NRCS Runoff Calculator
Runoff depth and volume by the NRCS curve number method, and the TR-55 graphical peak discharge, with rainfall from NOAA Atlas 14.
Results
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- Runoff equation
- Q = (P − Ia)2 / (P − Ia + S) for P > Ia, otherwise 0, with S = 1000/CN − 10 and Ia = 0.2S (NRCS TR-55, Eqs. 2-1 to 2-4). Depths are in inches; volume = Q × area.
- Weighted curve number
- Subarea curve numbers are weighted by area, as on TR-55 Worksheet 2. TR-55 adds that the CN procedure is less accurate when runoff is under 0.5 inch and should not be used when the weighted CN is under 40; both are flagged.
- Antecedent runoff condition
- TR-55 tables are for condition II (average). Condition I (dry) and III (wet) come from NRCS NEH Part 630 Chapter 10 Table 10-1, interpolated between the listed curve numbers.
- Graphical Peak Discharge
- qp = qu Am Q Fp (TR-55 Eq. 4-1), with the unit peak qu from the Appendix F coefficients for the rainfall type and Ia/P, Am in square miles and the pond and swamp factor Fp from Table 4-2.
- Rainfall from NOAA Atlas 14
- For a Texas location the 24-hour depth comes from NOAA Atlas 14 Volume 11 (partial-duration series), and runoff is listed for every return period.
NRCS TR-55 (1986) runoff curve numbers for antecedent runoff condition II and Ia = 0.2S, by hydrologic soil group. Type a value into the calculator; the hydrologic soil group for a Texas site is on the Texas Soil Screening Map.
Table 2-2a — Urban areas
| Cover | Condition | Impervious | A | B | C | D |
|---|---|---|---|---|---|---|
| Fully developed urban areas — open space (lawns, parks, golf courses, cemeteries, etc.): Poor condition (grass cover < 50%) | poor | — | 68 | 79 | 86 | 89 |
| Fully developed urban areas — open space (lawns, parks, golf courses, cemeteries, etc.): Fair condition (grass cover 50% to 75%) | fair | — | 49 | 69 | 79 | 84 |
| Fully developed urban areas — open space (lawns, parks, golf courses, cemeteries, etc.): Good condition (grass cover > 75%) | good | — | 39 | 61 | 74 | 80 |
| Fully developed urban areas — impervious areas: Paved parking lots, roofs, driveways, etc. (excluding right-of-way) | — | — | 98 | 98 | 98 | 98 |
| Fully developed urban areas — streets and roads: Paved; curbs and storm sewers (excluding right-of-way) | — | — | 98 | 98 | 98 | 98 |
| Fully developed urban areas — streets and roads: Paved; open ditches (including right-of-way) | — | — | 83 | 89 | 92 | 93 |
| Fully developed urban areas — streets and roads: Gravel (including right-of-way) | — | — | 76 | 85 | 89 | 91 |
| Fully developed urban areas — streets and roads: Dirt (including right-of-way) | — | — | 72 | 82 | 87 | 89 |
| Western desert urban areas: Natural desert landscaping (pervious areas only) | — | — | 63 | 77 | 85 | 88 |
| Western desert urban areas: Artificial desert landscaping (impervious weed barrier, desert shrub with 1- to 2-inch sand or gravel mulch and basin borders) | — | — | 96 | 96 | 96 | 96 |
| Urban districts: Commercial and business | — | 85% | 89 | 92 | 94 | 95 |
| Urban districts: Industrial | — | 72% | 81 | 88 | 91 | 93 |
| Residential districts by average lot size: 1/8 acre or less (town houses) | — | 65% | 77 | 85 | 90 | 92 |
| Residential districts by average lot size: 1/4 acre | — | 38% | 61 | 75 | 83 | 87 |
| Residential districts by average lot size: 1/3 acre | — | 30% | 57 | 72 | 81 | 86 |
| Residential districts by average lot size: 1/2 acre | — | 25% | 54 | 70 | 80 | 85 |
| Residential districts by average lot size: 1 acre | — | 20% | 51 | 68 | 79 | 84 |
| Residential districts by average lot size: 2 acres | — | 12% | 46 | 65 | 77 | 82 |
| Developing urban areas: Newly graded areas (pervious areas only, no vegetation) | — | — | 77 | 86 | 91 | 94 |
Table 2-2b — Cultivated agricultural lands
| Cover | Condition | Impervious | A | B | C | D |
|---|---|---|---|---|---|---|
| Fallow: Bare soil | — | — | 77 | 86 | 91 | 94 |
| Fallow: Crop residue cover (CR) | poor | — | 76 | 85 | 90 | 93 |
| Fallow: Crop residue cover (CR) | good | — | 74 | 83 | 88 | 90 |
| Row crops: Straight row (SR) | poor | — | 72 | 81 | 88 | 91 |
| Row crops: Straight row (SR) | good | — | 67 | 78 | 85 | 89 |
| Row crops: SR + CR | poor | — | 71 | 80 | 87 | 90 |
| Row crops: SR + CR | good | — | 64 | 75 | 82 | 85 |
| Row crops: Contoured (C) | poor | — | 70 | 79 | 84 | 88 |
| Row crops: Contoured (C) | good | — | 65 | 75 | 82 | 86 |
| Row crops: C + CR | poor | — | 69 | 78 | 83 | 87 |
| Row crops: C + CR | good | — | 64 | 74 | 81 | 85 |
| Row crops: Contoured & terraced (C&T) | poor | — | 66 | 74 | 80 | 82 |
| Row crops: Contoured & terraced (C&T) | good | — | 62 | 71 | 78 | 81 |
| Row crops: C&T + CR | poor | — | 65 | 73 | 79 | 81 |
| Row crops: C&T + CR | good | — | 61 | 70 | 77 | 80 |
| Small grain: Straight row (SR) | poor | — | 65 | 76 | 84 | 88 |
| Small grain: Straight row (SR) | good | — | 63 | 75 | 83 | 87 |
| Small grain: SR + CR | poor | — | 64 | 75 | 83 | 86 |
| Small grain: SR + CR | good | — | 60 | 72 | 80 | 84 |
| Small grain: Contoured (C) | poor | — | 63 | 74 | 82 | 85 |
| Small grain: Contoured (C) | good | — | 61 | 73 | 81 | 84 |
| Small grain: C + CR | poor | — | 62 | 73 | 81 | 84 |
| Small grain: C + CR | good | — | 60 | 72 | 80 | 83 |
| Small grain: Contoured & terraced (C&T) | poor | — | 61 | 72 | 79 | 82 |
| Small grain: Contoured & terraced (C&T) | good | — | 59 | 70 | 78 | 81 |
| Small grain: C&T + CR | poor | — | 60 | 71 | 78 | 81 |
| Small grain: C&T + CR | good | — | 58 | 69 | 77 | 80 |
| Close-seeded or broadcast legumes or rotation meadow: Straight row (SR) | poor | — | 66 | 77 | 85 | 89 |
| Close-seeded or broadcast legumes or rotation meadow: Straight row (SR) | good | — | 58 | 72 | 81 | 85 |
| Close-seeded or broadcast legumes or rotation meadow: Contoured (C) | poor | — | 64 | 75 | 83 | 85 |
| Close-seeded or broadcast legumes or rotation meadow: Contoured (C) | good | — | 55 | 69 | 78 | 83 |
| Close-seeded or broadcast legumes or rotation meadow: Contoured & terraced (C&T) | poor | — | 63 | 73 | 80 | 83 |
| Close-seeded or broadcast legumes or rotation meadow: Contoured & terraced (C&T) | good | — | 51 | 67 | 76 | 80 |
Table 2-2c — Other agricultural lands
| Cover | Condition | Impervious | A | B | C | D |
|---|---|---|---|---|---|---|
| Pasture, grassland, or range — continuous forage for grazing | poor | — | 68 | 79 | 86 | 89 |
| Pasture, grassland, or range — continuous forage for grazing | fair | — | 49 | 69 | 79 | 84 |
| Pasture, grassland, or range — continuous forage for grazing | good | — | 39 | 61 | 74 | 80 |
| Meadow — continuous grass, protected from grazing and generally mowed for hay | — | — | 30 | 58 | 71 | 78 |
| Brush — brush-weed-grass mixture with brush the major element | poor | — | 48 | 67 | 77 | 83 |
| Brush — brush-weed-grass mixture with brush the major element | fair | — | 35 | 56 | 70 | 77 |
| Brush — brush-weed-grass mixture with brush the major element | good | — | 30 | 48 | 65 | 73 |
| Woods — grass combination (orchard or tree farm): Woods-grass combination | poor | — | 57 | 73 | 82 | 86 |
| Woods — grass combination (orchard or tree farm): Woods-grass combination | fair | — | 43 | 65 | 76 | 82 |
| Woods — grass combination (orchard or tree farm): Woods-grass combination | good | — | 32 | 58 | 72 | 79 |
| Woods | poor | — | 45 | 66 | 77 | 83 |
| Woods | fair | — | 36 | 60 | 73 | 79 |
| Woods | good | — | 30 | 55 | 70 | 77 |
| Farmsteads — buildings, lanes, driveways, and surrounding lots | — | — | 59 | 74 | 82 | 86 |
Table 2-2d — Arid and semiarid rangelands
| Cover | Condition | Impervious | A | B | C | D |
|---|---|---|---|---|---|---|
| Herbaceous — mixture of grass, weeds, and low-growing brush, with brush the minor element | poor | — | — | 80 | 87 | 93 |
| Herbaceous — mixture of grass, weeds, and low-growing brush, with brush the minor element | fair | — | — | 71 | 81 | 89 |
| Herbaceous — mixture of grass, weeds, and low-growing brush, with brush the minor element | good | — | — | 62 | 74 | 85 |
| Oak-aspen — mountain brush mixture of oak brush, aspen, mountain mahogany, bitter brush, maple, and other brush | poor | — | — | 66 | 74 | 79 |
| Oak-aspen — mountain brush mixture of oak brush, aspen, mountain mahogany, bitter brush, maple, and other brush | fair | — | — | 48 | 57 | 63 |
| Oak-aspen — mountain brush mixture of oak brush, aspen, mountain mahogany, bitter brush, maple, and other brush | good | — | — | 30 | 41 | 48 |
| Pinyon-juniper — pinyon, juniper, or both; grass understory | poor | — | — | 75 | 85 | 89 |
| Pinyon-juniper — pinyon, juniper, or both; grass understory | fair | — | — | 58 | 73 | 80 |
| Pinyon-juniper — pinyon, juniper, or both; grass understory | good | — | — | 41 | 61 | 71 |
| Sagebrush with grass understory | poor | — | — | 67 | 80 | 85 |
| Sagebrush with grass understory | fair | — | — | 51 | 63 | 70 |
| Sagebrush with grass understory | good | — | — | 35 | 47 | 55 |
| Desert shrub — major plants include saltbush, greasewood, creosotebush, blackbrush, bursage, palo verde, mesquite, and cactus | poor | — | 63 | 77 | 85 | 88 |
| Desert shrub — major plants include saltbush, greasewood, creosotebush, blackbrush, bursage, palo verde, mesquite, and cactus | fair | — | 55 | 72 | 81 | 86 |
| Desert shrub — major plants include saltbush, greasewood, creosotebush, blackbrush, bursage, palo verde, mesquite, and cactus | good | — | 49 | 68 | 79 | 84 |
Average percent impervious area as listed in table 2-2a (urban districts and residential lots). Table 2-2c: actual curve number is less than 30; use CN = 30 for runoff computations. Table 2-2d: curve numbers for group A have been developed only for desert shrub. Source: USDA NRCS, Urban Hydrology for Small Watersheds, TR-55, 2nd ed., June 1986, tables 2-2a, 2-2b, 2-2c, 2-2d. U.S. Government work (public domain).
The calculator is tested against the cases below every time its code changes.
| Case | What | Reference | Values compared | Largest difference | Result |
|---|---|---|---|---|---|
| The table loads with the calculator. The same results are in verification.json. | |||||
Weighted CN, runoff and the graphical peak are compared with NRCS TR-55 Examples 2-1 and 4-1 to the precision they print (the unit peak qu in the example is read from a chart, so 3%); the rest are hand calculations of the published equations.
- Standard Ia = 0.2S only. The Ia = 0.05S alternative needs remapped curve numbers and is not included.
- The graphical peak is for one hydrologically homogeneous watershed with Tc from 0.1 to 10 hours, a weighted CN over 40, and the TR-55 Type II or III rainfall distribution; no hydrograph and no routing through storage. Fp applies only to ponds and swamps that are not in the Tc flow path.
- Curve numbers come from you or the TR-55 tables. The Texas Curve Number Screening Map gives screening values from land cover and soils for a site.
- What is a curve number?
- An index from 0 to 100 that combines land cover, soil group and condition to estimate how much rain becomes runoff: the higher the number, the more runoff. Impervious surfaces are about 98.
- Which hydrologic soil group is my site?
- Groups A to D rank soils from high to low infiltration. The Texas Soil Screening Map shows the NRCS SSURGO group for a Texas site.
- Why is there no runoff for small storms?
- Rain up to the initial abstraction Ia = 0.2S is held by interception, infiltration and surface storage before runoff begins. For CN 70, Ia is about 0.86 in.
- Can I attach the calculation sheet to a design report?
- You can print it and include it as a supporting calculation that you have checked. It is a calculation aid, not a sealed engineering document: the engineer of record reviews it and is responsible for the design, and the criteria of the reviewing agency decide which methods and values apply.
| Topic | Reference |
|---|---|
| Runoff and graphical peak | USDA NRCS (1986). Urban Hydrology for Small Watersheds, Technical Release 55, 2nd ed., Chapters 2 and 4, Appendix F. |
| Direct runoff | USDA NRCS (2004). National Engineering Handbook Part 630, Chapter 10: Estimation of Direct Runoff from Storm Rainfall. |
| Rainfall | Perica, S., et al. (2018). NOAA Atlas 14, Volume 11 Version 2.0: Texas. NOAA, National Weather Service. |
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A free calculation aid. This calculator applies the published methods named on this page to the inputs you enter. It is not a sealed engineering document, has not been reviewed by a licensed professional for your project, and no warranty is expressed or implied. Check the results independently before using them for design, bidding or permitting.
The criteria of the reviewing agency decide which methods and values apply to a design. The engineer of record is responsible for the design.
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