hydro-snap

Hydro-snap is an efficient tool for seamlessly aligning digital elevation models (DEMs) with mapped stream networks, ensuring accurate hydrological flow paths with minimal terrain alteration.

Example of a flow accumulation before (left) and after (right) alignment with hydro-snap. The DEM on the right has been aligned with the mapped stream network, ensuring accurate hydrological flow paths.

Hydro-snap allows users to:

• Align a DEM with a mapped stream network
• (Optionally) delineate a catchment from a provided outlet point
• (Optionally) force the flow accumulation to be consistent with a provided catchment boundary

The flow direction and accumulation are computed after the DEM has been aligned with the stream network, using the pysheds library.

The outputs of hydro-snap are:

• A reconditioned DEM (corrected_dem_final.tif)
• An intermediate pre-pysheds DEM (corrected_dem_pre_pysheds.tif) — retained in the output directory
• A flow direction raster (flow_direction.tif)
• A flow accumulation raster (flow_accumulation.tif)
• A catchment delineation raster (catchment.tif) — only when a shapefile of the outlet is provided
• The stream network shapefile with an additional incremental rank attribute (streams.shp)
• The stream start points shapefile (stream_starts.shp) — useful for identifying topology issues in the stream network
• The stream end points shapefile (stream_ends.shp) — useful for identifying topology issues in the stream network

Installation

Hydro-snap can be installed using pip:

pip install hydro-snap

The proj library is required to handle the spatial reference system of the DEM and stream network. If you are using Windows, you can install the proj library using conda:

conda install -c conda-forge proj

If you are using Linux, you can install the proj library using apt:

sudo apt-get install libproj-dev

If you are using macOS, you can install the proj library using brew:

brew install proj

Cannot find proj.db? Set the PROJ_DATA environment variable to the directory containing the proj.db file (see https://proj.org/en/9.4/faq.html#why-am-i-getting-the-error-cannot-find-proj-db).

Data requirements

You will need the following data to use hydro-snap:

• A digital elevation model (DEM) in GeoTIFF format (with a spatial reference system)
• A mapped stream network in shapefile format (with a spatial reference system). Lines should be oriented in the direction of flow (from upstream to downstream). If your lines go from downstream to upstream, use stream_orientation='upstream'.
• (Optional) A shapefile containing the outlet point of the catchment
• (Optional) A shapefile containing the catchment boundary
• (Optional) A shapefile containing breach lines in the catchment boundary. If not provided, the stream network is used as breaches; any catchment polygon with no stream crossing is opened at its lowest-elevation boundary cell automatically.

All inputs must share the same coordinate reference system (CRS).

Usage

Hydro-snap can be used to align a DEM with a mapped stream network using the following code:

from hydro_snap import recondition_dem

recondition_dem('path/to/DEM', 'path/to/streams.shp', 'output/dir')

If your stream lines are digitized from downstream to upstream, use stream_orientation='upstream':

from hydro_snap import recondition_dem

recondition_dem('path/to/DEM', 'path/to/streams.shp', 'output/dir',
                stream_orientation='upstream')

When the catchment outlet is provided, the catchment delineation raster is computed. The min_accumulation threshold controls how the outlet point is snapped to a stream cell:

from hydro_snap import recondition_dem

recondition_dem('path/to/DEM', 'path/to/streams.shp', 'output/dir',
                outlet_shp='path/to/outlet.shp',
                min_accumulation=10000)

A catchment boundary can be provided to constrain flow within it. Breaches (e.g. the river crossing the boundary) are detected from the stream network automatically, or can be supplied explicitly:

from hydro_snap import recondition_dem

# Stream network used as breaches automatically
recondition_dem('path/to/DEM', 'path/to/streams.shp', 'output/dir',
                catchment_shp='path/to/catchment.shp')

# Or provide explicit breach lines
recondition_dem('path/to/DEM', 'path/to/streams.shp', 'output/dir',
                catchment_shp='path/to/catchment.shp',
                breaches_shp='path/to/breaches.shp')

Parameters

Parameter	Default	Description
delta	0.0001	Elevation step (m) applied when lowering cells along the stream
walls_height	1000	Height (m) of temporary walls placed at the catchment border
epsg_code	None	EPSG code to assign when the CRS is missing from an input file
stream_orientation	'downstream'	'upstream' reverses line direction before processing
min_accumulation	10000	Minimum accumulation cell count for outlet snapping
simplification_tolerance	0.5	Simplify geometries before processing, as a fraction of the DEM pixel size. Set to 0 or None to disable. Improves performance on high-resolution datasets.

Diagnosing stream network issues

The start and end points of unconnected stream segments are written to stream_starts.shp and stream_ends.shp. You can also generate them independently to inspect your stream network before running recondition_dem:

import geopandas as gpd
from hydro_snap.hydro_snap import extract_stream_starts_ends

streams = gpd.read_file('path/to/streams.shp')
starts, ends = extract_stream_starts_ends(streams, output_dir='output/dir')

Unconnected start points indicate stream sources; unconnected end points indicate outlets or disconnected segments. Reviewing these can help identify gaps or direction errors in the stream network data.