OBSERVE DVARIABILIT YAN DTREND SO FEXTREM ERAINFAL LEVENT S OVE RBLU ENIL E

Abstract

Droughtsi nEthiopiacauseagreathumansufferingsandlosso flife .Suchconsequencesnotonly resultfrominsufficienttota lrainfallamountbutalsofiomlon gdryspellswithi ntherain y season.Henceassessingthevariabilityo fextremerainfal leventsi noverthebasiniscrucial.The aimo fthispaperisevaluatingthevariabilityo frainfal landtrendso fextremerainfal levents overBlueNilebasin.Thevariabilityo frainfal lislessi nthemainrainyseasona scomparedto thesmallrainyseason.Andi ntheKiremtseasonalmostal lthestationshavecoefficientso f variationlessthan20%.Trendsi nheavyrainfall ,totalrainfall,thenumbero fdrydays, maximum5daysconsecutiverainfalltotalsandsimpledailyintensityindex(SDII )havebeen analysedusingdailyrainfal lrecordsat26stations.BelgandKiremtseasonswereconsidered separatelyfortheperiodo f1980-2002.Heavyrainfal lisdistincta stheOO" 'percentileo fdaily rainfalli nthetw oseasons.Themagnitudeo ftrendswasdeterminedfiomYueetal2002andthe statisticalsignificaqjceweredeterminedbyMann-Kendaltest.Increasingtrendsi nheavyrainfal l andtotalrainfallhaveoccurredduringtheKiremtseason,butonly10-20%o fthestationshave statisticallysignificanttrends.Morenumbero fstationsshowssignificantincreasei ntrendso f heavyprecipitationii nBelgseasonthani ntheKiremtseason.Thegreatestsignificanti nheavy rainfallfrequencyhasoccurredi nthesoutheastandwesternparto ftheBlueNileBasin. Trendso fwetanddrydecadewereanalysedoverthebasin.Theresultshowsnearly75%o fthe driestyearsarefiom1981-1990whichagreeswiththedriestyearsthatoccurredoverthecountry lik e1984and1987.Thefrequencyo fthewettestyearsdecreasesfiom42%during1970-1980 and1981-1990to17%during1991-2002.