GCAM is an open-source, global, long-term, multi-sector human Earth system model; it contains representations of energy, economy, agriculture and land-use, and water systems for 32 geopolitical regions in the globe (Calvin et al. 2019). GCAM-USA divides the United States into 50 states and the District of Columbia (Iyer et al. 2015, Iyer et al. 2019, GCAM-USA documentation). GCAM-USA is embedded within the global GCAM model, so conditions within the United States are internally consistent with international conditions. The state-level regions contain more detailed representations of national-level economic features, including socioeconomics, energy transformation, carbon storage, renewable resources, electricity markets, and consumer end-use energy demands.

GCAM-USA represents energy-water-land dynamics at the level of states and the basins. The energy system includes detailed representations of fossil and renewable resources, conversion to usable forms (e.g. electricity), and end-uses. State-level assumptions about population and economic growth drive demands in three end-use sectors (buildings, industry, and transportation), each of which is modeled at the level of individual states. In addition, renewable resources, and energy conversion processes (electricity, and refining) are also modeled at the state-level. The model tracks water demands for power, domestic use, primary production, manufacturing, livestock, and irrigation at the state-level and includes supply constraints (with explicit representations of renewable and non-renewable supplies) at the basin scale. The land component of GCAM-USA calculates supply, demand, and land use for food, feed, fiber, forestry, and bioenergy products, as well as land cover for natural ecosystem types. Competition for land across various land uses (crops, pasture, commercial forest, and urban land) and land cover types (e.g., grass, shrub, tundra, non-commercial forest, and other arable land) is represented in 384 land-use regions. Representations of energy, water, and land systems dynamics are all hard-linked albeit being represented at different scales which offers users of GCAM-USA a unique ability to explore energy-water-land interactions at multiple scales in one consistent modeling framework.

The model has technological detail in all of the sectors. For example, the power generation sector includes details of about 30 different power generation technologies, four different types of thermal cooling systems, about 10 different building service types and building technologies that vary across fuel inputs, and about 40 different transportation technologies that vary across modes, vehicle class, vehicle size, and fuel input. GCAM-USA includes the ability to explore climate impacts on energy supply and demand, and agricultural yields and associated impacts land-use and agriculture trade. GCAM-USA is fully open source (along with its code) and can be downloaded from a github repository.