O presente trabalho desenvolve uma metodologia de estudo das necessidades de irrigação das culturas, através da simulação do uso de irrigação por meio de balanço hídrico diário. Os dados foram coletados no posto meteorológico da Estação Experimental de Ribeirão Preto, Instituto Agronômico de Campinas, em fevereiro e julho, no período de 1969 a 1990. Para atendimento a varias culturas, em diferentes fases de crescimento e tipos de solo, foram escolhidas laminas de consumo admissível de agua do solo (CAAS) de 5 a 50mm e os valores de coeficientes de cultura (kc) de 0,5 e 1,0. Com relação ao numero de irrigações observou-se a importância de se procurar aumentar o valor de CAAS quando este era pequeno, pois pode resultar em razoável redução no numero de irrigações por ano. Para os maiores valores de laminas de CAAS adotados essa variação não trouxe resultados expressivos. A introdução da precipitação, a simulação do balanço hídrico, e o uso de valores probabilísticos no calculo da lamina e do intervalo entre irrigações, fornecem mais subsídios para o planejamento e manejo de sistemas de irrigação, do que o simples uso da evapotranspiração media mensal

This work develops a methodology for estimating supplementary irrigation requirement in tropical and subtropical regions. Irrigations were simulated for February and July through simple water balance technique, based on 21 years of meteorological data of Ribeirão Preto, State of São Paulo, Brazil. In order to attempt different situations of crops and soils, it was chosen eight levels of allowable soil water depletion (ASWD) and two crop coefficients (kc). For the water balance it was assumed that soil water profile was at field capacity at the beginnlng of each month and after each simulated irrigation. For each month, irrigation intervals, number and depth of irrigation were annotated and its empirical probability calculated. These Information are the basis of the proposed methodology. The adopted criteria, that each simulated irrigation was done only when the allowed soil water depletion (ASWD) was exhausted, demanded an addition depth of irrigation over ASWD. This additional water might reach significant values and need to be predicted in the írrlgation system design. The inclusion of natural rainfall and the use of probabilistic values, as proposed. for calculating interval and depth of Irrigation turned out to be advantageous and more economic than using average monthly evapotranspiration, as commonly done. In relation to average number of irrigation per month it was observed, for small values of ASWD. That reasonable reduction could be attempted if ASWD was for extended. For large values of ASWD, little reduction in irrigation number per year was obtained by varying ASWD. The significant information obtained in this work recommend the use of the proposed methodology in irrigation design and management. The positive results also encourage to complete the simulation throughout the in Ribeirão Preto, as well as other locations in the State of São Paulo.