Rainfall variability in the Gezira irrigation scheme [Sudan]
1997
Ismail, M.A. (Sudan Gezira Board, Barakat (Sudan))
The rainfall data of the Gezira Scheme during the period 1970 to 1996 have been compiled and analyzed in this study. Rainfall is characterized by high fluctuations both over seasons and within the season with a downward trend. The rainfall distribution within the season has also changed. The occurrence of total annual rainfall levels of 343mm, 329mm, 316mm, 229mm, and 120mm are associated with exceedance probabilities of 0.01, 0.02, 0.10, 0.50, and 0.90 respectively. The respective return periods for these rainfall levels are 100, 50, 10, 2, and 1.11 years. Four homogeneous rainfall zones were delineated. The average annual rainfall for the period from 1979 to 1995 were 307mm, 234mm, 192mm, 124mm for the zone 1 to 4 respectively. The effect of rainfall variability on cotton pests was examined. Results of the analysis show that the levels of infestation of American Bollworm and Whitefly are positively correlated with rainfall. Whereas, there is a negative relationship between Jassid infestation and rainfall levels. The incidence of Aphids is not affected with rainfall variability. Moreover, the analysis shows that there is a positive significant relationship between the rainfall and number of sprays, implying that in heavy rainy seasons more sprays are needed to combat these pests. Cotton yield is, however, negatively correlated with July and August rainfall but positively correlated with September rainfall. This is because irrigation-water bottlenecks usually occur during September when all summer crops need irrigation. Thus, heavy rains during this month alleviate the problem
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