Advances of hydrology in a global vegetation model

LPJ-GUESS (Lund-Potsdam-Jena general ecosystem simulator) is a climate-driven dynamic ecosystem model, with explicit processes describing vegetation dynamics, soil biogeochemistry, land use, land management, and wetland. Carbon and nitrogen cycles in vegetation and soils have been included in the model. The model has been widely recognized and applied in different studies.
Similar to many other large-scale ecosystem models, LPJ-GUESS often runs at a resolution of large grid cells (e.g., 0.5 by 0.5 degrees) and there is no consideration of within-grid cell spatial variations caused by topography. Topography describes the shape of the earth surface and the topographical variation within each grid cell area can directly influence water redistribution and received radiation and as a result, vegetation distribution.

In this project we will implement two approaches in LPJ-GUESS to represent topographical variations within each grid cell. The first approach will apply a statistical index to describe topographical variability. This index will be read-in for each grid cell and used in the model to influence the location and slope of the simulated stands and patches within the grid cell. In this way, reconstructing topography within a grid cell will add the topographical impacts on different ecosystem processes, and will not add additional computing requirements. The second approach is through explicitly connecting sub-grid cells based on water flow directions and accumulation. The flow direction and accumulation will be computed for each grid cell based on the provided DEM data. The resolution of DEMs determines the number of sub-grid cells within each grid cell and the flow routing within each grid cell will be computed on daily basis. In this way, the model will be capable of dynamically tracing water movement within the grid cell and able to assess the impacts on the land cover changes.

In the second part of this project, we will further extend the model with lateral water flows among grid cells to accumulate and route water along the global river network. This scheme will use a global drainage map to divide the globe into different drainage basins, connect grid cells based on the river network, calculate river discharge, and account for lakes and reservoirs as additional water stores. Irrigation and human consumption of water will also be considered.

All these above-mentioned developments will also trace dissolved C and N changes in water. The model will be thoroughly tested at different sites, and be applied at large scales to assess the impacts on regional and global carbon and nitrogen budget.

We will improve the representation of hydrology in the LPJ-GUESS ecosystem model, and study the impacts on vegetation and ecosystems, including carbon and nutrient cycling.