Land use transitions

Simulating effects of land use in a BGC model ideally resolves sub-grid scale land use transitions (gross land use change). This is relevant when abandonment of cultivated fields and claiming new agricultural land occur in parallel within a single model gridcell, i.e. where shifting cultivation or fallow rotation is a common agricultural practice. For Landcover6K-BGC simulations, we encourage model simulations to resolve such gross land use change (simulation bgc_lu12_clim0, see Protocols). However, entry-card simulations do not require the resolution of gross land use change. Only net land use change is to be resolved for the “entry-card” simulation bgc_lu10_clim0. In the following, the definition of land use transitions as a function of the parameters \(\tau_f\) (fallow duration) and \(\tau_c\) (duration of cultivation between fallow periods, see forcing) is described.

BGC models simulating LULC commonly distinguish between natural land, croplands, and pasture (some additionally distinguish urban land) and require the fractional gridcell for each of these land use categories to be specified as an external forcing for each gridcell and time step. Let’s refer to these gridcell fractions as \(f_{\text{c}}, f_{\text{p}}, f_{\text{natural}}\). These add up to the total land area fraction in each gridcell, which is typically one (\(\sum_i f_i = 1\)).

Transition cropland to secondary

Models simulating gross land use change additionally require information about transitions between land use categories. For Landcover6K-BGC simulations, we consider only transitions from and to cropland, and encourage the distinction between primary (virgin) and secondary natural land (\(f_{\text{c}}, f_{\text{p}}, f_{\text{v}}, f_{\text{s}}\)). Abandoned cropland area (area fraction transiting away from croplands is to be added to secondary land and is referred to as \(f_{\text{c,s}}\). For a specific time step and gridcell, it is given by: \[ f_{\text{c,s}} = \frac{1}{\tau_\text{cult}} f_c + max(-\Delta f_c, 0) \] \(\tau_\text{cult}\) is the duration of cultivation between fallows (see forcing), and \(\Delta f_c\) is the net change in the cropland area fraction between time steps (positive values for expansion).

Transitions to cropland

Additional information is required to define whether land claimed for new cropland areas is to be taken from virgin (never converted previously), or from secondary land. If all land is to be taken from virgin land, land turnover in a (very) long simulation would eventually lead to all land at least once being used as cropland within one gridcell, and eventually all natural land is secondary. On the other extreme, if all land was claimed from secondary land, the regeneration during fallow may be very short and a large fraction of the gridcell area is spared as virgin land. The additional parameter \(\tau_\text{fallow}\) defines the period of fallow and is needed to inform this allocation rule. The area ratio of fallow to non-fallow fields is thus \(\tau_\text{fallow}/\tau_\text{cult}\) and the total land in all stages of fallow and cultivation plus pasture areas is: \[ f_\text{tot} = \left( \frac{\tau_\text{fallow}}{\tau_\text{cult}} + 1 \right) f_c + f_p \] Allocation rules that specify whether new cropland area is to be claimed from virigin or secondary land should thus be formulated so that secondary land (all stages of fallow) is (close to) \(\tau_\text{fallow}/\tau_\text{cult} \times f_c\). This can be achieved by prioritising claiming from virgin land, but restricting the total land that is claimable from virgin land based on \(f_\text{tot}\) and thereby maintain a given area ratio of fallow to non-fallow fields and sparing a fraction \((1 - f_\text{tot})\) from conversion (forced to remain virgin). This may be implemented by first defining the available virgin land for conversion \(f_\text{avl}\) as \[ f_\text{avl} = max( 0, f_v - (1 - f_\text{tot})) \] and the required land area fraction for new cropland as: \[ f_\text{req} = \frac{1}{\tau_\text{cult}} f_c + max(\Delta f_c, 0) \]

and then specifying the transition of virgin land to cropland as: \[ f_{v,c} = min( f_\text{req}, f_\text{avl}) \] and finally the transition of secondary to cropland to satisfy the remainder: \[ f_{s,c} = f_\text{req} - f_{v,c} \]