Estimativa da necessidade de irrigação suplementar para algumas culturas no Estado do Rio Grande do Norte.

Abstract

The estimated values of the precipitation at 80%, 75%, 60%, 50% and 25% probability levels were determined by employing the Incomplete Gamma Probability Distribution using the spacial averages of ten day precipitation totals of each meso-region of the state of Rio Grande do Norte, which is situated in the northeast of Brazil. The temperature estimates were used to calculate the potential évapotranspiration (ETP) using the methodology described by Thornthwaite (1948). The effective precipitation values were determined from the estimated values of ten day precipitation at different probability levels. The irrigation needs of the crops were obtained by using the crop coefficient values (Kc) suggested by Doorenbos and Kassam (1979) among other researchers. The methodology suggested by Dastane (1974) which employs the ratio between the potential évapotranspiration and the estimated precipitation was used to determine the effective precipitation (PE). The actual ten day potential évapotranspiration (ETRd) was obtained from the product of the crop coefficient (Kc) and the ten day potential évapotranspiration (ETPd). The supplimentary irrigation needs for ten day periods wthin the crop growing season were obtained by subtracting the effective precipitation values from the ten day actual évapotranspiration values. Depending on the aptness and water needs of the bean, corn, tomato and cotton crops, tables of the estimated values of the supplimentary irrigation needs are prepared for each of the ten day periods during the growing season in the sub-regions of "Alto Oeste" and "Baixo Oeste" and in the mesoregions of "Central, Agreste" and "Leste Potiguar" were prepared. The results show that the supplimentary irrigation needs of the four crops mentioned earlier are lesser in the meso-region of "Leste Potiguar' when compared with the other meso-regions studied in this research. All the crops studied needed the supplimentary irrigation when the precipitation at 75% probability level was considered. Of all the crops studied, the bean crop needed the minimum amount of water during its phenological cycle, irrespective of the region studied.