For the assessment of rain-fed land productivity, a water-balance model is used to quantify the beginning and duration of the period when sufficient water is available to sustain crop growth. Soil moisture conditions together with other climate characteristics (radiation and temperature) are used in a simplified and robust crop growth model to calculate potential biomass production and yield. For the assessment of irrigated land productivity[1] the duration of the period with temperatures conducive for crop growth is used for matching the crop cycle length and for the calculation of biomass production and yield.
The calculated potential yields are subsequently combined in a semi-quantitative manner with a number of reduction factors directly or indirectly related to climate (e.g., pest and diseases), and with soil and terrain conditions. The reduction factors, which are successively applied to the potential yields, vary with crop type, the environment (in terms of climate, soil and terrain conditions) and assumptions on level of inputs/management.
In order to ensure that the results of the suitability assessment relate to production achievable on a sustainable basis, (i) fallow periods have been imposed, and (ii) terrain slopes have been excluded when inadequate for the assumed level of inputs/management or too susceptible to topsoil erosion.
[1] For
determining irrigated land productivity potentials, it has been assumed that
(i) water resources of good quality are available, and (ii) irrigation infrastructure
is in place. In other words, the procedures identify areas where climate,
soils and physiography permit irrigated crop cultivation, but do not assess
availability of sufficient water supply. Note, however, that Global AEZ could
readily be linked to watershed data to define limits to water availability.
