Hydrologic Indices
Hydrologic Indices turn the rasters produced by flow routing into measures that mean something on the ground: where the soil stays wet, where a stream has the power to erode, how far above the nearest watercourse a place sits.
Most of them combine a flow accumulation raster and a slope raster, so run Flow Routing first, and Slope from Terrain derivatives.
Start here
Section titled “Start here”Wetness Index
Section titled “Wetness Index”The topographic wetness index, TWI, the most widely used index in the toolbox. It combines the area draining to a cell with its steepness: a lot of upstream area on gentle ground means water accumulates and lingers.
High values mark valley bottoms, seepage zones and saturated soils. Low values mark steep, well drained slopes.
It is a topographic index, not a measurement of moisture. It assumes steady state conditions and knows nothing about soil, geology or rainfall, so read it as a relative ranking of wetness potential rather than as a water content.
Values are usually computed with a dispersive flow method such as FD8, which spreads water over the hillside more realistically than D8.
Depth To Water
Section titled “Depth To Water”The cartographic depth to water index. Starting from streams and lakes, it computes the least cost climb away from them, giving for each cell an estimate of how far below the surface the water table lies.
Widely used in forestry to map wet ground and plan machine traffic, and easier to interpret on the ground than TWI.
Elevation Above Stream
Section titled “Elevation Above Stream”Also known as height above nearest drainage. For each cell it gives the vertical difference between that cell and the stream it drains into, following the flow path.
It is the simplest first pass flood exposure map you can build from a DEM: everything under two metres above the stream is where water goes first.
The Euclidean variant measures to the nearest stream in a straight line rather than along the flow path. Faster, less faithful in complex terrain.
Relative Stream Power Index and Sediment Transport Index
Section titled “Relative Stream Power Index and Sediment Transport Index”Two erosion related indices. Stream power combines catchment area and slope to estimate the erosive energy of flowing water, marking the places where channels incise. Sediment transport, the LS factor of the universal soil loss equation, estimates the capacity to move material, and feeds soil erosion modelling.
All the tools
Section titled “All the tools”| Tool | What it does |
|---|---|
| Wetness Index | Topographic wetness index, from specific catchment area and slope. |
| Relative Stream Power Index | Erosive power of flow, from catchment area and slope. |
| Sediment Transport Index | The LS factor, estimating sediment transport capacity. |
| Depth To Water | Cartographic depth to water, by least cost accumulation from streams and lakes. |
| Elevation Above Stream | Height above the nearest stream, measured along the flow path. |
| Elevation Above Stream Euclidean | Height above the nearest stream, measured in a straight line. |
| Downslope Distance To Stream | Distance along the flow path from each cell to the nearest stream. |
| Distance To Outlet | Downstream distance from each stream cell to the outlet. |
| Downslope Index | The Hjerdt index: the distance needed to descend a given elevation drop, a measure of hydraulic gradient. |
| Elev Relative To Watershed Min Max | Position of each cell within its own watershed’s elevation range, as a percentage. |
| Hydrologic Connectivity | Connectivity indices describing how well parts of the catchment are hydrologically linked. |
| Edge Contamination | Marks the cells whose upstream area extends beyond the edge of the raster. |
| Find Noflow Cells | Finds cells with no lower neighbour, which reveals remaining depressions. |
| Find Parallel Flow | Finds stream cells whose flow directions run parallel, a known D8 artefact. |
Check your conditioning first. Find Noflow Cells and Edge Contamination are diagnostic tools. Run them before trusting any index: a raster full of no flow cells means the DEM was not properly conditioned, and heavy edge contamination means your extent is too tight.
Indices are relative. TWI, stream power and the rest are comparable within one raster at one resolution. Comparing values between two studies at different resolutions is meaningless.