Terrestrial Hydrology Model with Biogeochemistry

The Center for Sustainability and the Global Environment (SAGE), part of the Gaylord Nelson Institute for Environmental Studies, University of Wisconsin-Madison, has developed a new generation of surface terrestrial hydrology transport model, called Terrestrial Hydrology Model with Biogeochemistry (THMB 1.2) [formerly, HYDrologic Routing Algorithm (HYDRA)]. THMB simulates the time-varying flow and storage of water in terrestrial hydrological systems, including rivers, wetlands, lakes, and human-made reservoirs, as a linked system. The model currently operates on the global scale on a 5 minute latitude by longitude grid (~10 km at the equator) spatial resolution and with a 1-hour step time. THMB requires the following boundary conditions: topography (from digial elevation models); evaporation from water surfaces (estimated from climate data, using a simple Penman energy balance model); surface runoff [supplied by the SAGE IBIS (Integrated BIosphere Simulator), a land surface biophysics/ecosystem process model]; base flow (drainage from the soil column, supplied by IBIS); and precipitation (from climate data).

The THMB model derives river paths and potential lake and wetland volumes from the digital elevation model (DEM) representation of the land surface. The physical land surface of THMB is coupled to a linear reservoir model to simulate (1) the discharge of river systems, and (2) the spatial distribution (and volume) of large lakes and wetland complexes. Rivers, lakes, and wetlands are defined as a continuous hydrologic network in which locally derived runoff accumulates and is transported across the land surface via rivers, fills lakes and wetlands, and is eventually transported to the ocean or is evaporated from an inland water body.

The THMB river direction file (see the rivdir.nc file) parameterizes which of 8 possible directions that water will flow across the land surface, pointing to the downstream grid cell. The values range from 1 to 8, with 1 being North, progressing clockwise to 8 which is Nortwest.

THMB has been tested globally, against observed annual mean discharge and lake area and has been used to investigate the accuracy of general circulation model simulation of equilibrium surface hydrology. HYDRA and IBIS have been used together to evaluate the simulated hydrology of the National Center for Environment Prediction (NCEP)/ National Center for Atmospheric Research (NCAR) climate reanalysis for the period 1963-1995 and to investigate the time-transient response of a linked river, lake, and wetland system (e.g., changes to the water balance of the Lake Chad drainage basin between 1953 and 1995). Work in progress includes using THMB and IBIS to investigate how variations in climate and changes in vegetation cover affect the hydrologic response of the Mississippi River basin, and to study the export of nitrate to the ocean, with particular focus on the Mississippi River system and the Gulf of Mexico.