Abstract. Numerical simulation of the form and characteristics of Earth's surface provides insight into its evolution. Landlab is an Open Source Python package that contains modularized elements of numerical models for Earth's surface, thus reducing time required for researchers to create new or reimplement existing models. Landlab contains a gridding engine which represents the model domain as a dual graph of structured quadrilaterals (e.g., raster) or irregular Voronoi polygon-Delaunay triangle mesh (e.g., regular hexagons, radially symmetric meshes, fully irregular meshes). Landlab also contains components – modular implementations of single physical processes – and a suite of utilities which support numerical methods, input/output, and visualization. This contribution describes package development since version 1.0 and backward-compatibility breaking changes which necessitates the new major release, version 2.0. Substantial changes include refactoring the grid, improving the component standard interface, dropping Python 2 support, and creating 30 new components – for a total of 57 components in the Landlab package. We describe reasons why many changes were made in order to provide insight to designers of future packages. We conclude by discussing lessons about the dynamics of scientific software development gained from the experience of using, developing, maintaining, and teaching with Landlab.;
