Catchment

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.

How to use it

1. Open Catchment and click the outlet: the culvert inlet, the gully mouth or the point on the stream you care about.
2. Set the terrain search radius. Default 2 km; up to 5 km.
3. Leave outlet snapping on so a click on a bank or a sandbank 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.

How it works

A square of LiDAR bare-earth terrain is loaded around the click. Small pits are filled, each cell is assigned its steepest downhill neighbour, 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 hectares and square metres.

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

Limits and warnings

• Terrain is loaded to 5 km at most, so a river's catchment will always be reported as partial.
• Culverts, pipes and stopbanks are not in the terrain. Water is routed over roads and through embankments.
• On flat ground a few centimetres decide which way water goes, and the boundary can look arbitrary.

Data and attribution

Results come from the LINZ national 1 m LiDAR bare-earth elevation model, read in your browser, with the national 8 m model filling any gaps. Tools that use the surface model (Sunlight, Viewshed, Object Heights) read the matching LiDAR surface tiles.

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

Related

Neighbouring tools

• Water & Drainage — Extracts the drainage channels across an area as an ordered stream network, and maps 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.