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Water · Click an outlet

Catchment area calculator for culverts, draws and streams

This catchment area calculator maps all the land upstream of a point you click, a culvert inlet, the mouth of a draw or a stream crossing, and reports the contributing area in acres. It walks the drainage directions of the USGS 3DEP lidar model back up from the outlet.

Search results for “catchment calculator” are mostly roof rainwater tools. This is the ground catchment, also called the drainage area, which is the number culvert, pond and detention sizing starts from.

Free to run · pay to download

See the full result on the map free, with no account. Download the catchment grid as a GeoTIFF with a pass: US$29 for 24 hours, US$99 for 30 days or US$399 for 12 months, a one-off payment in US dollars that never renews. Every download on the site is included while the pass lasts.

Open Catchment on the map →Pass prices and what’s included

Who uses this

  • Engineers and designers. Get the contributing area for culvert and stormwater sizing.
  • Farmers and ranchers. Size a culvert or a stock pond by the land that drains to it.
  • Owners. See whether runoff comes from a neighbor’s property or the road.

When to use it

  • Sizing a culvert, a pond or a detention basin by the area draining to it.
  • Seeing whether runoff onto your land comes from a neighbor’s property or the road.
  • Checking how much land drains to a low corner or a stream crossing.

Tools for your job

Catchment is one step in a bigger question. These are the jobs it helps with, and the tools that go with it.

For the job: Drainage, flooding and wet ground

Where does the water go, and where will it sit?

  1. 1.Water & Drainage — The drainage channels across an area, and where water ponds once low spots fill.
  2. 2.Wetness Map — Which parts of a field the shape of the land keeps wet.
  3. 3.Catchment (this tool) — All the land that drains to one culvert, draw or low corner.
  4. 4.Landforms — Hollows, valleys and footslopes at a glance: where water gathers.

How to use it

  1. Open Catchment and click the outlet: the culvert inlet, the mouth of a draw or the point on the stream you care about.
  2. Set the terrain search radius: 1.24 mi by default, up to 3.11 mi.
  3. Leave outlet snapping on so a click on a bank still lands on the channel, or set it to Off to use the exact cell.
  4. Run. The blue area drains to the outlet. If it touches the edge of the loaded terrain, increase the radius and run again.

Step by step: Mount Hood

We find all the land that drains to a culvert where a creek crosses the road below Timberline Lodge.

1.Open Catchment. Leave outlet snapping on and click where the water leaves: a culvert inlet, the mouth of a draw or a low corner.

2.Run the analysis. The panel shows how far the outlet was moved to the nearest channel, and the area that drains to it.

What it showed

  • If the blue area touches the edge of the loaded terrain, increase the search radius and run again.
Try it yourself on Mount Hood →

How it works

A square of USGS 3DEP bare-earth terrain is loaded around the click. Small pits are filled, each cell is assigned its steepest downhill neighbor, and the tool walks back up those directions from the outlet, collecting every cell that eventually drains into it.

With snapping on, the tool first looks within the chosen distance for the cell that collects the most upstream water, and moves the outlet there if it carries at least ten times what the clicked cell does. The moved outlet is marked on the map and the distance reported.

Reading the results

Contributing area is given in acres, or square feet for a small catchment.

A partial catchment warning means the true catchment extends beyond the loaded terrain, so the area is a lower bound.

Limits and caveats

  • Terrain is loaded to 3.11 mi at most, so a river’s catchment will always be reported as partial.
  • Culverts, pipes and levees are not in the terrain. Water is routed over roads and through embankments.
  • On flat valley floors a fraction of an inch decides which way water goes, and the boundary can look arbitrary.

Data and attribution

Results come from the USGS 3D Elevation Program (3DEP) bare-earth elevation model: lidar at 3.3 ft per pixel where it has been flown, and the 1/3 arc-second model, about 33 ft per pixel, elsewhere, with elevations above NAVD 88. The tools with a surface option (Sunlight, Viewshed, Sightline, Object Heights) read DOGAMI’s statewide digital surface model of trees and buildings.

USGS 3DEP elevation data and USGS maps are U.S. Government works in the public domain: you may use the results, including commercially. A credit to the USGS 3D Elevation Program is appreciated, and every file you download carries it. DOGAMI lidar products are published by the State of Oregon; credit "DOGAMI / Oregon Lidar Consortium". County taxlot boundaries are the assessor's GIS data, not surveys, and each county publishes its own disclaimer, shown with the parcel. The aerial basemap is from Esri and follows Esri's terms; it is not included in downloads.

Questions

Does this size the culvert for me?

No. It gives the drainage area, which is the first input to a culvert or pond design. Rainfall intensity, runoff coefficients and pipe capacity come next, usually from an engineer or your county’s road standards.

Why does it say “partial catchment”?

The catchment reaches the edge of the terrain loaded, up to 3.11 mi, so the true area is larger. Increase the radius, or accept that a river’s catchment is too big for this tool.

Ready to try it?

Open Catchment on the map →

Free to use, no account. The analysis runs in your browser on public-domain USGS 3DEP data. Downloading the results needs a pass: see pricing.