Environmental Protection Agency (EPA) to determine GHG reductions from various biofuel pathways. As indicated above, FASOM is the only one of the models that looks forward and solves all years simultaneously.
GTAP is both a database and a modeling framework. The GTAP database can be used with other models, and the basic GTAP model can be adapted to particular questions of interest. GTAP adopts the Armington approach to international trade, which means that domestic and imported goods are differentiated. Changes in the structure of international trade are less pronounced than for models with the Heckscher-Ohlin assumption of homogeneous goods. There are 57 sectors plus 3 biofuel sectors in the standard GTAP database. However, most research is done with far fewer regions and sectors. Aggregation tools are available to permit users to specify whatever regional and sector aggregation they prefer. Global land is divided into 18 agroecological zones, and land covers of cropland, pasture, and forest are included. GTAP simulations (using the comparative static approach) essentially estimate the global economic effects from whatever policy or technology shock is of interest. For example, GTAP has been used to estimate the effects of a 15-billion gallon mandate for corn-grain ethanol. To do so, the results of the baseline model run are compared to the results of introducing a shock into the model that requires 15 billion gallons of corn-grain ethanol be produced to satisfy the Renewable Fuel Standard as amended under the Energy Security and Independence Act of 2007 (RFS2) while holding other demands and requirements constant. The results of these two model runs are then compared with respect to corn price, corn exports and imports, other agricultural prices, land-use change, and other relevant economic variables.
POLYSYS includes eight U.S. crops (corn, sorghum, barley, oats, cotton, rice, wheat, and soybean) and seven livestock sectors (beef, pork, turkeys, broilers, eggs, lamb and mutton, and dairy). The model divides the United States into 305 crop-reporting districts; its current version is able to analyze county-level effects if needed. The POLYSYS simulations are anchored to USDA’s published crop-projection baseline, and the simulations provide deviations from that baseline for the shock of interest. Crop and livestock supply is regionally generated, and demand is handled through a system of simultaneous equations. POLYSYS also tracks farm income, government payments, and several environmental variables. For environmental variables, POLYSYS interacts with the Environmental Policy Integrated Climate (EPIC) model (Williams et al., 1990, p. 18). The model can be and commonly is run stochastically to capture the effects of uncertainty and nonlinearities in some of the functions. In recent years, POLYSYS has been modified to include crop residues and dedicated energy crops as biomass feedstock supply options.
LAND COVER IN ECONOMIC MODELS
The models differ in the extent to which types of cover are included. GTAP and FASOM have forestland, pasture, and cropland, although it is predominantly only U.S. land in FASOM. POLYSYS includes cropland and pasture, but not forestland at present, although it may be added in the near future. FAPRI has cropland and pasture for the United States and Brazil but not forestland.