This tutorial demonstrates implementation of the Cellular Automaton Tree-GRass-Shrub Simulator (CATGRaSS) [Zhou et al., 2013] on a digital elevation model (DEM). This model is built using components from the Landlab component library. CATGRaSS is a spatially explicit model of plant coexistence. It simulates local ecohydrologic dynamics (soil moisture, transpiration, biomass) and spatial evolution of tree, grass, and shrub Plant Functional Types (PFT) driven by rainfall and solar radiation.

Each cell in the model grid can hold a single PFT or remain empty. Tree and shrub plants disperse seeds to their neighbors. Grass seeds are assumed to be available at each cell. Establishment of plants in empty cells is determined probabilistically based on water stress for each PFT. Plants with lower water stress have higher probability of establishment. Plant mortality is simulated probabilistically as a result of aging and drought stress. Fires and grazing will be added to this model soon.

This model (driver) contains:
- A local vegetation dynamics model that simulates storm and inter-storm water balance and ecohydrologic fluxes (ET, runoff), and plant biomass dynamics by coupling the following components:
- PrecipitationDistribution
- Radiation
- PotentialEvapotranspiration
- SoilMoisture
- Vegetation

- A spatially explicit probabilistic cellular automaton component that simulates plant competition by tracking establishment and mortality of plants based on soil moisture stress:
- VegCA

To run this Jupyter notebook, please make sure that the following files are in the same folder:
- cellular_automaton_vegetation_DEM.ipynb (this notebook)
- Inputs_Vegetation_CA.txt (Input parameters for the model)
- Ecohyd_functions_DEM.py (Utility functions)

[Ref: Zhou, X, E. Istanbulluoglu, and E.R. Vivoni. "Modeling the ecohydrological role of aspect-controlled radiation on tree-grass-shrub coexistence in a semiarid climate." Water Resources Research 49.5 (2013): 2872-2895]