4. Algorithm
4.1. Overview
HexWatershed includes major classicial watershed delineation algorithms with modifications.
These algorithms are implemented within both the Python frontend and the C++ backend.
4.2. Algorithms
4.2.1. Distance
Distance is calculated using the great-circle distance, orthodromic distance, or spherical distance is the distance along a great circle.
4.2.2. Area
A closed geometric figure on the surface of a sphere which is formed by the arcs of great circles.
4.2.3. Angle
A spherical angle is a particular dihedral angle; it is the angle between two intersecting arcs of great circles on a sphere.
4.2.4. Flow simplication
Flowline simplication is achieved through the PyFlowline Python package (https://anaconda.org/conda-forge/pyflowline)
4.2.5. Mesh generation
Mesh generation is achieved through the PyFlowline Python package (https://anaconda.org/conda-forge/pyflowline)
4.2.6. Topological relationship reconstruction
Topological relationship reconstruction is achieved through the PyFlowline Python package (https://anaconda.org/conda-forge/pyflowline)
4.2.7. DEM resampling
HexWatershed provides two resampling methods:
4.2.7.1. Nearest resampling
In the nearest resampling, the model defines the cell elevation in the following steps:
obtain cell center location in longitude and latitude
convert location into DEM row and column indices
obtain DEM pixel value and assign as the cell elevation
4.2.7.2. Zonal mean resampling
In the zonal mean resampling, the model defines the cell elevation in the following steps:
obtain all the cell vertex locations in longitude and latitude
define a polygon using the vertex locations
extract the DEM using the polygon
calculate the mean extracted DEM and assign as the cell elevation
4.2.8. Stream burning
A topological relationship-based stream burning algorithm is implemented in HexWatershed v3.0.
4.2.9. Depression filling
In general, the depression filling algorithm is similar to that in the HexWatershed v1.0.
The major difference is related to the stream burning algorithm.
4.2.10. Flow direction
The flow direction is defined based on elevation differences and distances.
Currently, only the deepest slope is used to define the single flow direction.
Multi-flow directions will be supported in future versions.
4.2.11. Flow accumulation
The actual drainage area is used instead of flow accumulation cell number. This is because each cell may have different cell area.
4.2.12. Stream grid
The flow accumulation threshold is used to define the stream grid.
4.2.13. River confluence
If a stream grid has more than one upstream, this stream grid is defined as a river confluence.
4.2.14. Stream segment
Each individual stream line between headwater-confluence is defined as a stream segment
4.2.15. Subbasin
All the cells that contributes to a stream segment are used to define a subbasin.
4.2.16. Watershed
All the cells that contributes to a river outlet are used to define a watershed.