External Inputs to the Land Model

GCAM’s external land inputs include information on land, carbon, other emissions, and the value of unmanaged land in the historical period. These inputs are required for each global land unit and historical year.

External Inputs

Description

Table 1: External inputs used by the land model ¹

Name	Description	Type	Source	Resolution	Unit
Historical Land Use and Land Cover	Land area by region, land type and year. Land cover data is provided beginning in 1700 in order to spin-up the carbon cycle within GCAM. Crop-specific harvested area is used to downscale FAO data to a subnational level; however this data is only available for a single year. Similarly, the division between irrigated and rainfed land is only available for a single year only.	External Data Set	Moirai	Raw data is by country, water basin and land type.	thousand \(km^2\)
Historical Harvested Area	Harvested land area by country, crop, and year.	External Data Set	FAO, gcamdata-faostat	Raw data is by country, crop, and year	Hectares
Historical Cropland Cover	Arable land, temporary crops, and temporary fallow land area by country and year.	External Data Set	FAO, gcamdata-faostat	Raw data is by country and year	Hectares
Terrestrial Carbon Information	Inputs include potential vegetation and soil carbon density (i.e., carbon density if the land grew to equilibrium), and mature age for vegetation carbon. Note that vegetation carbon contents for crops are calculated from crop yields. All other carbon parameters are external inputs.	External Data Set	Houghton (1999) and King (1997)	Land type and Biome	Various
Soil time scale	Inputs include the number of years for soil carbon changes to occur. Note that this is not the time to equilibrium, which is much longer.	Assumption		GCAM geopolitical region	Years
Value of Unmanaged Land	GCAM requires profit rates for all land types in the historical period for calibration. Managed land profit is calculated in the supply model. For unmanaged land, however, the value is input into the model.	External Data Set	Moirai	Raw data is by country, water basin, and land use.	million USD
Share Parameters	GCAM requires the user to specify the logit exponents that determine the substitutability between different leaves and nodes in the land model. These parameters were chosen to produce land supply elasticities comparable to those found in the literature, although it should be noted that there is not a transformation between logit exponents and supply elasticities for all land types.	Assumption		Specified by land node	N/A
Parameters to introduce a new land type	For land types that do not exist in the historical period, GCAM requires parameters to introduce these land types in the future. Specifically, GCAM needs to know how that land type will compete with other land types in its nest if it were to have equal profit.	Assumption		Currently specified globally, but could be specified by region and water basin	Share

1: Note that this table differs from the one provided on the Land Modeling Page in that it only lists external inputs to the land module (either data sources or assumptions). Additionally, the units listed are the units of the raw inputs, rather than the units the GCAM requires.

Note that for the Shared Socioeconomic Pathways (SSPs), different inputs are used for some variables. See SSPs for more information.

Data

Historical Land Use and Land Cover

The raw data used for historical land cover is mainly provided in Land_type_area_ha.csv.

The arable land, temporary crops, and temporary fallow land are provided in GCAMDATA_FAOSTAT_LandCover_229Regs_3Covers_1973to2020.csv.

Historical Harvested Area

Historical harvested area by crop and country is provided in GCAMDATA_FAOSTAT_ProdArea_195Regs_271Prod160AreaItems_1973to2020.csv.gz and GCAMDATA_FAOSTAT_ProdArea_96Regs_16FodderItems_1973to2020.csv.gz.

The data used to separate crops into irrigated and rainfed is provided in MIRCA_irrHA_ha.csv and MIRCA_rfdHA_ha.csv.

Terrestrial Carbon Information

The mature age and carbon density information used in GCAM is provided in Various_CarbonData_LTsage.csv. Crops that use C4 photosynthesis and tree crops are each disaggregated in the land allocator; the specific FAO crops that are mapped to these categories are indicated in FAO_ag_items_PRODSTAT.csv. The vegetation carbon densities of tree crops are estimated according to an assumed ratio between crop yield and vegetation carbon density for each specific type of tree crop, in Various_Tree_C_yield_ratios.csv

Soil time scale

The soil time scale is provided in A_soil_time_scale_R.csv

Value of Unmanaged Land

The unmanaged land value is directly computed based on GCAM data since it is more consistent and avoids potential aggregation uncertainties in processing the dated SAGE-GTAP data. See additional details in CMP #393.

The logit exponents used in the land allocation module are provide in A_LandNode_logit.csv and A_LandNode_logit_irr.csv.

Parameters to introduce a new land type

The calibration parameters for bioenergy are in A_bio_ghost_share.csv.