WATER RESOURCES RESEARCH, VOL. 33, NO. 7, PAGES 1707-1719, JULY 1997 Hydrograph separations in an Arctic watershed using mixing model and graphical techniques JamesP. McNamara, Douglas L. Kane, and Larry D. Hinzman Water and EnvironmentalResearch Center, University of Alaska Fairbanks Abstract.Storm hydrographs in the Upper Kuparuk River basin (142 km 2) in northern Alaska were separatedinto sourcecomponents using a mixingmodel and by recession analysis.In non-Arcticregions, storm flow is commonlydominated by old water, that is, water that existedin the basinbefore the storm.We suspectedthat this may not be true in Arctic regionswhere permafrostdiminishes subsurface storage capacity. Streamflow during the snowmeltperiod was nearly all new water. However, all summerstorms were dominatedby old water. Stormsin a neighboringbasin were dominatedby new water but much lessthan was the snowmeltevent. Thus a large increasein old water contributions occurredfollowing the snowmeltperiod. This increasecontinued moderately through the summerin 1994 but not in 1995. We credit the seasonalchanges in old water contributionsto increasedsubsurface storage capacity due to thawing of the active layer. Introduction suspectedthat storm flow may not be dominatedby old water, as is commonlyobserved. Permafrostis a ubiquitouspresence in the Arctic that influ- An analog for permafrostbasins may be watershedson the encesnearly all physicaland biologicalecosystem processes. southernCanadian Shield, where impermeablebedrock under- Severalstudies have shownthat permafrosthas significanthy- lies shallow soils. However, several workers have shown that drologicalconsequences which resultprimarily from the min- storm flow is indeed composedprimarily of old water in Ca- imal subsurfacestorage capacity due to frozen ground [Hin- nadian Shield watersheds,even with their diminished old water zman et al., 1993; Dingman, 1970; McNamara et al., 1997; reservoirs[Peters et al., 1995;Bottomley et al., 1986; Welset al., Rouletand Woo, 1988; Woo and Steer,1983]. This is of partic- 1991; Hinton et al., 1994]. An important distinctionbetween ular concernto the NSF Land-Air-Ice-Interaction(LAII) Arc- tic Flux Studyoperating in the Kuparuk River basinin north- Canadian Shield watershedsand the Kuparuk River basin is ern Alaska.The goal of the Arctic Flux Studyis to estimatethe that the subsurfacereservoir and consequentbasin storage regionalfluxes of massand energyin the Kuparuk River basin capacityin the Kuparuk River basin increasesas the ground between the land, the atmosphere,and the Arctic Ocean thawsduring the summermonths from essentiallyzero depth [Welleret al., 1995].This requiresa comprehensiveunderstand- in the spring to depths approachingthose in the Canadian ing of the mechanismsand pathwaysby which water travels Shield watersheds late in the summer. Other studies have throughthe system.Hence we investigatedthe compositionof shown that certain hydrologicprocesses undergo coincident stormflow in the Kuparuk River basinby askingthe following changeswith the thawingactive layer. Hinzman et al. [1991] questions:Is storm flow primarily composedof precipitation, showedthat the portion of the soil profile that contributesto callednew water, or subsurfacewater previouslyexisting in the hillslope runoff increasesthrough the summer. Further, Mc- basin, called old water, and what influence does permafrost Namara et al. [1997] suggestedthat runoff/precipitationratios have on storm flow composition?An understandingof both may decreaseas the active layer thicknessincreases. Thus we the partitioningof hydrographsand the mechanismsresponsi- suspectedthat the systematicincrease in activelayer thickness ble for the partitioningis a prerequisiteto understandingthe would produceconsequent changes in stormhydrograph com- relationshipsthat existbetween terrestrial and aquaticsystems. positionsthrough the summer. Severalcase studies in variousnonpermafrost regions have The specificobjectives of this paper are (1) determinethe shownthat old water typicallydominates storm hydrographs, proportionsof old and new water in stormflow in the Kuparuk includingsnowmelt events [Buttle and Sami, 1992;Bottomley et River basinduring 1994 and 1995,(2) investigatethe potential al., 1986;Dincer et al., 1970;Eshleman et al., 1993;Kennedy et influences,particularly of permafrost,on storm flow composi- al., 1986;Kobayashi et al., 1993;McDonnell et al., 1991;Rodhe, tion. Storm flow compositionswere determined from hydro- 1981;Peters et al., 1995]. This may have significantinfluences grhph separationsusing mixing model and graphical tech- on the transportof nutrientsfrom the terrestrialto the aquatic niques. The influences on storm flow composition were system,a primary area of researchin the Kuparuk River study. investigatedby constructingcorrelation matrices with variables The old water reservoirin a basinwith permafrostis severely restricteddue to the frozen ground.Essentially all subsurface includingold water composition,precipitation characteristics, flow occursin a shallowzone called the active layer that un- total flow, and storm date as a surrogatefor activelayer thick- dergoesannual freezing and thawingcycles. Consequently, we ness.Runoff generatingmechanisms were qualitativelyevalu- ated usinga techniquedeveloped by Eshlemanet al. [1993] to Copyright1997 by the American GeophysicalUnion. computecontributing areas based on hydrographseparations. Paper number 97WR01033. We focusedon summer stormsin the Upper Kuparuk River 0043-1397/97/97WR-01033509.00 basin(Figure 1), with a limited analysisof snowmeltprocesses. 1707 1708 MCNAMARA ET AL.: HYDROGRAPH SEPARATIONS IN AN ARCTIC WATERSHED Alaska Location Map Inmavait Creek Watershed Area = 2.2 k•n2 •••'•Water track 7 = .026 k• / \ / Fairbanks Watershed N Boundary / 0 500 km / •=•o. Kuparuk Watershed '• U•perKuparuk Watershe•/' Area •Area = '142 kmz ./ . •. Arctic / west DOCk•• ••ean // Watershed . w•s•. g •1 Sa•w•n / K••kU99er •I=avait Creek A A A A A A - A -- • . A .... • Watershed I I • • • Boundary Figure 1. Map showinglocations of the studysites in the KuparukRiver basin, northern Alaska. This study focusedon the Upper Kuparuk River basin (142 km2), with a limitedanalysis of ImnavaitCreek (2.2 km2). Additional analyseswere performed in the much smaller This studyfocused on the Upper KuparukRiver, a headwa- neighboringImnavait Creek (2.2 km2). ter basin which drains 142 km 2 in the northern foothills of the BrooksRange. The slopesin the basin are coveredwith till from two glacialadvances, Sagavanirktok and Itkillik, from the Site Description middle and late Pleistocene[Hamilton, 1986]. At the intersec- The KuparukRiver flowsfrom the glaciatedfoothills of the tion with the Dalton highwaythe Upper Kuparuk River is a BrooksRange throughthe tundra flats of the coastalplain to fourth order stream on a U.S. Geological Survey (USGS) the Arctic Ocean near PrudhoeBay (Figure 1). The entire 1:63360 map. However, the hillslopesand tributary valleys regionlacks trees, is underlainby continuouspermafrost, and containa complexnetwork of smallstreams that do not appear is coveredwith snowfor 7-9 monthseach year. The snowmelt on mapsat that scale.Two dominantstreams join togetherat event is generallythe dominanthydrologic event eachyear, the base of steep hills in the upper basin forming the main which typicallyoccurs over a 7-10 day period betweenearly channelwhich occupies a north-northwesttrending valley. The May andearly June [Kane et al., 1991].Approximately 30-40% main basinlength is 16 km, with a channellength of 25 km. of the annual precipitation falls as snow from September Vegetation in the basin consistsof alpine communitiesat throughMay. The averagesummer rainfall is around18 cm in higher elevationsand moist tundra communities,predomi- the foothills of the Brooks Range. The maximum snowfallis nantly tussocksedge tundra, at lower elevations.Patches of typically10-14 cm of water equivalent.Summer temperatures dwarfwillows and birchesup to 1 m in heightoccupy portions are typicallybetween 6 ø and 18øC,and winter temperatures are commonlyaround -15 øto -25øC. Permafrostthickness ranges of the banks.The averageelevation of the basin is 967 m. from around 300 m near the foothills to over 600 m near the ImnavaitCreek (2.2 km 2) is a smallbeaded stream occupy- coast[Osterkamp and Payne,1981]. Hence the regionis effec- ing a north-northwesttrending glacial valley which was formed tivelyisolated from deepgroundwater. Subsurface flow occurs during the Sagavanirktokglaciation (middle Pleistocene) in a shallowzone abovethe permafrostcalled the activelayer [Hamilton, 1986]. The dominantvegetation in the Imnavait whichundergoes annual freezing and thawing.Soils typically basinis tussocksedge tundra covering the hillslopes[Walker et thaw to maximumdepths of 25-40 cm but can thaw to 100 cm al., 1989].An organiclayer typically near 10 cm thick,but up to dependingon severalenvironmental factors including soil type, 50 cm thick in the valleybottom, overlies glacial till, where the slope,aspect, and soilmoisture [Hinzman et al., 1991]. soil rarely thawsdeeper than the extent of the organicpeat MCNAMARA ET AL.: HYDROGRAPH SEPARATIONS IN AN ARCTIC WATERSHED 1709 layer. The creek is essentiallya chain of ponds,called beads, flow, the new componentof the flow, and the old component that formed where the stream has eroded and melted massive of the flow, respectively.The streamflowattributed to old wa- ground-icedeposits. The streambottom rarely cutsthrough to ter at any