Calculator Inputs
Use the polygon, grid, or traced-map method. The responsive form displays three columns on large screens, two on smaller screens, and one on mobile.
Example Data Table
| Parameter | Example Value | Notes |
|---|---|---|
| Method | Coordinate Polygon | Use boundary points in meters. |
| Coordinates | (0,0), (520,0), (780,250), (610,640), (180,520) | Five vertices define the catchment polygon. |
| Rainfall Depth | 110 mm | Storm depth over the watershed. |
| Runoff Coefficient | 0.48 | Represents rainfall converted to direct runoff. |
| Main Channel Length | 1450 m | Used for width, slope, and time estimates. |
| Elevation Drop | 84 m | Outlet-to-headwater relief. |
| Computed Area | 339,350 m² | Equivalent to 33.935 ha. |
| Computed Perimeter | 2,302.838 m | Derived from polygon side lengths. |
| Runoff Volume | 17,917.680 m³ | Based on rainfall depth and runoff coefficient. |
| Time of Concentration | 15.845 min | Estimated with the Kirpich relation. |
Formula Used
1) Coordinate polygon area: A = 0.5 × |Σ(xiyi+1 - yixi+1)|
2) Polygon perimeter: P = Σ √[(xi+1-xi)² + (yi+1-yi)²]
3) Grid method area: A = (Nfull + w × Npartial) × acell
4) Trace method area: Aground = Amap × (scale / 100)², where map area is in cm² and ground area is in m².
5) Runoff volume: V = C × P × A, where C is runoff coefficient, P is rainfall depth in meters, and A is area in m².
6) Average width: W = A / L
7) Watershed slope: S = H / L and S% = (H / L) × 100
8) Time of concentration: Tc = 0.01947 × L0.77 × S-0.385
9) Form factor: Ff = A / L²
10) Compactness coefficient: Cc = P / [2 × √(πA)]
11) Drainage density: Dd = Lstreams / A
12) Rational peak flow: Q = 0.278 × C × I × A, with I in mm/hr and A in km².
How to Use This Calculator
Choose the method matching your source data. Use the coordinate method for surveyed or GIS-style boundary points. Use the grid method when estimating area from counted cells. Use the trace method when you know the traced map area and map scale.
Enter rainfall depth and runoff coefficient to estimate direct runoff volume. Add rainfall intensity for a rational-method peak runoff estimate. Enter main channel length and elevation drop when you also want slope, average width, and time of concentration.
Press the calculate button. The result section appears above the form and below the header. Review the summary cards, detailed result table, and Plotly graph. Use the export buttons to save the result table as CSV or PDF.
For best accuracy, use consistent units, close the polygon properly, verify scale values, and avoid rough partial-cell estimates when surveyed coordinates are available.
FAQs
1) What is a watershed area?
A watershed area is the land surface draining runoff to one outlet. It controls runoff volume, timing, storage behavior, and hydraulic design decisions.
2) Which method is the most accurate?
The coordinate polygon method is usually the most accurate when boundary points come from surveying, CAD, GIS, or digitized mapping.
3) When should I use the grid method?
Use the grid method when you estimate catchment size from gridded mapping sheets or transparent overlays and do not have traced coordinates.
4) Why does runoff volume depend on runoff coefficient?
The runoff coefficient represents losses from infiltration, storage, and surface retention. Higher coefficients convert more rainfall into direct runoff volume.
5) What does time of concentration mean?
Time of concentration is the travel time for runoff from the hydraulically most distant point to reach the outlet.
6) Why is perimeter useful in watershed work?
Perimeter supports compactness analysis and helps describe basin shape. Shape strongly affects peak response and hydrograph concentration.
7) Can I use feet or kilometers for coordinates?
Yes. The calculator converts meters, kilometers, and feet internally, then reports standard engineering output units consistently.
8) Are these outputs enough for final design?
They are useful screening and planning values. Final stormwater or hydrology design should also include field checks, loss modeling, and local standards.