The objective of this work was to estimate the relative water consumption (ETr/ETm) for the critical period of common bean in several sowing dates, and different water availability in soil for tree 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 0.68 in the sowing date of November, in Julio de Castilhos, for water availability in soil of 50mm, to 0.88 in the sowing of September in Passo Fundo, for water availability in soil of 70mm. The medium values, obtained for each local and sowing date, are inside of favorable index for the common bean (ETr/ETm > 0.60). The largest values of the relative water consumption where obtained in sowing date of September, with intermediary values for October and smallest values in sowing of November. The relative water consumption was smaller than 0.20, in years of strong drought, evidencing situations of high risk to grain production. In 20.6% of the cases, the relative water consumption was classified as of high or medium risk (index ETr/ETm £ 0.60 during the critical period), for water availability in soil of 50mm. Cruz Alta presented the highest risk to grain production, while Passo Fundo presented the smallest risk. The sowing date with the highest risk was November while September had the lowest risk. For the analyzed places, it can be expected that, of every ten years, in about two should happen reduction of common bean yield due to water deficiency, with variable intensity, depending on the place, year and sowing date.
maximum evapotranspiration; real evapotranspiration; relative water consumption; sowing date; common bean
