NIDISWeeklyClimate,WaterandDroughtAssessmentSummary

UpperColoradoRiverBasinSeptember7,2010

PrecipitaFonandSnowpack

ForthemonthofAugust,theheaviestamountsofprecipitaFonfellintheSanJuanbasininColoradoandtheLowerGreenRiverbasininUtah(Fig.1),wheresomeareassawcloseto4inches.SweetwaterCounty,WYwasthedriestcountyintheUpperColoradoRiverBasin(UCRB)withlessthananinchofprecipitaFonthroughoutthecounty.AsidefromSweetwaterCountyandSanJuanCounty,UT,therestofthebasinsawnearoraboveaverageprecipitaFonforthemonth(Fig.2).

ThestartofSeptemberhasbeenabnormallydryformostoftheUCRB(Fig.3),withmanyareasnotreceivinganyprecipitaFonforthepastweek.TheLowerGreenRiverbasin,theSanJuanbasin,andtheYampa‐WhitebasinreceivedthemostprecipitaFonlastweek,totalingaroundaquarterofaninchineachbasin.

Fig.2:AugustprecipaspercentofaverageFig.1:Augustprecipininches Fig.3:Aug30–Sep5precipininches

Fig.4:SnotelWYTDprecipitaFonpercentofaveragechangefromlastweek.

Fig.5:SnotelWYTDprecipitaFonpercenFles(50%ismedian,21‐30%isDroughtMonitor’sD0category).

Snotelwater‐year‐to‐date(WYTD)precipitaFonpercentsofaveragedecreasedfromlastweekacrosstheenFreUCRB(Fig.4).AlongthewesternborderoftheUCRB,2–3%decreasesfromlastweekwereexperienced(afairlylargedropinpercentageforthislateinthewateryear).TheSanJuanbasininsouthwestColoradoalsoexperiencedlargedecreases—around2–3%fromlastweek.

Water‐year‐to‐date(WYTD)percenFlesfortheSnotelsitesintheUCRBshowthelowestvaluescorrespondingwiththelocaFonsofcurrentabnormaldryness(D0orbelowthe30thpercenFle)ontheU.S.DroughtMonitormap—intheRioGrandebasintothesouth,theUpperandLowerGreenRiverbasinsandneartheColoradoheadwatersregion(Fig.5).TheremainingSnotelsitesshowpercenFleshighenoughtonotbeconsideredfordroughtdesignaFons.

StreamflowAbout85%oftheUSGSstreamgagesintheUCRBarereporFngnormal(inthe25–75thpercenFlerange)orabove7‐dayaverageflowsasofSeptember6th(Fig.6)—thisisadecreasefromtheover90%reportedonAugust24th.ThemajorityofbelownormalstaFonsarelocatedintheLowerGreenRiverbasininUtah.SeveralgagesalongtheWhiteRiverarealsorecordingbelownormal7‐dayaveragestreamflows.

Hydrographsacrossthebasinshow7‐dayaveragestreamflowsconFnuingtheirreturntobaseflows(Fig.7).ThestreamgagesontheColoradoRiverattheCO‐UTstatelineandontheSanJuanRivernearBluff,UTshowsecondarypeakflows(asaresultofasurgeinmonsoonalmoisture)inlateJuly/earlyAugust,followedbyadecreaseinflows,thoughbotharesFllwithinthenormalrange.ThegageontheGreenRiveratGreenRiver,UTalsoshowsnearnormalstreamflow.

Fig.6:USGS7‐dayaveragestreamflowcomparedtohistoricalstreamflowforSeptember6thintheUCRB.

Fig.7:USGS7‐dayaveragedischargeoverFmeattheCO‐UTstateline(top),GreenRiver,UT(middle)andBluff,UT(bogom).

WaterSupplyandDemandNearaveragetemperatureswereseenaroundmuchoftheUCRBlastweek,withbelowaveragetemperaturesintheUpperGreenRiverbasinandaboveaveragetemperaturesalongthefrontrangeandeasternplains.SoilmoisturecondiFonsconFnuetodegradeinnorthernColorado.AlsofiredangerhasbeenprevalentthispastweekalongtheeasternborderoftheUCRBandthroughoutthefrontrangecounFes.

AllofthemajorreservoirsintheUCRBconFnuedtoseedecreasesinlakelevelsoverlastweek.LakeDillonissFlloperaFngnearcapacitywithonlyslightdecreasesinstorage.Allofthemajorreservoirs,asidefromLakePowell,areabovetheiraverageSeptemberlevels.AugustinflowvolumeintoLakePowellwas82%ofaverage,whichisbelowwhathadbeenprojected.ReleaseshavedecreasedwiththebeginningofSeptember,andsteadyreleaseswillconFnuethroughOctober,notfluctuaFngforpowerproducFon.LakePowelllevelsare77%oftheSeptemberaverageand63%ofcapacity.

PrecipitaFonForecastLatesummerpagernofweaktrough/ridgewillconFnuethroughthenextweek.WhilecurrentforecastkeepsT.S.Hermineandassociatedmoisturewelltotheeastoftheregion,sub‐tropicalmoistureoverthefourcornerswillbegintoincreaseinthesouthwestflowfollowingthepassageofaridgetoday.ThiswillsetthestageforsomescageredshowersandthunderstormsinwesternColoradoandeasternUtahonWednesday.QuanFtaFveprecipitaFonfieldsshowamountsbetween0.25and0.5inchescenteredovertheSanJuanmountainsthroughWednesdaynight,withnortheasternUtahandnorthwesternColoradopossiblyseeingsomeshowerslingerintoThursdaywithpassageofPacifictrough.ThisfeaturewillswingacrossthenorthernUCRBduringthedayonThursdayanddryouttheatmospherefortheupcomingweekend.Withliglemoistureavailable,expectdrycondiFonstoprevailthroughtheweekendintoearlynextweek.

DroughtandWaterDiscussion

TheBoulderWFOhasrecommendedthatD0beintroducedintoGrand,Jackson,Summit,Larimer,Boulder,Gilpin,ClearCreek,Jefferson,Park,Denver,AdamsandArapahoeCounFesinnorthernColorado,basedonshorttermdrynessoverthepast30days(Fig.8).Overthepastweek,thisregionhasseenligletonoprecipitaFon,verylowhumidiFes,highwinds,andhighdangerforfire(thereiscurrentlya7,000acrefireinBoulderCounty).TheDMauthor’sopinionisthatthisisaverywidespreadexpansionthatdoesn’thavemuchjusFficaFoninthesoilmoisture,SPI,precip,orstreamflowproducts.Firedangercanbeusedasadroughtindicator,butistypicallynot.Currently,VegetaFonHealthIndexpointstoshorttermdrynessintheuppersoillayersinthisregion.Theauthorsuggestsholdingoffanotherweek,andtheareawillbecloselymonitoredbeforethisD0isadded.Alsorecommendedistore‐expandtheD0throughoutalloftheRioGrandebasininsouthernColorado(Fig.8)basedonabnormaldrynessoverthepast30daysandalsointhelonger‐term.

StatusquoisrecommendedfortheremainderoftheUCRB.

Fig.8:August31releaseofU.S.DroughtMonitorfortheUCRB

DroughtcategoriesandtheirassociatedpercenFles