DayCent Computer Model Compares Biofuels' Impacts

By Jerry W. Kram
People have many reasons for wanting to switch from fuels and products made from petroleum and coal to those made from biomass. Some are concerned with the inevitable decline of fossil fuel resources and their growing cost. Others worry about greenhouse gases and climate change. For the latter, there is a thorny question: Do alternative fuels actually reduce the likelihood of global warming?

While that question is still somewhat controversial, researchers are generating new data that is starting to fill in the blanks and lead toward a definitive answer. One tool in the search for an answer is a computer simulation of how agricultural crops, such as corn used for ethanol production, affect the release of greenhouse gases from the soil. The program is called DayCent, and one of its developers is Stephen Del Grosso of the USDA Agricultural Research Service.

The program simulates the production of greenhouse gases from the soil and also simulates the growth of crops, Del Grosso says. That gives scientists the data they need to compare the impact of different biofuels feedstocks. "It gives you your soil emissions and your crop yields," he explains. "If you have your crop yield, you know how much fossil fuel you are displacing. Then there are some other models that come into play."

In order for the model's estimates to be accurate, a large amount of background data must be gathered, analyzed and incorporated into the program. "We don't feel comfortable running the model with any arbitrary crop that we haven't compared with data," Del Grosso says. "So that is the first thing we want to establish-that the model does perform reasonably well."

One of the model's recent tests was a comparison of biofuel feedstocks in Pennsylvania. The state was chosen because data was available for yields of potential biofuel crops, such as switchgrass, along with comprehensive data on soil types and conditions. "We could do what we called ‘model validation,' comparing the model to the data and tuning the model for different crops," Del Grosso says. "[For example,] were pretty satisfied with how the model predicted [nitrogen dioxide]. That's not to imply it's anywhere close to perfect, but compared to other models of similar sophistication, it does pretty well." Other research groups are testing the model in locales from Canada to New Zealand.

Nitrous oxides, such as nitrogen dioxide, are potent greenhouse gases. "In these types of systems, [nitrogen dioxide] is by far the biggest source because it has a global warming potential of 300 [times the same amount of carbon dioxide]," Del Grosso says. "Even though the actual fluxes of [carbon dioxide] might be higher, once you account for the global warming potential of [nitrogen dioxide], it totally swamps things out." So far, DayCent has matched or beaten other models when its predictions are compared with actual data. The Pennsylvania study indicated that all the examined biofuel feedstocks reduced net greenhouse gas emissions when compared with fossil fuels.

The next stage of the project is to validate the model for different areas of the United States. "There are switchgrass plots in the Midwest, [particularly] Iowa and the Dakotas," Del Grosso says. "We are in the process of running the model in those areas to make sure we get reasonable results. So I think in a year or two we will have [verified the model] that can produce switchgrass yields in a couple areas of the United States. The next big goal will be to run DayCent countrywide in areas where these biofuels are feasible and try to come up with a rough estimate on a national scale of how much fuel we could reasonably create."