Catchment area calculator for culverts, gullies and streams

Who uses this

• Engineers and designers. Get the contributing area for culvert and stormwater sizing.
• Farmers. Size a culvert or a dam by the land that drains to it.
• Owners. See whether runoff comes from a neighbour's block or the road.

When to use it

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

Which tool 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.

How to use it

1. Open Catchment and click near the spot you care about: the culvert inlet, the gully mouth or the point on the stream. An orange dot marks your click.
2. After a moment, blue water paths appear around your click and a dark dot shows your click snapped onto the nearest one. Click a different line to choose another spot. To measure at the exact click instead, set "Snap my click to a water path" to Off.
3. Run. The blue area drains to the dark dot. If the catchment continues past the area analysed, a red line marks where and the area is reported as a minimum.

Step by step: Mt Iron, Wānaka

We find all the land that drains to a gully at the foot of Mt Iron.

What it showed

• If the catchment runs past the area analysed, a red line shows where it continues and the area is reported as a minimum.

Screenshots from NZ Elevation Tools. Aerial imagery from LINZ Basemaps. Sourced from the LINZ Data Service and licensed by Environment Southland, Otago Regional Council and Queenstown-Lakes District Council for re-use under the Creative Commons Attribution 4.0 International licence.

How it works

A catchment is all the land whose rainwater ends up at one point, so the answer depends on which path that point sits on. That is why the tool draws the paths first and lets you choose.

LiDAR bare-earth terrain is loaded around your click, in cells about 0.9 m across. Small pits are filled so water can always keep moving, and each cell sends its water to its steepest downhill neighbour. The blue lines are where at least 0.1 ha has collected, the same calculation and the same minimum as the Flow Paths tool.

Your click moves to the nearest blue line within the distance you set, and you see it move before anything runs. Where lines run side by side a few metres apart, it takes the one draining the most land. Running the analysis then walks back uphill from that point, collecting every cell that eventually drains into it. Those cells are the catchment, and their total is the area.

If the catchment reaches the edge of the terrain, the tool repeats over a wider area with coarser cells: about 1.8 m, 3.6 m and finally 7 m, out to 5 km from your click. The results state the detail actually used. If a wider, coarser pass reports less land than a finer one, the finer figure is kept and flagged as a minimum.

Method, sources and who built it

Reading the results

Contributing area is given in hectares and square metres, with the detail and the size of the area analysed.

A "Not the whole catchment" warning means the true catchment extends beyond the terrain analysed, so the area is a minimum. The red line shows where it continues.

The area depends heavily on which water path the point sits on: two points a few metres apart, on different paths, can give very different figures. A "Check your click" warning means under 0.1 ha drains to the spot, which usually means it is not on a path. For a figure you will rely on, check the dark dot sits on the path you mean.

With a pass, download the catchment outline: DXF in NZTM2000 metres for CAD, or GeoJSON and KML in WGS84. It follows the edges of the cells counted, holes and separate pieces included, so its area is the figure shown: the cell count times the cell area. Land joined only through a diagonal cell corner comes out as separate pieces touching at that corner. Measured in NZTM2000, the outline’s area reads slightly different, from about 0.3% smaller in Northland to about 0.6% larger in Fiordland, because of the projection’s scale and the cell size, which is a spherical approximation; each file states the difference its own outline measures. You can simplify it at 1, 2 or 5 m; the file then records how much the outline area changed. A file from a partial catchment is marked as not complete, and a click that probably missed the water path carries that warning. The grid itself downloads as a GeoTIFF.

Limits and warnings

• Terrain is analysed to 5 km from the click at most, so a river's catchment will always be reported as partial.
• Flat water and braided riverbeds have no clear downhill path, so clicks there can report far too little land, sometimes with no warning. This tool is for gullies, drains and small streams, not for measuring a river.
• Culverts, pipes and stopbanks are not in the terrain. Water is routed over roads and through embankments, and fine detail shows embankments sharply, so check any result around a road crossing.
• On flat ground a few centimetres decide which way water goes, and the boundary can look arbitrary.

Compare quotes without ringing around

Catchment gives you a fast, indicative answer. When the job needs pricing, designing or signing off, getting quotes usually means calling around and explaining the same site over and over. Instead, run the tool on your site and choose Compare quotes under the result. Describe the job once and we’ll send it to up to 3 local stormwater engineers, with an aerial map of the site, a Google Earth file and your results attached, so they can price it properly from the start. They quote you directly. It’s free and there’s no obligation.

Data and attribution

Heights come from the LINZ elevation tiles: LINZ’s national bare-earth elevation model (LiDAR where it has been flown, the 8 m model elsewhere), stored in 0.1 m steps, read in your browser in a square around your click: cells of about 0.9 m out to 800 m, then about 1.8, 3.6 or 7 m when a larger catchment needs a wider area. Over the sea the tiles read 0 m; they hold no seabed.

Elevation and cadastral data is sourced from the LINZ Data Service and licensed for re-use under the Creative Commons Attribution 4.0 International licence. Copyright in each LiDAR survey belongs to the body that commissioned it — a regional or district council, Waka Kotahi, NIWA and around thirty others — rather than to LINZ, which hosts and processes the national dataset. Every file exported from this site names the licensors whose surveys cover the area, and anything you publish from those files carries the same attribution requirement. See https://www.linz.govt.nz/products-services/data/licensing-and-using-data/attributing-elevation-or-aerial-imagery-data

Questions

Related

Neighbouring tools

• Flow Paths — Maps the overland flow paths across an area, each sized by the hectares of land draining through it, and where water would pond once hollows fill, with depth, area and volume.
• Wetness Map — Maps where the shape of the land tends to concentrate moisture, from how much ground drains to each point and how flat that point is.