## 2.27.1. Annual Transient Land Use and Land Cover Data[¶](#annual-transient-land-use-and-land-cover-data "Permalink to this headline")
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The changes in area over time associated with changes in natural and crop vegetation and the land use on that vegetation are prescribed through a forcing dataset, referred to here as the _landuse.timeseries_ dataset. The _landuse.timeseries_ dataset consists of an annual time series of global grids, where each annual time slice describes the fractional area occupied by all PFTs and CFTs along with the nitrogen fertilizer and irrigation fraction of each crop CFT, and the annual wood harvest applied to tree PFTs. Changes in area of PFTs and CFTs are performed annually on the first time step of January 1 of the year. Wood harvest for each PFT is also performed on the first time step of the year. Fertilizer application and irrigation for each CFT are performed at each model time step depending on rules from the crop model. Fertilizer application rates are set annually. The irrigation fraction is also set annually; irrigated crops are placed on separate columns from their unirrigated counterparts, so changes in irrigated fraction triggers the changes in subgrid areas discussed below (sections [2.27.2](#transient-landcover-reconciling-changes-in-area) and [2.27.3](#transient-landcover-mass-and-energy-conservation)).

As a special case, when the time dimension of the _landuse.timeseries_ dataset starts at a later year than the current model time step, the first time slice from the _landuse.timeseries_ dataset is used to represent the current time step PFT and CFT fractional area distributions. Similarly, when the time dimension of the _landuse.timeseries_ dataset stops at an earlier year than the current model time step, the last time slice of the _landuse.timeseries_ dataset is used. Thus, the simulation will have invariant representations of PFT and CFT distributions through time for the periods prior to and following the time duration of the _landuse.timeseries_ dataset, with transient PFT and CFT distributions during the period covered by the _landuse.timeseries_ dataset.

