PROBABILITY OF A MONTHLY RAINFALL BEING EQUAL TO OR G REATER THAN POTENTIAL EVAPOTRANSPIRATION DURING THE GROWING SEASON OF SPRING-SUMMER CROPS IN THE STATE OF RIO GRANDE DO SUL
Abstract
The state of Rio Grande do Sul is responsable for approximately 25% of the national grain production. With the exception of rico, practically all the others agricultura' crops are grown without irrigation therefore depending on the natural rainfall regime. The simplest way of cstimating the climatic water availability for crops is through comparison of the amount of rainfall with the potential evapotranspiration. Our objetives were to estimate the probability of rainfall bcing cqual to or greatcr than the monthly average potential evapotranspiration during the growing season of the spring-summer crops in the state (September to May). Our study was bascd on 27 meteorological stations well distributcd in the state and with a homogcncous historie series. The probability of monthly rainfall bcing grcater than the potential evapotranspiration decreases as we approach the summer months. In the ctitical months accordind to the water availability (December, January and February), for the spring-summer crops, Chis probability is equal to or less than 60% in a great portion of the state, including the principal grain producing regions
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