Estimation of probable maximum precipitation: a case study for Bilate sub river basin, Rift Valley Basin, Ethiopia

Abstract

Probable maximum precipitation is a basic and fundamental data for determining the probable maximum flood in the design of hydraulic structures. It is defined as theoretically greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographical location at a certain time of the year. Developing Isohyetal map was to overcome the problem of inadequate information and to facilitate quick estimation of PMP values for ungauged catchments in the basin. The General objective of this study was to estimate probable maximum precipitation and development of Isohyetal map for bilate sub river basin. It was provided to overcome the problem of inadequate information and to facilitate quick estimation of PMP values for ungauged catchments in the basin. Statistical package for social science20, Global mapper18, Digital Elevation model and MATLAB R2018a were the material used to progress the work. L-moment was a parameter for selection of distribution as candidate distribution. normal, log normal, Gamma, Extreme value type one and General extreme value was used, and values were subjected to goodness of fit tests of chi-square and Kolmogorov-smirnov tests to assess how the data has been the best fits. Diagnostic D-index was used to separate best distribution to predict different year return period rainfall depth from fitted distribution. Accordingly, the minimum D-index value was for General extreme value and parameter of Probability weighting method to station of Angecha with value of 0.024. Isohyetal map over bilate have been generated by means of Arc Map 10.4.1. Estimated value of Probable maximum precipitation was found that from 71.581mm to 128.571mm of stations Durame and Humbo respectively for 1-day duration and the statistical Hershfield Km was estimated as 1.717 to 3.231, belongs to stations of Durame and Hossana with an average value of 2.418 for 1-day duration. the overall monitoring data series at stations are taken into account the Probable maximum precipitation value calculated by statistical method is more suitable.