What is "HexWatershed"?

HexWatershed is a mesh independent flow direction model for hydrologic models.

Why do we develop HexWatershed?

Spatial discretization is the cornerstone of all spatially-distributed numerical simulations including watershed hydrology. Traditional square grid spatial discretization has several limitations:

  1. It cannot represent adjacency uniformly;

  2. It leads to the "island effect" and the diagonal travel path issue in D8 scheme;

  3. It cannot provide a spherical coverage without significant spatial distortion;

  4. It cannot be coupled with other unstructured mesh-based models such as the oceanic models.

Therefore, we developed a watershed delineation model (HexWatershed) based on the hexagon mesh spatial discretization.

We further improve HexWatershed to fully unstructured mesh-based to support variable-resolution meshes such as the MPAS mesh.

We propose that spatially distributed hydrologic simulations should consider using a hexagon grid spatial discretization.

How we did it?

We rewrote all the core watershed delineation algorithms on a completely different data structure.

What is the current status of "HexWatershed"?

We are adding more features to the HexWatershed ecosystem:

  • The zonal resampling within the unstructured mesh is supported and other geostatistics are supported as well;

  • HexWatershed is ready for continental and global scale simulation (e.g., through the MPAS mesh);

  • HexWatershed uses the state-of-art topology method to conduct stream burning and depression filling;

  • We are evaluating hydrology simulations using the HexWatershed routing maps.


  • Chang Liao. (2022). HexWatershed: a mesh independent flow direction model for hydrologic models (0.1.1). Zenodo.

  • Chang Liao, & Matt Cooper. (2022). Pyflowline a mesh independent river network generator for hydrologic models. (0.1.24). Zenodo.

  • Chang Liao, Tian Zhou, Donghui Xu, Richard Barnes, Gautam Bisht, Hong-Yi Li, Zeli Tan, Teklu Tesfa, Zhuoran Duan, Darren Engwirda, and L. Ruby Leung. Advances in hexagon mesh based flow direction modeling. Advances in Water Resources (2021).