Watershed delineation – the catchment of any point from one click

To size a culvert, a ditch, a storm-water pond or a small bridge, or to answer a hydrology assignment, you first need the catchment: all the land whose rain runs to your point. GIS programs can delineate it, but that means finding an elevation model, filling its sinks, computing flow directions and accumulation and snapping a pour point, which takes an afternoon the first time. This tool does the same steps in your browser. Click on the stream where the water should be measured, and the heights around the point are read from Terrain Tiles, a free worldwide elevation dataset built from national lidar models, EU-DEM, SRTM and others. Depressions in the data are filled so that water can always flow on, every cell is given the direction of its steepest downhill neighbour (the D8 method used by ArcGIS, QGIS and GRASS), and the cells upstream of your point form the catchment. The outline is drawn on the map with the longest flow path and the stream network, and the panel gives the numbers that design methods and assignments ask for. Download the catchment as GeoJSON, KML or a shapefile for QGIS, ArcGIS or CAD, or copy a link that reopens the same result.

How to use it

1. Search for a town or postcode, or pan the map, and zoom in until you can see the stream or the road crossing you need.
2. Click exactly on the stream, the bridge, the culvert or the lake outlet; the point is moved onto the nearest stream cell within 60 m.
3. Wait a few seconds while the elevation data is read; the window grows from 4 × 4 km to at most 20 × 20 km until the whole catchment fits.
4. Read the catchment area, perimeter, longest flow path, heights, mean slope and the shape indices in the panel.
5. Choose how dense the stream network should be, then download GeoJSON, KML or the shapefile, or copy the share link.

How accurate is it?

Where the heights come from

Terrain Tiles mix the best open elevation model for each place: the Environment Agency's 2 m lidar in England, USGS 3DEP at about 10 m in the United States, 10 m national models in Austria and Norway, EU-DEM at 25 m in most of the rest of Europe and SRTM at about 30 m in much of the world. The tool reads them on a square grid in the outlet's UTM zone with at most 500 cells along each side: 8 m cells in the first 4 × 4 km window, 40 m in the largest 20 × 20 km window, never finer than one pixel of the tiles. A catchment is only as good as the ridges between valleys in that data; in hills and mountains the ridges are clear and the outline usually follows the true divide within one or two cells.

How the water is routed

First every depression in the grid is filled with the Priority-Flood algorithm of Barnes, Lehman and Mulla (2014), working inwards from the edge of the window, and flats are given a tiny slope so they drain; then each cell drains to the one of its eight neighbours with the steepest drop (D8), and the cells whose path passes through the outlet are the catchment. This is the standard method in ArcGIS (Fill, Flow Direction, Watershed), QGIS and GRASS r.watershed. D8 sends all water one way, so on smooth convex slopes the divide can wobble by a cell. Area and perimeter are measured geodesically on the WGS84 ellipsoid from the outline, whose staircase of cell edges is smoothed by at most three quarters of a cell so the perimeter is not overstated.

Flat land, towns and drains

Where the land is flat, as in Denmark, the Netherlands, marshes and river plains, height differences of a few decimetres decide where water runs, and a 10–30 m model cannot see them. Ditches, drain pipes, sewers, culverts under roads and pumping stations also move water across divides that the terrain does not show. In those places the outline can be badly wrong, and you should use a 1–2 m lidar model (Denmark's DHM from Dataforsyningen, France's RGE ALTI, Germany's state DGM1, Spain's CNIG MDT) or the official catchment maps of your water authority, and for any permit or design calculation the authority's own catchment.

Large catchments

The window grows from 4 × 4 km to 8, 14 and finally 20 × 20 km, moving towards the catchment found so far. When the catchment still reaches the edge of the largest window, the tool says so and shows only the part it found: the area, flow length and indices are then too small. For a large river use a national catchment map or a global dataset such as HydroSHEDS.

Good to know

Clicking beside the stream

A click on a slope next to the stream gives the small catchment of that slope. The tool moves the point to the nearest cell that drains a lot within 60 m and tells you how far it moved it; if the result is tiny, click on the stream itself, or a little further downstream.

Lakes and reservoirs

Water surfaces are flat in the data. The tool lets a lake drain where its shore is lowest, which is usually its real outlet, but a dam or a weir that is not in the elevation model can send the water another way. Click below the outlet of the lake to include everything that drains into it.

The stream network is a model

The blue lines are cells that collect more than the chosen area upstream, not mapped streams. Where they leave the real streams on the base map, the elevation data is too coarse there, which is also a warning about the outline.