Estimation of Probable Maximum Precipitation: A Case Study on Lower to Middle Awash River Basin, Ethiopia.

Abstract

Probable maximum precipitation (PMP) is theoretically the greatest depth of precipitation for a given duration that is physically possible over a given size of storm area at a particular geographical location at a particular time of year. The PMP helps for the design of a civil structure appropriately in the study area. The PMP for rainfall stations in Ethiopia have been estimated by Herhfield’s statistical method with frequency factor determined by the Hershfield’s chart. But different studies show that the vale of frequency factor founded from the chart was not reliable for PMP estimation for country like Ethiopia having variable climating condition. Therefore, the main purpose of thse study was to evaluate the frequency factor obtained from the hershfield’s chart and to provide reliable estimate of PMP using insitu and global land data assimilation system (GLDAS) reanalysis global precipitation products for middle to lower Awash River basin (MLAWB) and evaluate PMP value of reanalysis global precipitation products with the insitu PMP value. The Hershfiled’s empirical formula and chart method were applied for Km and PMP calculation. R-studio 2012, MATLAB-2013 and ARCGIS are tools applied to work with the input data. The study shows both insitu and GLDAS reanalysis product for 1 day, 2 days and 3 days the Km and PMP value are not more than 5 and 222 repectively and GLDAS reanalysis precipitation product is adequately capture PMP with respect to the the insitu PMP result in the study area for stations such as Dupty, Combolcha-1, Nazret, Mojo, Metehara, Meiso, Koka dam and Awash 7 kilo within 0.6 and 25% of deviations. The rest of the stations in the basin such as Assebeteferi, Combolcha-2, Gewane, Haik, and Kemise are not adequately captured by GLDAS reanalysis precipitation produt PMP value with respect to the stations insitu PMP value and the reanalysis precipitation product for these stations is not practical unless it is improved by minor and major data improvemet techniques. Comparison of the PMP value using the new Km in MLAWB and the chart value for insitu data exhibit differences in between 128 to 307% which is very much exagurated. This result confirmed the Hershfield’s chart overestimated PMP value which has far reaching consequences in total cost of dam and spillway projects in the study area. The average ratio of PMP value to 10,000 years return period quantiles for insitu and GLDAS reanalysis precipitation product for the study area was found tobe 2.0 and 1.81 respectively. This shows GLDAS reanalysis product is adequately captured the result of average ratio value of the insitu data.