This research was conducted with the objective of estimating supplementary irrigation need when sprinkler irrigation is used on soybeans (Glycine max L. Merrill) crops, in the agroecological regions of Rio Grande do Sul, Brazil, called "Planalto Médio" and "Missões". The water deficit simulated combining different sites, sowing seasons, irrigation management levels and risk levels. Results indicated that the needs for supplementary irrigation were higher when sowing was performed on October 15. The lower values were observed when sowing was performed on December 15. Supplementary irrigation requirements were greater under the agroecological conditions of the "Missões" as compared to the "Planalto Médio". The supplementary amounts of irrigation estimated increase as the level of risk diminishes.
