The objective of this work was to estimate the relative water consumption (ETr/ETm) for the critical period of soybean in several sowing dates for ten places of Rio Grande do Sul state, Brazil. The maximum evapotranspiration (ETm) was estimated using the crop coefficient (Kc) while the real evapotranspiration (ETr) was estimated through the water balance, calculated for the period from 1975/76 to 2000/2001. Considering the average values of the periods, the relative water consumption varied from 0.46 in the sowing of October, in Rio Grande, to 0.74 in the sowing of December in Veranópolis. São Gabriel and Rio Grande are the places that present the smallest values of relative water consumption, therefore with larger risk for the production of grains. In years of strong drought, the relative water consumption, in some situations, was smaller than 0.20, evidencing the problem of water deficiency. In 46.3% of the cases the relative water consumption was classified as of high or medium risk (index ETr/ETm < 0.60 during the critical period). Rio Grande presented 63.3% of the cases with index equal to or smaller than 0.60, followed by São Gabriel with 61.1%, Encruzilhada do Sul with 52.6% and Júlio de Castilhos with 50.8%. The places of smaller risk were Passo Fundo, Veranópolis and Santa Rosa, with indexes equal to or smaller than 0.60 happening in 30.3%, 37.5% and 38% of the cases, respectively. The sowing date with higher risk was October and with lower risk was December. For the group of the places, it can be expected that every twenty years, in about nine reduction of soybean yield should happen due to water deficiency, with variable intensity, depending on the place, year and sowing date.
maximum evapotranspiration; real evapotranspiratiom; soybean; relative evapotranspiration; sowing date
