Hydrological Sciences

Flow calculation

r.carve: Takes vector stream data, transforms it to raster and subtracts depth from the output DEM.
r.drain: Traces a flow through an elevation model on a raster map.
r.fillnulls: Fills no-data areas in raster maps using v.surf.rst splines interpolation
r.fill.dir: Filters and generates a depressionless elevation map and a flow direction map from a given elevation layer.
r.flow: Construction of slope curves (flowlines), flowpath lengths, and flowline densities (upslope areas) from a raster digital elevation model (DEM)
r.topidx: Creates topographic index [ln(a/tan(beta))] map from elevation map.
r.terraflow: Flow computation for massive grids.

r.gwflow: Numerical calculation program for transient, confined and unconfined groundwater flow in two dimensions.
r3.gwflow: Numerical calculation program for transient, confined groundwater flow in three dimensions.

r.sim.sediment: Sediment transport and erosion/deposition simulation using path sampling method (SIMWE)
r.sim.water: Overland flow hydrologic simulation using path sampling method (SIMWE)

For an overview, see R.stream.*.

r.stream.angle: Route azimuth, direction and relation to streams of higher order
r.stream.basins: Calculate basins according user input
r.stream.del: Calculate basins according user input
r.stream.distance: Calculate distance to and elevation above streams and outlets according user input. It can work in stream mode where target are streams and outlets mode where targets are outlets
r.stream.extract: Stream network extraction
r.stream.order: Calculate Strahler's and Horton's stream order Hack's main streams and Shreeve's stream magnitude. It use r.watershed or r.stream.extract output files: stream, direction and optionally accumulation. Output data can be either from r.watershed or r.stream.extract but not from both together
r.stream.pos: Route azimuth, direction and relation to streams of higher order
r.stream.stats:Calculate Horton's and optionally Hack's statistics according to user input

r.basin.fill: Generates a raster map layer showing watershed subbasins.
r.water.outlet: Generates a watershed basin from a drainage direction map (from r.watershed) and a set of coordinates representing the outlet point of watershed.
r.watershed: Watershed basin analysis program.
r.basin: Generates the main morphometric parameters of the basin.

Tutorials:

Metz, M. et al 2009: Fast Stream Extraction from Large, Radar-Based Elevation Models with Variable Level of Detail (PDF)
Metz, M. et al 2010: Accurate stream extraction from large, radar-based elevation models (PDF)
J Jasiewicz, M Metz, 2011: A new GRASS GIS toolkit for Hortonian analysis of drainage networks, Computers & Geosciences. DOI