Development of One Day Probable Maximum Precipitation (PMP) and Isohyetal Map for Tigray Region, Ethiopia

Yohannes Gerezihier Gebremedhin


Water is a prime requirement for the existence of life; however uncontrollable amounts of water can adversely affects the survival of living beings. Due to wide range of precipitation variability, drought and extreme floods, the study of one day Probable Maximum Precipitation (PMP) for Tigray region is necessary. In an attempt to develop PMP and Isohyetal map for one day duration, using annual daily extreme rainfall series of 22 stations were subjected to statistical analysis using Hershfield formula adapted version of chow. Stations having inadequate daily records were identified and estimated using Normal Ratio Method, and Double mass curve was employed to check for the consistency of the data. An appropriate frequency factor (Km) was displayed as a function of the mean of the annual maxima for rainfall observations and the PMP for one day duration, and the highest value of Km was found to be 5.91. It was found that PMP vary from 70.06 to 144.51 mm, and the ratio of estimated one-day PMP and highest observed rainfall varied from 1.04 to 1.42. To predict extreme daily rainfall for each station normal, log normal, log Pearson type-III and Gumbel probability distribution functions were used, and values were subjected to goodness of fit tests of  chi-square, correlation coefficient and coefficient of determination tests to assess how best the fits had been. Results revealed that the log Pearson type-III distribution performed the best, with average return period 2.7*103 years. The ratio of one-day PMP to rainfall depth for frequencies return period of 5, 10, 50, 100, 1000 and 10000 year floods had been estimated and found to vary from 33.29 to 175.92 mm. The predicted PMP value to depths of various years return period ratios were computed and found to vary between 0.5132 and 2.712. Isohyetal map over Tigray region was generated by means of Arc Map10, IDW interpolation approach and the PMP Isohyetal lines were vary from 80 to 135mm. The high PMP Isohyetal values were observed in the Southern, Central and Eastern Zone and decreases in South-East Zone. For more reliable finding it is better to deal with uniformly distributed stations and larger update data as the climate pattern of the region is dynamic.

Keywords: PMP[1], Probability Distribution Function, Goodness of Fit Test, Return Period, Isohyetal Map, Tigray Region

[1]Probable Maximum Precipitation-PMP

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