R.richdem

DESCRIPTION

r.richdem is a collection of seven GRASS GIS modules that expose algorithms from the RichDEM terrain-analysis library through standard GRASS raster and vector interfaces. RichDEM provides high-performance, research-grade algorithms for hydrological conditioning of digital elevation models (DEMs), flow routing, and terrain attribute calculation, with particular strength in handling real topographic depressions rather than removing them.

Modules

Module	Description
r.richdem.filldepressions	Fill depressions in a DEM using the Priority-Flood algorithm; optional ε-gradient for flat areas
r.richdem.breachdepressions	Breach depressions by carving least-cost channels (Lindsay 2016)
r.richdem.resolveflats	Impose a local ε-gradient on flat areas to ensure unique flow directions
r.richdem.flowaccumulation	Flow accumulation with 13 SFD/MFD algorithms (D8, D-infinity, Quinn, Holmgren, Freeman, ...)
r.richdem.terrainattribute	Terrain attributes: slope (4 unit systems), aspect, total/planform/profile curvature
r.richdem.dephier	Compute the depression hierarchy of a DEM (required first step for Fill--Spill--Merge)
r.richdem.fsm	Fill--Spill--Merge: route surface water through the depression hierarchy without removing depressions

All modules share a librichdem helper library for GRASS ↔ RichDEM raster and vector conversion, installed to $GISBASE/etc/r.richdem/librichdem/.

Workflows

Classic flow-accumulation workflow

Depressions must be removed before computing flow accumulation with most algorithms. Two approaches:

# Option A: fill depressions, impose epsilon gradient on flat areas
r.richdem.filldepressions -e input=dem output=dem_filled
r.richdem.flowaccumulation input=dem_filled output=flow_accum

# Option B: breach depressions, resolve remaining flats
r.richdem.breachdepressions input=dem output=dem_breached
r.richdem.resolveflats input=dem_breached output=dem_conditioned
r.richdem.flowaccumulation input=dem_conditioned output=flow_accum method=Dinf

Fill--Spill--Merge workflow

Fill--Spill--Merge (Barnes et al., 2021) routes water through real depressions without removing them, producing an equilibrium water-table depth map. This requires first computing the depression hierarchy:

# Step 1: build the depression hierarchy (run once per DEM)
r.richdem.dephier input=dem \
    output_labels=dep_labels \
    output_flowdirs=dep_flowdirs \
    output_hierarchy=dep_hierarchy

# Step 2: create an initial water-table depth map
r.mapcalc "wtd = -0.5"

# Step 3: run Fill-Spill-Merge
r.richdem.fsm input=dem \
    labels=dep_labels \
    flowdirs=dep_flowdirs \
    hierarchy=dep_hierarchy \
    water_depth=wtd \
    output=wtd_after

REQUIREMENTS

All modules in this collection require the RichDEM Python package, which is not a standard GRASS GIS dependency and must be installed separately:

pip install richdem

If pip install richdem fails (the package requires a C++ compiler), build from source:

git clone https://github.com/r-barnes/richdem.git
cd richdem/wrappers/pyrichdem
pip install -e .

Ensure that RichDEM is installed into the same Python environment used by GRASS GIS.

REFERENCES

Barnes, R., Callaghan, K.L., Wickert, A.D. (2021). Computing water flow through complex landscapes -- Part 3: Fill--Spill--Merge: flow routing in depression hierarchies. Earth Surface Dynamics 9(1), 105--121. DOI: 10.5194/esurf-9-105-2021
Barnes, R., Callaghan, K.L., Wickert, A.D. (2020). Computing water flow through complex landscapes -- Part 2: Finding hierarchies in depressions and morphological segmentations. Earth Surface Dynamics 8(2), 431--445. DOI: 10.5194/esurf-8-431-2020
Lindsay, J.B. (2016). Efficient hybrid breaching-filling sink removal methods for flow path enforcement in digital elevation models. Hydrological Processes 30(6), 846--857. DOI: 10.1002/hyp.10648
Barnes, R. (2016). RichDEM: Terrain Analysis Software. URL: http://github.com/r-barnes/richdem

AUTHORS

Richard Barnes, Kerry L. Callaghan, Andrew D. Wickert (algorithm design and RichDEM library --- see references above)

GRASS GIS bindings: Andrew D. Wickert, with assistance from Claude Sonnet 4.6

SOURCE CODE

Available at: r.richdem source code (history)
Latest change: Sunday May 31 20:21:19 2026 in commit 2163cf6