Ecohydrological characterization of the Nyando wetland, Lake Victoria, Kenya: A State of System (SoS) analysis

摘要

LakeVictoriafloodplainwetlandshaveacomplexhydrologicalsettingcharacterizedby transitionfromaterrestrialtoanaquaticenvironment.Astate-of-system(SoS)analysiswascarriedoutinapapyrusdominatedwetlandintheNyandoRiverDelta,onthe easternshoresofLakeVictoria,Kenya,tocharacterizeandprovidedatafordetailed ecohydrologicalstudies.Theobjectivesofthestudywereto:(1)determinethespatio- temporalchanges inthewetlandevolutionand(2) analyzethemainhydrological factorsthathaveinfluencedwetlandevolution.Multi-temporaldry-seasonLandsatMSS,LandsatTMandLandsatETM+imagerycoveringNyandoWetlandandits surroundingareawereprocessedandanalyzedtogeneratetimeseriespolygonand polylinemapsofthewetland and river.Resultsshowthatthewetlandincreasedinsizefrom 5,925hain1950to9,925hain1973,anddeclinedto4,527hain2008.Inthelast60years,NyandoRiverhasmigratedinageneraleastwarddirection.Timeseries hydrologicaldata(1950-2009)werestatisticallytestedforhomogeneityusingthe Spearman’sranktestforlineartrends,PettittestandStandardNormalHomogeneitytest (SNHT)forchangepointanalysis,andsplit-recordtestsperformedforvariance(F-test) andmean (t-test). In addition,datawere analyzed using descriptive statistics andfrequencyanalyses.Statisticaltestresultsshowthatthehydrologicaldata serieswere homogeneous.Resultsofchangepointanalysesindicatethattotalannualrainfallin Nyandodeclinedin1979,whilethemeanannualdischargeforNyandoRiverandLake Victorialevelshadsignificantupwardshiftsin1961.Thedecadalmeandischargesvariedsignificantlyovertimeandincreasedby80%from11.45m3/sobservedinthe 1950-1961subset,reducingby11.4and21.9%inthenexttwodecadalsub-sets, beforerisingby35.0% in1990sanddroppingby 24.0%inthelastdecade.Thedecadalmeanannuallakelevelsincreasedfrom1134.0to1135.43minthe1951-1961and 1962-1972andremainedabovethelong-termmeanof1135.0mfor43yearssince 1962beforedroppingdrasticallyby1.4mtoanaverageof1134m/yearin2005-2009.The highestrecordedlakelevelatKisumuStationwas1136.2min1964afterincreasingby2.5 mfrom1961.Dischargedataexhibittrimodalseasonalpatterns,whilethelake levelshadtwo peaks.Thelakelevelsaremoresensitive to direct lake rainfall. Changes in the Nyando wetland area are linked to the seasonal and episodic flood and drought events coupled with anthropogenic activities (regulation of lake levels, modification of river including cut-off meanders, river training andconstructionofdykes,drainageofwetlandforcultivation,settlement andlivestockgrazing,abstractionofwaterforirrigation).Acombinationofthese hydrologicalandhumanfactorsisthemaincauseoftheNyandoWetlandevolution.Ifthe landusetrendcontinuesunabated,thentheincreaseinpapyruslosseswillposeabig challengetotheecologicalfunctioningofthewetlandanditssupporttosustaining communitylivelihoods.Keywords:NyandoWetland, River, ecohydrology, Lake Victoria.