Estimation Of Probable Maximum Precipitation: - A Case Study On Abelti Watershed, Omo Gibe River Basin, Ethiopia

Abstract

Hydrological studies are useful in planning, designing and managing water resources, infrastructures. Probable maximum precipitation is ideally the maximum precipitation depth for a given duration at a definite geographical location at a specific time in a year or season. PMP value should be estimated by more than one method for critical hydrologic regulations, and preferably by both statistical and moisture maximization methods. The aim of this study is to estimate and compare the reliable probable maximum precipitation values and computing Various return period Rainfall Depth for estimated PMP values for Abelti watershed, Omo Gibe River basin, in South Western Ethiopia. Daily Rain fall, maximum temperature and relative humidity and Micro soft excel-2016, R-package, digital elevation model and Arc Map 10.4.1 were data and software used to analysis the study respectively. In this research, annual maximum one day duration rainfall data of 31 years from 8 stations based on statistical and three station based on moisture maximization methods in the Abelti catchment were analyzed to estimate PMP respectively. Probability distribution such as normal, lognormal, gamma and Weibull values were subjected to GoF test and Chi-square, Anderson-Darling and Kolmogorov Smirnov tests to assess how best the fits had been. Results showed that the lognormal distribution performed best for study area. The statistical Hershfield’s Km varied from a minimum of 2.16 at Sekoru station to a maximum of 5.0 at Gedo for a 1-day rainfall duration. So, the maximum frequency factor (Km) obtained from statistical method on the study area was 5.0. The maximum PMP values estimated from statistical method for 1- day durations were 173mm observed at Limu Genet station. The PMP values Estimated by statistical method was 1.63 times higher than the maximum rainfall record. The PMP estimated from the Moisture Maximization method is average of 1.94 times higher than the PMP founded from statistical approach. The PMP values were also derived for different return period values and found to vary between 45.5 mm at 2 years return period for Gedo station and 194.5 mm at 10,000 years return period at Wolkite station.