EPA's draft NONROAD2004 emission inventory model calculates nonroad equipment populations by age (i.e., an age distribution) for given equipment types and scenario years. This calculation is necessary for the model to account for factors which affect nonroad emissions over time as the in-use fleet ages and turns over to newer equipment, including emissions deterioration, new emissions standards, technology changes, and changes in equipment population resulting from sales growth trends. The NONROAD model calculates equipment age distributions for each horsepower range of each equipment type based on the median life (hours at full load), activity (hours of use per year), load factor, and current growth rate for that equipment, combined with the model's generalized scrappage function. Age distribution refers to the fractions of a given equipment's population that are one, two, three, etc., years old in a given target year. This age distribution is treated as constant regardless of the target calendar year being evaluated, as described in more detail below. This methodology differs from the previous versions of NONROAD, which attempted to calculate age distributions for future years by stepping through each year between the base year and future evaluation year, calculating equipment populations and scrappage for each model year of equipment still in service, and the necessary equipment sales to meet the projected population.