCEI GHCN-Daily (station record through ) station USC00352997, 90th percentile of interevent dry period length retrieved RANK 4 Irrigation water demand defaulted 5,116 cu ft/day Can change the irrigation demand in the governing month by up to 4.0 times. CurveNumber default value (“Plant factor in a landscape water budget”), which is an assumption and not a measurement. RANK 5 Green roof evapotranspiration rate defaulted 0.116 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 6 Contributing flow path length defaulted 1,060 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. RANK 7 Design storm depth derived 3.72 in Can change the design storm depth at a named return period by up to 1.0 times. NOAA Atlas 2 (1973) (Volume X (Oregon), 1973; HDSC and Bureau of Reclamation GIS grids, 2001, stored with CurveNumber (downloaded )) 24-hr duration, 25 year ARI, partial duration series, Oregon: read off the return-period diagram (Figure 6) on the straight line through the 2 year (2.25 in) and 100 year (4.62 in) 24-hr depths, at 0.621 of the way from the 2 year line, as the Atlas directs; those depths read at the grid cell under the site centroid retrieved Design storm return period SUPPLIED 25 yr Supplied by CurveNumber Examples on , and CurveNumber recomputed from it. Nothing was overwritten: this is an argument CurveNumber takes, not a value it produced. It is an override all the same, because a person put it into this report, and it is in the overrides section below with every other one. Reason: Illustrative rule for this example only: the local detention standard is taken to require the 25-year, 24-hour storm. Check the governing standard for a real site. Temporal distribution SERVICE NOT ESTIMATED BECAUSE A DATASET DID NOT ANSWER. An estimate is not made from a service that was not read. Not estimated: the Atlas 14 temporal distribution files gave no single answer for this site. NOAA Atlas 14 does not cover this location (latitude 45.5210, longitude -123.0185). NOAA Atlas 14 covers every state in the contiguous United States except Washington and Oregon, where CurveNumber reads NOAA Atlas 2 (1973) instead, and neither atlas has a depth at this point. It is usually a point offshore. Enter the design depth from your local criteria manual. 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 valuesdual group C/D on map unit Aloha silt loam (1) overridden hydrologic soil group decision recorded; the letter used for each map unit is in the soil map unit table in the datasets section Replaced by a later entry for the same quantity, listed below. This row is history and is not the value this report was computed from. Not recorded against any one area: this override is against the site. Changed by CurveNumber Examples on Sep 26, 2026, 4:05 PM PDT, CurveNumber's default value “Dual hydrologic soil group resolution” Reason given: Drain tile record: The owner's drain tile plan (fictional, dated 1998, for this example) shows 4 inch clay tile at 40 foot spacing under the whole pasture, outletting to the roadside ditch, and the tile was found working on the site walk. Drained, so group C. dual group C/D on map unit Aloha silt loam (1) overridden hydrologic soil group decision recorded; the letter used for each map unit is in the soil map unit table in the datasets section Not recorded against any one area: this override is against the site. Changed by CurveNumber Examples on Sep 26, 2026, 4:05 PM PDT, CurveNumber's default value “Dual hydrologic soil group resolution” Reason given: Drain tile record: The owner's drain tile plan (fictional, dated 1998, for this example) shows 4 inch clay tile at 40 foot spacing under the whole pasture, outletting to the roadside ditch, and the tile was found working on the site walk. Drained, so group C. dual group C/D on map unit Wapato silty clay loam (43) overridden hydrologic soil group decision recorded; the letter used for each map unit is in the soil map unit table in the datasets section Not recorded against any one area: this override is against the site. Changed by CurveNumber Examples on Sep 26, 2026, 4:05 PM PDT, CurveNumber's default value “Dual hydrologic soil group resolution” Reason given: Observed on site: The Wapato soil is the low swale along the south edge, which the tile plan does not reach and which held standing water on the site walk. Undrained, so group D. Design storm return period supplied set to 25 yr A person supplied this input and CurveNumber recomputed the estimate from it. Nothing was overwritten: the figure it displaced is CurveNumber’s own, and it is printed with the estimates above. Not recorded against any one area: this override is against the site. Supplied by CurveNumber Examples on Sep 26, 2026, 4:06 PM PDT Reason given: Illustrative rule for this example only: the local detention standard is taken to require the 25-year, 24-hour storm. Check the governing standard for a real site. Records that are not overrides. The product wrote these when a condition was copied, or they are the design of a practice. Proposed condition seeded from existing by CurveNumber Examples on Sep 26, 2026, 4:06 PM PDT: 12.874 ac copied. Nothing copied has been edited since, so the surfaces are the same in both conditions and any change is the practice. The aerial photograph, and what was read off itNo 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 aboutNo 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 resolvedNo 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 waivedNo 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 answerHydrologic condition defaulted Fair (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 defaulted 2 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 defaulted 5 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 defaulted 0 (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. Dual hydrologic soil group resolution defaulted the undrained letter (D) unless drainage is attested (a judgment with no published source; low confidence) PENDINGno acceptance recorded A person replaced this default on part of the site (CurveNumber Examples, Sep 26, 2026, 4:05 PM PDT); see the overrides section. It still carries the answer on the rest of the site, so it is counted above. Worth in the answer: Enormous. Open space in good condition on A/D is CN 39 drained against CN 80 undrained. At P = 3 in, pasture on B against D is 0.365 in against 1.250 in, a factor of 3.42. Displaced by: User attestation that artificial drainage is present, or a drain tile record. Cover type, NLCD 21 developed open space defaulted Open space, Fair, on the pervious part (a judgment with no published source; moderate confidence) PENDINGno acceptance recorded Worth in the answer: Against using the TR-55 residential 1 acre lot row as the pervious cover, which already contains 20 percent impervious, group B composite is 65.0 against 71.3 at the measured 10.9 percent impervious, and 0.507 in against 0.773 in, 52 percent more runoff. The family choice itself is nearly free, because the open space and pasture rows are identical digit for digit; the condition axis is not, at 69 against 61 on group B and 0.670 in against 0.365 in. Displaced by: Site imagery or a site visit establishing the condition of the pervious cover, or a high resolution land cover product that separates turf from tree canopy. 5 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 11 of them, 0 have been acknowledged and 11 have not; the machine readable companion to this report lists every one with its state. Assumptions with no external sourceThese 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 defaulted Fair (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 defaulted 2 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 defaulted 5 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 defaulted 0 (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. Dual hydrologic soil group resolution defaulted the undrained letter (D) unless drainage is attested (a judgment with no published source) NO EXTERNAL SOURCE EXISTS. The conservative choice, matching the physical condition absent evidence of drainage. Worth in the answer: Enormous. Open space in good condition on A/D is CN 39 drained against CN 80 undrained. At P = 3 in, pasture on B against D is 0.365 in against 1.250 in, a factor of 3.42. Displaced by: User attestation that artificial drainage is present, or a drain tile record. Cover type, NLCD 21 developed open space defaulted Open space, Fair, on the pervious part (a judgment with no published source) NO EXTERNAL SOURCE EXISTS. Class 21 is vegetation on lots under 20 percent impervious: parks, golf courses, large lot residential, roadside verge. TR-55 open space, described as lawns, parks, golf courses and cemeteries, is the row that names that vegetation. Maps to open space with the condition left to “Hydrologic condition”, which deliberately does not follow TR-55's own assumption that urban pervious area is in good condition. Worth in the answer: Against using the TR-55 residential 1 acre lot row as the pervious cover, which already contains 20 percent impervious, group B composite is 65.0 against 71.3 at the measured 10.9 percent impervious, and 0.507 in against 0.773 in, 52 percent more runoff. The family choice itself is nearly free, because the open space and pasture rows are identical digit for digit; the condition axis is not, at 69 against 61 on group B and 0.670 in against 0.365 in. Displaced by: Site imagery or a site visit establishing the condition of the pervious cover, or a high resolution land cover product that separates turf from tree canopy. Cover type, NLCD 22 developed low intensity defaulted Open space, Fair, on the pervious part (a judgment with no published source) NO EXTERNAL SOURCE EXISTS. Maps to open space for the PERVIOUS remainder only, with the measured impervious fraction carried as its own area. The composite residential rows are not used at all, because they are a class midpoint for imperviousness with a cover assumption stapled to it. Worth in the answer: 9.2 CN units on group B against the double counted alternative. The residential 1/4 acre row is exactly 0.38 x 98 + 0.62 x 61 = 75.06, so it already holds its impervious share; using it as the pervious cover and adding the measured 34.6 percent impervious as its own area gives 83.0 where the correct composite is 73.8, and 1.444 in against 0.898 in, 61 percent high. Both sides are on TR-55's own good condition assumption for urban pervious area, so the figure isolates the double counting from the condition choice; under the defaulted fair condition it is 3.9 CN units and 21.5 percent. Preferring the class midpoint of 38 percent to the measured 34.6 is worth a further 1.3 CN units and 7.3 percent. Displaced by: A high resolution land cover product that resolves roofs and driveways, or a site plan giving the actual lot coverage. Cover type, NLCD 81 pasture and hay defaulted Pasture, grassland or range, Fair (a judgment with no published source) NO EXTERNAL SOURCE EXISTS, though this is the closest thing to a name match in the crosswalk. NLCD merges grazed pasture with hay and TR-55 separates them: meadow is protected from grazing and mowed, and is published as a single Good condition row. Maps to pasture with the condition left to “Hydrologic condition”, because choosing meadow would assert a good condition as well as a cover type. Worth in the answer: 11 CN units on group B against the meadow row: pasture Fair 69 against meadow 58, which is 0.670 in against 0.274 in, a factor of 2.4. Displaced by: Whether the field is grazed or cut for hay, which the operator knows and no raster records. Cover type, NLCD 23 developed medium intensity defaulted Open space, Fair, on the pervious part (a judgment with no published source) NO EXTERNAL SOURCE EXISTS. Maps to open space for the pervious remainder. At this intensity the remnant is a third of the area and is more often compacted than established, which argues for Poor rather than the defaulted Fair, at 79 against 69 on group B. Worth in the answer: 8.1 CN units on group B against reading the 1/8 acre lot row as the pervious cover and adding the measured 66.4 percent impervious on top: 93.6 against 85.6, and 2.314 in against 1.630 in, 42 percent high. Class midpoint against measured imperviousness is small on this class, 0.55 CN units and 2.5 percent at the defaulted fair condition, because 64.5 and the measured 66.4 nearly agree. Displaced by: A high resolution land cover product, or a site plan. The pervious condition is displaced by imagery showing whether the remnant is established turf or compacted fill. Inputs, for recomputationExisting condition
Proposed condition
How to read the provenance marksentereddouble 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
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. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||