Agro-climatic constraints

Matching the climatic characteristics by grid-cell with the crop temperature profile requirements and the calculation of net biomass and yield provides quantification of crop production that can be anticipated under conditions that are essentially free from soil constraints but also from agro-climatic constraints.

Agro-climatic constraints cause direct or indirect losses in the yield and quality of produce. Yields losses in a rain-fed crop due to agro-climatic constraints have been formulated based on principles and procedures proposed in FAO (1978-81a). For details of the conditions that are influencing yield losses and a listing of the agro-climatic constraint parameters for all the crop/LUTs considered, reference can be made to .

In the present study, covering also temperate and boreal environments, one additional category of agro-climatic constraints has been added to the four categories in the original study. This fifth so-called ‘e’ constraint covers yield losses due to the occurrence of early or late frosts. The five categories are:

(a)	Yield losses due to water-stress constraints on crop growth (e.g., rainfall variability);
(b)	Yield losses due to the effect of pests, diseases and weed constraints on crop growth;
(c)	Yield losses due to water-stress, pest and diseases constraints on yield components and yield formation of produce (e.g., affecting quality of produce);
(d)	Yield losses due to workability constraints (e.g., wetness rendering produce handling difficulties); and
(e)	Yield losses due to occurrence of early or late frosts.

The availability of historical rainfall data has made it possible to derive the effect of rainfall variability through year-by-year calculation of yield losses due to water stress. Therefore the (a) constraint, related to rainfall variability is no longer applied. However the (a) constraint has been retained in the agro-climatic constraints database for use with data sets containing average rainfall data and for comparison with results of the presently used year-by-year analysis.

The (b), (c) and (d) constraints are related to wetness and the ratings of these constraints have been linked to the LGP. It appears however, that in different climate zones, wetness conditions, traditionally expressed as P/ETo ratios, vary considerably for similar LGPs. Long LGPs with relatively low P/ETo ratios occur generally in subtropical, temperate and boreal zones, while relatively high ratios occur in the tropics.

To account for these significant differences in wetness conditions of long LGPs (> 225 days), agro-climatic constraints have been related to P/ETo ratios by calculating equivalent LGPs, i.e., adjustments where P/ETo ratios where below average. The equivalent LGPs are then used in the application of the (b) (c) and (d) constraints. This presents an example of agro-climatic constraints for winter wheat. For irrigated production only the agro-climatic constraints related to excess wetness apply.

The application of the agro-climatic constraints to the combined results of temperature suitability and the biomass and yield calculations provides agro-climatic suitabilities. and present examples of agro-climatic suitability maps for rain-fed and rain-fed plus irrigated wheat production at the high level of inputs.