Aquihood: Watershed Catchment Scaling
According to the USGS:
A watershed is an area of land that drains all the streams and rainfall to a common outlet, such as the outflow of a reservoir, the mouth of a bay, or any point along a stream channel. Watersheds can be as small as a footprint or large enough to encompass all the land that drains into major river systems. The word watershed is sometimes used interchangeably with drainage basin or catchment. Watersheds consist of surface water--lakes, streams, reservoirs, and wetlands--and all the underlying groundwater.
In terms of sizing, Aquihood is concerned with defining a watershed at the five-minute walkable level.
When I say level, I mean the smallest scale that I am currently working with, even though both smaller and larger watershed resolutions are possible. As the USGS definition explains, a watershed can be small or large depending on the drainage outlet, or, in GIS terminology, the pour point.
For the poster I prepared for the Annual River Restoration Centre Conference 2026, I explored the following nested scales:
Nested Scale | Confluence Spacing | Catchment Perimeter | Time | Speed | Mode |
Aquihood Rivulet | ~400 m | ~1.6 km | 5 min | 5 km/h | Walking |
Quarter Brook | ~1.25 km | ~5 km | 5 min | 15 km/h | Cycling |
District Creek | ~2.5 km | ~10 km | 5 min | 30 km/h | Local driving |
Borough Stream | ~4.2 km | ~16.8 km | 5 min | 50 km/h | Inter-district driving |
Ward Tributary | ~7.5 km | ~30 km | 5 min | 90 km/h | Motorway |
Prefecture River | ~12.5 km | ~50 km | 5 min | 150 km/h | Regional train |
Watersheds come in many different shapes depending on the slope and form of the topography, and how the geology has been shaped ,squeezed or flattened over millions of years.
To create a standardised system of nested scales, I have simplified the geometry so that the length of the drainage segment is also the side length of a four-sided basin. At the Aquihood level, the perimeter of the rhombus is therefore 400 m × 4 = 1.6 km, which is equivalent to approximately a 20-minute walk.
The area of the basin is estimated using three simple shapes based on the same 400 m yardstick:
Circle (400 m diameter): 12.57 hectares
Rhombus (400 m sides, 60° angles): 13.86 hectares
Square (400 m sides): 16.00 hectares
The Aquihood area therefore lies somewhere between these values, with exceptions, of course.
The main emphasis, however, is not the exact geometry but the five-minute walkable watershed.
The challenge now is how to generate these five-minute walkable watersheds automatically in GIS while remaining true to both the natural confluence network and the upstream-to-downstream movement of surface water.
The diagram below illustrates the nesting of these scales. The fractal progression does not occur only at the main outlet but is repeated throughout the network, towards both the edges and the centre of every watershed.
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