The probability of the occurrence of a dry spell period during the growing cycle of wheat was determined using the gamma distrobition with gamma = 1, suitable for data with an inverted-J curve frequency distribution. The maximum evapotranspiration was estimated for 10-day periods, switching stepwise the sowing date from March to May, in several localities of the São Paulo State, Brazil. Assuming that the available water is the major factor which affects yield, the probability of attending the crop water requirements was estimated using the maximum evapotranspiration and rainfall for a given period. It was observed that in the Paranapanema Valley and in the southwestern part of the State the probability of water supply for this crop is higher than the other regions and the best sowing date lies between March 11 and April 1.
wheat; planting date; maximum evapotranspiration; gamma-distribution