Abstract:
Probable Maximum Precipitation (PMP) is theoretically maximum depth of precipitation for a
given duration: It is physically possible over a given size storm area at a specific geographical
location at a particular time of year. The PMP is used for the design of a hydraulic structure
appropriately in the study area. The procedure for estimation of PMP used in the country is based
on Hershfield’s graphical estimation, but different studies show that the value founded from the
chart was not reliable. Therefore, the main objective of this study is to Estimate the Probable
Maximum Precipitation and develop Isohyetal Map in Didessa Sub Basin using Hershfield’s
statical method. Microsoft excel, RAINBOW, Easy fit and Arc GIS software where the materials
used for this study Double mass curve is employed for consistency test. The study shows that for
1day, 2and 3day the maximum Frequency factor Km and PMP value obtained by Hershfield’s
statical method are 5.10 and 255.57mm respectively. Whereas, the computed maximum km and
PMP from the chart is 16 and 513.96. The values exhibited deviation about 213.73 for km and
101.10% for PMP. This result confirmed that the value of PMP from chart is over estimated which
can leads to uneconomical designs in the Didessa Sub Basin. The ratio of one day rainfall depth
for a return period of 2,5,10,25,50,100,200,500,1000,2000,5000 and 10,000 year floods had been
estimated and found to vary from minimum 52.69mm to maximum 210.74m. The estimated PMP
values to 10,000 years return period founded by Hershfield’s statical methods was 1.12, 1.17 and
1.26 times that of 10,000 years return period rainfall depth for 1day 2day and 3day duration
respectively in the DSB. Isohyete maps were generated by IDW interpolation method and PMP
grid values were varying between 124mm to 167mm. This study clearly indicates that the
Hershfield chart gives highest values of PMP due to highest value of Km from the chart. So,
further researches should be conducted on the rest of Ethiopian basins for fixing the country’s
reliable maximum Frequency fator (km).
