Evaluation of Extreme flow Quantiles estimated from Global Reanalysis runoff data: A case study of Blue Nile River Basin
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Date
2014-06
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Addis Ababa University
Abstract
Stream discharge data have long been measured and used by engineers in the design of hydraulic
structures and flood protection works, and in planning for flood plain use. A flood frequency
analysis is the basis for the engineering design of many projects and economic analysis of flood
control projects. The study was to evaluate the performance of earth2observe global reanalysis
runoff products obtained from ECMWF and UNIVK reanalysis in reproducing the peak
discharges for potential use in the planning and design of water resources structures. The daily
peak discharge data from 5 stations; Gilgel Abbay, Kessie, Bahirdar, Main Beles and Border in
the Blue Nile basin were used for the evaluation. The stations were selected based on
independence and randomness tests, recorded length of data and catchment area of the station.
The reanalysis and observed AM discharge were analyzed from daily flow for estimation of
extreme flow quantiles at a return period, using at-site flood frequency analysis. The widely used
distributions; GEV, LN3, LP3 and EV1 were used to estimate flow quantiles. For selection of
best-fit distributions, method of parameters estimation and parameter estimation Easy-Fit
statistical software was employed. Goodness of-fit-test; Chi-square, Kolmogorov-smirnov and
Coefficient of determination were used for selection of best fit distributions. The results indicate
that the reanalysis runoff products (ECMWF and UNIVK) have limitation in reproducing the
peak discharges. The ECMWF reanalysis AM discharge was overestimated for Kessie, Gilgel
Abbay and Border stations by PBIAS of -4.43, -22.30 and -131.41% respectively and
underestimated for Main Beles and Bahirdar stations by PBIAS of 30.08 and 39.9% respectively.
The UNIVK reanalysis AM discharge was overestimated for Bahirdar, Gilgel Abbay and Border
stations by PBIAS of -10.65, -28.90 and -68.30% and underestimated for Main Beles and Kessie
by PBIAS of 28.71 and 4.28% respectively. The annual maximum discharges extracted from
both ECMWF and UNIVK runoff products were not found reliable to estimate flood quantiles
for design purposes. Therefore, both data have severe limitation in reproducing the peak
discharges and estimation of flood quantiles and can’t be used for design purpose before major
correction is done.
Keywords: Global reanalysis runoff, ECMWF and UNIVK reanalysis, Flow quantiles,
Earth2Observe, Blue Nile
Description
Keywords
Global reanalysis runoff, Ecmwf and Univk reanalysis Flow quantiles, Earth2Observe, Blue Nile