**3. General issues**

The climate exerts the greatest limiting effect on yield. Thus, proper management of the soybean crop depends on climate information at the site of interest. Agroclimatic zoning is used to provide climate data to the agricultural community [14].

Cardoso et al. [15] verified that in general the precipitation climate forecasts contribution to the improvement of estimated soybean productivity, primarily in periods when the crop is more sensitive to water deficit. The improvement of estimated soybean productivity may give a contribution to agribusiness sector, in order to turn more realistic expectations available and assist on the strategic planning for the decisions maker.

Knowledge of favorable periods for soybean culture, such as timing of the rainy season and duration of intermittent dry wet spells is very important for planning various agronomic operations such as preparing a seedbed, proper maturity, sowing, weeding, harvesting, threshing and drying. This minimizes crop risk and optimizes use of resources such as water, labor, fertilizer, herbicides and insecticides. Knowledge of the critical periods for crop development in conjunction with knowledge of climatic factors is very helpful for adjusting management practices for best crop yield and quality.

As already highlighted climate stresses such as drought, excessive rain, extreme temperatures and low irradiance can significantly reduce yields of crops and restrict areas and timings of production. The most important factors useful to know for soybean production are air temperature, photoperiod and water supply [10].

In Brazil, a system has been developed for identification of suitable areas for planting soybean. The Agricultural Zoning Program has been coordinated by the Ministry of Agriculture and EMBRAPA since 1996. This program has the objective of defining planting seasons guarantee at least an 80% probability of having an adequate water supply for the growing season. The planting periods were defined through the simulation of a climatic water balance that gives an index of water supply called by the Portuguese initials as ISNA (Water Necessity Satisfac‐ tion Index) using historical rainfall data, potential evapotranspiration, physiological charac‐ teristics of each crop and water retention by the soil [16].
