ABSTRACT The effective rainfall amounts (ER) as a part of the irrigation requirement were estimated for the premature and serotinous varieties of rice in the Mazandaran Plain , using different methods. Finally the “Dependable Rain” method were selected for the estimation.Comparison of the maps, reveals that the ER amounts are more in the western part of the plain than the eastern part ; Consequently , the net irrigation requirement is low in the western part. Because knowing the minimum and maximum values of the ER with specific confidence, helps the planners in different decisions, the ER amounts were estimated at 90 , 95 and 99 percent confidence intervals. The related maps show that the confidence for ER amounts is low for both premature and serotinous varieties in the eastern part; Meanwhile the ER amounts are almost 50 milimeters more for serotinous variety than premature variety at different confidence intervals in the whole region. Also, The maps of return periods, show that the ER amounts are higher in the western and central parts than the eastern part and that the accessibility of ER, varies from lower than 80 to more than 420 milimeters in the growing season in terms of various return periods and different parts of the region.

Keywords: KEYWORDS : effective rainfall, return period, confidence interval, Rice, Mazandaran Plain

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