Drainage Area to a Culvert, Pond or Stream Crossing
Where does the water at the end of the driveway come from, and how much land feeds the pond you want to dig? Click the point and map all the land upstream of it, anywhere in Wisconsin.
Find a drainage area now
Open Catchment, click the culvert or the mouth of the draw, and run it. The area appears in acres.
Open Catchment →Why the drainage area comes first
A culvert that washes out, a pond that never fills or a basement that takes water every spring usually comes down to the same question: how much land drains to this point? The answer is rarely obvious from the ground. A draw that looks small can collect the runoff from a whole hillside, a road ditch can carry water from the neighbor's field, and on gentle glacial ground the divide between two drainages can sit somewhere you would never guess.
The Catchment tool answers it from the lidar terrain. Note that search results for “catchment calculator” are mostly roof rainwater tools; this is the ground catchment, the land surface draining to a point.
What people use it for
- Culverts under a driveway, a field lane or a town road: the area is the first input to the sizing.
- Ponds and basins: how much land feeds a farm pond or a stormwater basin.
- Runoff onto your land: whether the water crossing a parcel comes from a neighbor, the road or your own slope.
- Stream crossings for a trail bridge or a ford.
- A low corner that floods: how much ground is sending water to it.
How to find the drainage area
- Open Catchment and click the outlet: the culvert inlet, the mouth of a draw, a low corner or the point on the stream you care about. Zoom in on the aerial basemap to click the right spot.
- Set the terrain search radius, in miles. It decides how much terrain is loaded around the click, up to 3.11 mi. Start small for a driveway culvert and larger for a stream.
- Choose outlet snapping. Leave Snap the outlet to the nearest channel on so a click on a bank or a gravel bar still lands on the channel, or set it to Off to use the exact click.
- Choose Run analysis. The blue area drains to the outlet, and the panel reports the contributing area in acres, or square feet for a small catchment, and how far a snapped outlet moved.
- Check the edge. If the blue area touches the edge of the loaded terrain, the panel warns of a partial catchment. Increase the radius and run again.
See the rest of the water picture
The drainage area is where the water comes from. These show where it runs and where it gathers.
How it works
A square of USGS 3DEP bare-earth terrain is loaded around your click. Small pits are filled so water does not get stuck in them, each cell is given its steepest downhill neighbor, and the tool walks back up those directions from the outlet, collecting every cell that eventually drains into it. The cells it collects are the catchment.
Where it can mislead
Culverts, pipes and tile are invisible
The model is the ground surface. A road embankment looks like a dam, and water that in reality goes through a culvert is routed around or over the road. Storm sewers in towns and drainage tile in fields move water the terrain does not show.
Flat ground is decided by fractions of an inch
On flat valley floors, marshy lowlands and the broad flats of central Wisconsin, a tiny difference in elevation decides which way water goes, and the boundary can look arbitrary. Walk the divide after a rain if it matters.
Rivers are too big
Terrain is loaded to 3.11 mi at most. That covers a driveway culvert or a draw on a hillside above the Wisconsin River comfortably, but the river itself drains far more land, and its catchment will always be reported as partial.
For anything that carries public road traffic or could flood a neighbor, the drainage area is the start of the design, not the design. An engineer or your county highway or town road office will say what standard applies, and the FEMA flood maps show the mapped flood hazard along larger streams.
About the data
Ground elevations come from the USGS 3D Elevation Program (3DEP) seamless bare-earth model, with elevations above NAVD 88. Source cells are 3.3 ft per pixel in the finest work units, with about 9.8 ft per pixel and 33 ft per pixel sources elsewhere. USGS elevation data is public domain. Lidar coverage and source dates vary across Wisconsin. Check your site against the USGS 3DEP work-unit index; an unavailable lookup means coverage is unknown. The catchment is computed on the bare-earth model, so it follows the ground surface as it was when the lidar was flown. More from the USGS 3D Elevation Program and Wisconsin elevation resources.
Frequently asked questions
What is a drainage area?
All the land upstream of a point, from which rain and snowmelt eventually flow to it. It is also called the catchment, the contributing area or the watershed of that point, and it is the first number in sizing a culvert, a pond or a detention basin.
Does this size the culvert for me?
No. It gives the drainage area. Rainfall intensity, runoff from the land cover and soils, and pipe capacity come next, usually from an engineer or your county or town road standards.
Why does it say partial catchment?
The area reaches the edge of the terrain loaded, so the true catchment is larger and the figure is a lower bound. Increase the terrain search radius and run again. Terrain is loaded to 3.11 mi at most, so a river’s catchment will always be partial.
Why did my outlet move?
Outlet snapping looked near your click for a cell that collects far more water, at least ten times what the clicked cell does, and moved the outlet there so a click on a bank still lands on the channel. The moved outlet is marked on the map and the distance is reported. Set snapping to Off to use the exact cell you clicked.
Does it know about culverts and storm sewers?
No. Culverts, pipes, tile and levees are not in the terrain, so water is routed over roads and through embankments. Click on the upstream side of a road culvert, where the water arrives, to get the area that feeds it.
Can I download the catchment?
Yes, as a GeoTIFF grid for GIS, from the results panel. Downloading needs a pass; the analysis is free to run.
Related guides
- Find wet ground with the topographic wetness index — where water gathers across a property.
- Terrain checks before buying rural land — drainage as part of the pre-offer pass.
- How to create an elevation profile — the fall along a ditch or a pipe.