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Probability (%)10080604020VegetativeReproductiveGrain filling0 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5Probability (%)100Vegetative80 ReproductiveGrain filling60402002 3 4 5 2 3 4 5 2 3 4 5Consecutive 5-d drought periodsNumber of 5-d drought periodsFig. 2. Probability of 5-d droughts during thecrop season.Fig. 3. Probability of consecutive 5-d droughtsduring the crop season.5-d droughts can be expected during the grain-filling stage compared with none duringboth the vegetative and reproductive stages.Figures 2 and 3 show the probability analysis of 5-d droughts and consecutive5-d droughts. The probability of two 5-d droughts (not consecutive) during the grainfillingstage is more than 80%. The probability of 10-d and 15-d droughts (two andthree consecutive 5-d droughts) during the grain-filling stage is 73% and 53%, respectively.Thus, a 2-wk period without rain is expected once in two years and can bedetrimental to crop yield.Figures 4 and 5 show a scatter diagram of seasonal rainfall (15 July-15 November)and total numbers of 5-d droughts and consecutive 5-d droughts. There is a faircorrelation (r = 0.56 and significant at the 1% level) between total seasonal rain andtotal number of 5-d droughts. Seasonal rainfall and total number of consecutive 5-ddroughts, however, are uncorrelated.The 50% and 80% dependable 5-d rainfalls for May-November were determinedand are shown in Figure 6, which shows that in the study area little or norainfall is expected after early October. Since early October is the beginning of thegrain-filling stage, the crop is vulnerable to damage by drought.Figure 7 shows the probability of getting the 400 mm of rainfall required fortransplanting at different times at the beginning of the crop season. In an average year(50% probability), the 400 mm of rainfall required for transplanting can be expectedby 15 July. Twice in ten years (80% probability), however, this amount may not beavailable before 15 August and transplanting may be delayed by one month.A water adequacy analysis was carried out for crop field duration by calculatingthe relative water supply using equation 5 in each 5-d period. The average seasonalRWS value at 50% probability, when transplanting is completed by 15 July(average year), is 0.79. This means that, even in an average year, the water supplyfrom rainfall is inadequate to meet the crop water requirement. If transplanting isdelayed because of inadequate rainfall at the beginning of the season (completed afterAgrohydrologic and drought risk analyses of rainfed cultivation . . . 239