Digital Corn
Using Computer Models to Simulate Agricultural Ecosystems
The power of modern computing has enabled scientists to develop large-scale ecosystem models capable of simulating natural processes over regional and even global scales and long periods of time. Thes models are invaluable tools for exploring the interaction of human actions with the environment.
A recent example comes from the work of scientists Simon Donner and Christopher Kucharik (S. Donner and C. Kucharik, 2008, 'Corn-based ethanol production compromises goal of reducing nitrogen export by the Mississippi River', Proceedings of the National Academy of Sciences, 105(11), pages 4513 to 4518).
They showed how the increased production of corn for ethanol feedstock in the American Midwest influenced nitrogen levels in the Mississippi River by linking, or coupling, a large-scale terrestrial ecosystem model (Agro-IBIS) and a hydrological transport model (THMB)
The Integrated Biosphere Simulator modified for Agricultural Systems (Agro-IBIS)
Agro-IBIS is a version of the Integrated Biosphere Simulator (IBIS) modified for agricultural systems. IBIS is one of a class of large-scale models called Dynamic Global Ecosystem Models (DGEMs) used to simulate the impacts of climate change on terrestrial ecosystems.
IBIS simulates select biophysical processes of the terrestrial ecosystem such as the movement of water between the soil, plants and the atmosphere, seasonal plant growth, carbon and nutrient cycling across the landscape, including nitrogen, and surface and subsurface water runoff.
Agro-IBIS includes these processes too, but can also model corn, wheat and soybean crop systems, growing virtual crops based on climate conditions, when and where the crops are planted and how much fertilizer is applied, tracking crop growth and nutrient uptake across the agricultural landscape. One output of particular importance for Donner and Kucharik’s analysis was the amount of surface and subsurface water runoff to the aquatic ecosystem and its nitrogen content.
The Terrestrial Hydrology Model with Biogeochemistry (THMB)
Once this virtual water and nitrogen enters the simulated aquatic ecosystem, the Terrestrial Hydrology Model with Biogeochemistry, or THMB, moves it down the Mississippi River in discrete steps. At each step, it accounts for the river flow from upstream, any new runoff and nitrogen entering the river system, precipitation, evaporation, and nitrogen losses due to natural processes. THMB’s final output is an estimate of how much nitrogen enters the Gulf of Mexico.
The Agro-IBIS/THMB system was used to model agricultural processes over the entire Mississippi watershed, an area of 3.2 million square kilometres with calculations performed for discrete cells, each representing an area of approximately 10 square kilometres. This equates to about 320,000 cells and clearly illustrates the importance of computers to modern ecosystem modeling.
While in this particular example Donner and Kucharik used the Agro-IBIS/THMB system to analyze the conflict between corn-ethanol and nitrogen pollution, it has more general applicatations as a tool for exploring the impact of a changing climate on the agricultural landscape and changing agricultural techniques on the environment.
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