Calculator Inputs
Example Data Table
| Area name | Area | Cover | Soil group | Impervious % | Expected role |
|---|---|---|---|---|---|
| Residential lawns | 8.00 | Open space, fair condition | B | 10 | Moderate runoff source |
| Roofs and drives | 2.00 | Paved parking, roofs, drives | B | 100 | High runoff source |
| Woods buffer | 5.00 | Woods, good condition | B | 0 | Lower runoff source |
Formula Used
Weighted curve number:
CNw = Σ Area × CN / Σ Area
Potential maximum retention:
S = 1000 / CNw - 10
Initial abstraction:
Ia = λS
Runoff depth:
Q = (P - Ia)² / (P + S - Ia), when P is greater than Ia.
If rainfall is not greater than initial abstraction, Q is zero.
Dry and wet condition adjustments use common curve number conversion equations. These convert average condition values into approximate dry or wet values.
How to Use This Calculator
- Enter the project name and storm rainfall depth.
- Choose the initial abstraction ratio. The common value is 0.20.
- Select dry, average, or wet antecedent runoff condition.
- Add each land cover area in a separate row.
- Select the land cover type and hydrologic soil group.
- Use manual CN override only when your study requires it.
- Enter connected impervious percentage for each area.
- Press the calculate button and review the result above the form.
- Download CSV or PDF output for reporting.
TR-55 Curve Number Planning Guide
Why Curve Number Matters
TR-55 curve number work helps estimate direct runoff from a storm. The method is common in small watershed design. It links land cover, soil behavior, and rainfall into one practical index. A higher curve number means less infiltration. It also means more runoff. A lower value suggests better storage and slower response.
Weighted Watershed Inputs
Real sites rarely use one land cover. A basin may include lawns, roofs, woods, drives, and open fields. This calculator accepts several area rows. Each row can use a manual curve number or a selected land cover value. The final curve number is area weighted. Large areas influence the result more than small patches.
Hydrologic Soil Groups
Soil group selection supports better screening. Group A soils drain quickly. Group D soils drain slowly. Groups B and C sit between those limits. Land cover quality also matters. Good grass or woods can reduce runoff. Poor condition can raise it. Connected impervious area usually creates the fastest response.
Runoff Depth Review
After the weighted curve number is found, the tool estimates potential retention. It also calculates initial abstraction. That abstraction represents early losses before runoff begins. These losses can include interception, surface storage, and early infiltration. When rainfall is below abstraction, estimated runoff is zero.
Practical Use
Use this page for planning, comparison, and report preparation. Try several development scenarios. Compare existing and proposed cover mixes. Export the result for records. Check every input before design use. Local rules may require adjusted rainfall, approved curve tables, or separate modeling steps. The calculation is useful, but it should not replace professional judgment.
Reporting Tips
Keep all area units consistent. Acres are common for TR-55 studies. Hectares also work when every row uses hectares. Do not mix units in one run. Use notes to record assumptions. Include source maps, soil survey references, and field observations. When impervious cover is treated with special drainage controls, describe that treatment clearly.
Model Limits
The method gives event runoff depth, not a full hydrograph. Time of concentration, peak discharge, and routing need additional steps. Snowmelt, frozen ground, and unusual urban drainage can need other methods. Use the result as a starting point. Then verify it with local hydrology guidance.
FAQs
What is a TR-55 curve number?
It is an index that represents runoff potential. It combines soil group, land cover, treatment, and condition into one value.
Can I use acres and hectares?
Yes. Use one area unit consistently. The weighted curve number works with any area unit when all rows use the same unit.
What does a higher curve number mean?
A higher value means greater runoff potential. It usually shows lower infiltration, more impervious cover, or poorer soil drainage.
What is hydrologic soil group D?
Group D has slow infiltration and high runoff potential. Clayey soils and shallow restrictive layers often fall in this group.
Should I enter manual curve numbers?
Use manual values when a local table, approved study, or agency standard gives a specific curve number for your site.
What is initial abstraction?
Initial abstraction represents early rainfall losses. These include interception, depression storage, and infiltration before runoff starts.
Does this calculate peak discharge?
No. This calculator estimates weighted curve number and runoff depth. Peak discharge needs time of concentration and hydrograph steps.
Can this replace a drainage report?
No. It supports planning and checking. Final designs should follow local standards and be reviewed by a qualified professional.