High uncertainty in 21st century runoff projections from glacierized basins

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1 High uncertinty in 1st century runoff projections from glcierized sins Mtthis Huss 1,, Michel Zemp, Philip C. Joerg nd Ndine Slzmnn 1 1 Deprtment of Geosciences, University of Friourg, CH-17 Friourg, Switzerlnd Deprtment of Geogrphy, University of Zurich, CH-87 Zurich, Switzerlnd E-mil: mtthis.huss@unifr.ch Astrct Glcier response to chnging climte nd its impct on runoff is understood in generl terms, ut model-sed projections re ffected y considerle uncertinties. They originte from the driving climte model, input dt qulity, nd simplifictions in the glcio-hydrologicl model nd hmper the reliility of the simultions. Here, n integrtive ssessment of the uncertinty in 1st century glcier runoff is provided sed on experiments using the Glcier Evolution Runoff Model (GERM) pplied to the ctchment of Findelengletscher, Switzerlnd. GERM is clirted nd vlidted in multi-ojective pproch nd is run using 9 Regionl Climte Models (RCMs) until 1. Among others, the hydrologicl impcts of the RCM downscling procedure, the winter snow ccumultion, the surfce ledo nd the clcultion of ice melt nd glcier retret re investigted. All experiments indicte rpid glcier wstge nd trnsient runoff increse followed y reduced melt seson dischrge. However, mjor uncertinties in, e.g., glcier re loss ( 1% to 63%) nd the chnge in nnul runoff ( 7% to +% reltive to tody) y 1 re found. The impct of model ssumptions on chnges in August runoff is even higher ( 94% to %). The spred in RCM results ccounts for -% of the overll uncertinty in modelled dischrge. Initil ice thickness, the mount nd sptil distriution of winter snow nd the glcier retret model hve the lrgest effect on the projections, wheres the RCM downscling procedure, clirtion dt qulity nd the melt model (energy lnce vs degree-dy pproch) re of secondry importnce. Keywords: runoff projection, glcier, uncertinty, modelling, climte chnge Introduction Ongoing nd future climte chnge hs mjor impct on lpine environments (e.g., Beniston, 3). The potentil loss of sustntil frction of glcier ice volume until the end of this century will significntly lter the runoff chrcteristics of mountinous dringe sins (e.g., Brun et l., ; Brnett et l., ; Huss, 11). Due to sesonl shortge of wter supply, downstrem impcts of chnges in the cryosphere might e considerle in terms of irrigtion for griculture, hydropower production, river trnsporttion nd ecology (Xu et l., 9; Immerzeel et l., 1; Kser et l., 1; Viviroli nd others, 11). Numerous model studies for wide rnge of climtic settings hve een performed, estimting future trends in the hydrology of glcierized sins (e.g., Juen et l., 7; Sthl et l., 8; Weer et l., 1; Hgg et l., 13; Bvy et l., 13; Rgettli et l., 13). As roust result, shift in the runoff regime nd decrese in melting seson dischrge is found on the long run. However, n integrtive uncertinty ssessment of modelled future runoff from high-mountin ctchments hs not een performed to dte. The uncertinty in projected runoff is the comined effect of the spred in climte model results, the downscling procedure, input dt qulity, s well s simplifictions nd poorly understood feedcks in the modelling of glcier chnge nd runoff. Although the individul uncertinties might cncel out ech other, some prmeteriztions in the impct models might led to systemtic over- or underestimtion of future runoff, nd thus require creful evlution. Knowledge out the integrted uncertinties is vitl for mking runoff projections useful in terms of dpttions in the wter resource mngement. Severl individul components of the uncertinty in glcier runoff projections hve recently een ssessed. The impct of differences in ir temperture nd precipittion trends projected y Regionl Climte Models (RCMs) or Glol Circultion Models (GCMs) on the runoff regime of glcierized ctchments ws ddressed in different regions (e.g., Sthl et l., 8; Frinotti et l., 1; Lutz et l., 13; Rgettli et l., 13). Dedicted studies hve investigted the effect of climte model dt downscling procedures on clculted glcier mss lnce (e.g., Rdić nd Hock, 6; Kotlrski et l., 1; Slzmnn et l., 1), nd the field dt requirements for n unmiguous clirtion of hydrologicl models (Konz nd Seiert, 1; Schefli nd Huss, 11). It hs een recognized tht the estimte of the initil glcier ice volume (Gi et l., 1), nd the pproch to clculte glcier geometry chnge hve strong impct on clculted future glcier re nd runoff (Huss et l., 1; Linsuer et l., 13). Mny studies hve focused on uncertinties in the modelling of snow nd ice melt sed on the surfce energy lnce or temperture-index models (e.g., Klok nd Oerlemns, 4; Hock, ; Pellicciotti et l., ; Koiersk et l., 13). Other fctors such s the effect of the sptil snow ccumultion distriution, nd chnges Preprint sumitted to Journl of Hydrology Octoer, 13

2 in deris-covered glcier surfces on modelled dischrge hve not yet een specificlly ddressed y glcio-hydrologicl studies to our knowledge. This pper ims t detiled ssessment of the mjor uncertinties in the modelling of future runoff from glcierized dringe sins, nd quntifies potentil uncertinty rnges sed on n extensive set of model experiments. This llows identifying the fctors nd processes tht re the lest constrined y the stte-of-the-rt glcio-hydrologicl model pproches nd re most influentil for the overll uncertinty in 1st century runoff projections. Our study is focused on the high-lpine ctchment of Findelengletscher, Swiss Alps, for which welth of dt on glcier mss lnce nd dischrge over severl decdes is ville. The sin thus represents n idel test site for this comprehensive modelling study.. Study site nd dt.1. Geogrphicl setting Findelengletscher is lrge temperte vlley glcier in the southern Swiss Alps (46 N, 7 E). The region is chrcterized y glcier equilirium line ltitudes of round 33 m.s.l., eing mong the highest in the Alps (Misch et l., ). The ctchment of the hydrologicl sttion rnges from 484 to 4173 m.s.l., nd hs n re of 1. km (Fig. 1). The sin is locted in the hedwters of the Rhone River nd ws 74% glcierized in 1, leding to distinctly glcil runoff regime. Findelengletscher (13. km in 1) occupies the lrgest prt of the wtershed. Adlergletscher (. km ) nd few smller glciers mke up for the rest of the glcieriztion (Fig. 1)... Studies on Findelengletscher Over the lst yers, considerle knowledge out glciologicl nd hydrologicl processes nd chnges in the sin of Findelengletscher hs een ccumulted representing strting point for this study. Mchguth et l. (6) nd Sold et l. (13) investigted the sptil distriution of winter snow on Findelengletscher. Long-term series of glcier mss lnce since 19 were derived y Huss et l. (1). Severl uthors hve ddressed the future hydrology of the ctchment. Frinotti et l. (1) clculted glcier retret nd runoff over the 1st century using 1 RCMs of the ENSEMBLES project (vn der Linden nd Mitchell, 9). Uhlmnn et l. (13,) clirted hydrologicl model to dischrge dt nd performed model run until 1 using results of one RCM from the PRU- DENCE project (Christensen nd Christensen, 7)..3. Field dt A mss lnce monitoring progrm is mintined on Findelen- nd Adlergletscher since 4 (Mchguth, 8). Extrpoltion of mss lnce mesured t network of 13 stkes nd snow pits (Fig. 1) over the glcier yields the nnul mss udget of Findelengletscher, s well s the ltitudinl distriution of melt nd ccumultion. Winter lnce is determined since 9 y -1 snow pits nd 4-7 mnul snow proings per survey, distriuted over the entire elevtion rnge The monitoring of snow ccumultion distriution is further supported y helicopter-orne ground-penetrting rdr (GPR) since 1 providing snow depth on severl tens of kilometers of continuous trcks (Sold et l., 13). Five Digitl Elevtion Models (DEMs) documenting glcier surfce elevtion chnges re ville for Findelen- nd Adlergletscher. DEMs for 198 nd 7 with sptil resolution of m nd n estimted ccurcy of ±. m were derived sed on irorne photogrmmetry (Buder et l., 7). For, 9 nd 1, high-resolution Light Detection And Rnging (LiDAR) DEMs with rndom error of <.1 m were estlished (Joerg et l., 1). Glcier outlines were mpped for ech DEM (Fig. 1). Differencing of repeted DEMs llows the clcultion of ice volume chnges. A LiDAR DEM is lso ville for April 1 (Joerg et l., 1). By compring this terrin model with the DEM from Octoer 9 nd pplying correction for sumergence nd emergence of the glcier surfce over the winter seson, fully distriuted mp of snow depth on 1 1 m grid over the entire glcier ws derived (Sold et l., 13). Glcier ice thickness nd volume ws determined sed on comintion of different Ground Penetrting Rdr (GPR) mesurement cmpigns. Two profiles with low-frequency helicopter-orne GPR device were cquired for the center of the glcier in 8 (Frinotti et l., 9). In Mrch 1, ground-sed GPR mesurements on the glcier tongue, nd out 3 km of longitudinl nd cross-glcier trcks were relized with helicopter-orne GPR. Wheres the ltion re is well covered with mesurements, edrock reflections were prtly wek in the ccumultion re. Therefore, n pproch to invert ice thickness from surfce topogrphy sed on the principles of flow dynmics (Huss nd Frinotti, 1) ws first clirted to the oservtions nd then used s complementry dt source in regions for which the numer of direct mesurements ws insufficient (out % of the glcier surfce). The comined thickness mp yields mximum ice depth of m, nd totl ice volume of 1.6 km 3 in the dringe sin. Locl ice thickness uncertinty is estimted s ±1% for regions with GPR dt, nd ±% for unmesured regions. Continuous dischrge mesurements recorded y Grnde Dixence SA re ville for t dily resolution. The guging sttion is locted t distnce of 1 km from the present glcier terminus (Fig. 1). There re no runoff dt during the winter months (i.e. etween Novemer nd April/My) for most yers. Hence, we do not use nnul runoff volumes for vlidtion, lthough dischrge is normlly smll during wintertime. For the summer months, dt qulity is high with only very few dt gps. Dily men ir temperture, glol incoming short-wve rdition nd precipittion re ville from MeteoSwiss wether sttion t Zermtt (1638 m.s.l., 6 km from glcier terminus). The dt cover the period In ddition, dily ir temperture for from Gornergrt (313 m.s.l.) t distnce of 4 km from Findelengletscher, nd Test Grigi (3479 m.s.l., 1 km, 191-) re ville (Fig. 1).

3 3 Zermtt Findelengletscher Elevtion chnge -1 (m) Gornergletscher Gornergrt Adlergletscher Test Grigi Guging sttion Findelengletscher Elevtion (m.s.l.) Elevtion chnge (m) m Figure 1: Recent chnges in the geometry of Findelengletscher. Colours show glcier surfce elevtion chnges etween Octoer nd Septemer 1 sed on comprison of two DEMs (Joerg et l., 1). Stkes nd snow pits for the mesurement of nnul mss lnce re depicted with dimonds, snow proings relized in April 1 re shown (crosses). The loction of wether sttions round the study site is indicted in inset (). Inset () provides the ltitudinl distriution of surfce elevtion chnge. The vriility (± stndrd devitions) within ech nd is shown Climte scenrios Scenrios of future climte chnge re otined from the project CH11 (e.g., Bosshrd et l., 11) tht presents n nlysis of results of the ENSEMBLES climte model runs (vn der Linden nd Mitchell, 9). ENSEMBLES RCMs re driven y different Glol Circultion Models (GCMs) (Tle 1) tht re forced y the SRES A1B emission scenrio (IPCC, 7). The A1B emission scenrio represents n evolution close to the medin of other storylines nd fetures rpid economic growth s well s lnced use of fossil nd non-fossil fuels. The chnges in ir temperture nd precipittion used in this study re sed on the delt chnge pproch (see e.g., Hy et l., ; Slzmnn et l., 7) nd hve een evluted y Bosshrd et l. (11) for the grid points round Findelengletscher for 1 RCMs. The delt chnge pproch expresses the effect of climte chnge etween two periods in terms of the difference in the men of given vrile. The periods hve the sme length (3 yers) nd re divided into reference period (198-9) nd three scenrio periods (1- / 4-74 / 7-99). Chnges in dily men ir temperture nd precipittion etween the reference nd the scenrio period re ggregted to monthly vlues. Note tht temperture chnges re dditive, nd precipittion chnges re multiplictive, oth reltive to the period All RCM results prescrie significnt increse in ir temperture (Tle 1). For the period 7-99, chnges in nnul temperture reltive to re etween +. nd +4. C. Expected trends in nnul precipittion re minor nd re inconsistent etween the RCMs. Temperture increse in summer is projected to e lrgest y ll models (+.94 C ove the nnul verge), nd most RCMs indi Tle 1: Regionl climte scenrios of the ENSEMBLES project with the responsile institution (Inst.) nd revitions for the GCMs nd the RCMs used. Projected chnges in men winter (Nov-Apr.) nd summer (My-Oct.) ir temperture (in C), T w nd T s, nd precipittion (in %), P w nd P s, for Findelengletscher in 7-99 re given reltive to The medin scenrio is written in old fce, nd two extreme scenrios in itlic. Inst. GCM RCM T w T s P w P s MPI ECHAM REMO SMHI ECHAM RCA KNMI ECHAM RACMO ICTP ECHAM REGCM ETHZ HdCM3Q CLM HC HdCM3Q HdRM3Q SMHI HdCM3Q3 RCA CNRM ARPEGE ALADIN SMHI BCM RCA cte incresed winter precipittion nd reduction in summer (Tle 1). From the 1 ENSEMBLES RCMs we select medin scenrio tht is henceforth used s reference climte evolution (MPI ECHAM REMO), nd two extreme scenrios providing lower ound (SMHI BCM RCA) nd n upper ound (HC HdCM3Q HdRM3Q) of expected climte chnge. We completely exclude the results from the DMI ECHAM HIRHAM model in our study. Close inspection of the rw results of this RCM indicted the possiility of model rtifcts close to the study region. As peks in ir temperture seemed to e cut off for the grid cells close to Findelengletscher, unrelisticlly smll increses in summer tempertures in comprison to ll other RCMs were evident.

4 Methods 3.1. Glcier model Glcier mss lnce, retret nd runoff is clculted using the Glcier Evolution Runoff Model (GERM, Huss et l., 8). This glcio-hydrologicl model is designed to clculte pst nd future runoff from glcierized dringe sins nd includes components for snow ccumultion distriution, snow nd ice melt, 3D glcier geometry chnge, evpotrnspirtion, nd runoff routing. A detiled description of the model components is given in Huss et l. (8, 1) nd Frinotti et l. (1). Herefter, the most importnt model prmeteriztions re riefly descried nd further developments of the originl model re highlighted. GERM is run on regulr m grid for the dringe sin of Findelengletscher. The model is driven y dily men temperture, glol rdition nd precipittion recorded t Zermtt. In order to reduce uncertinties due to extrpoltion with elevtion, mesured tempertures re shifted to the Medin Elevtion (ME, 33 m.s.l.) of the ctchment y using oserved monthly temperture grdients etween the vlley sttion Zermtt, nd Gornergrt nd Test Grigi (Fig. 1) tht re oth locted t n elevtion roughly corresponding to the equilirium line of Findelengletscher. Tempertures for time step t re then extrpolted to every gridcell (x, y) y ssuming n nnully constnt lpse rte dt/dz s T(x, y, t) = T ME (t) + (z(x, y) z ME ) dt/dz. (1) Snow ccumultion C (in wter equivlent) is clculted sed on the mesured precipittion P Z (t) t Zermtt occurring t tempertures T(x, y, t) < T thr s C(x, y, t) = P Z (t) c prec D(x, y). () T thr is the threshold temperture etween solid nd liquid precipittion with liner trnsition rnge of ±1 C. The fctor c prec llows the djustment of mesured precipittion sums to the dringe sin nd ccounts for guge underctch. Sptil vritions in ccumultion due to preferentil snow deposition nd wind-driven snow redistriution re tken into ccount y using snow distriution multiplier D(x, y) (Troton et l., 199; Frinotti et l., 1). D(x, y) is specificlly derived for Findelengletscher from direct oservtions of ccumultion vriility (Sold et l., 13) comined with smll-scle terrin chrcteristics nd hs vlues of etween nd. Snow nd ice melt is clculted sed on simplified formultion of the energy lnce eqution proposed y Oerlemns (1). The energy ville for melt Q M = Q M (x, y, t) is otined y Q M = (1 α) G + k + k 1 T, (3) where α=α(x, y, t) is the locl surfce ledo t dy t, G = G(x, y, t) in W m is the glol incoming short-wve rdition, nd k + k 1 T is the sum of the long-wve rdition lnce nd the turulent het exchnge linerized round the melting point (Oerlemns, 1; Mchguth et l., 6), with k nd k 1 s constnt prmeters, nd T = T(x, y, t) the ir temperture As no contiuous nd homogeneous rdition mesurements re ville for the dringe sin, G is computed s G(x, y, t) = r(t) I pot (x, y, t), (4) with I pot the potentil cler-sky solr rdition nd r(t) the dily rtio of oserved to potentil glol rdition derived from the mesurements t Zermtt. If Q M is greter thn zero, the melt rte is otined with the ltent het of fusion. The temporl chnge in snow ledo α snow is clculted fter Oerlemns nd Knp (1998) s α snow = α firn + (α snow, α firn ) exp ( d d ), () where we ssume α firn =. the ledo of firn, nd α snow, =.9 the ledo of fresh snow (e.g., Cuffey nd Pterson, 1). d is the numer of dys since the lst snowfll nd d = is typicl time scle (Oerlemns nd Knp, 1998). Glcier geometry chnge occurring in response to surfce mss lnce forcing is clculted sed of the hprmeteriztion (Huss et l., 1). With simple empiricl function, glcier surfce elevtion chnge is relted to the ltitudinl rnge of the glcier yielding mximum ice thickness chnges t the glcier snout nd smll vritions in the ccumultion re. Prescriing mss conservtion, the nnul chnge in ice volume clculted with the mss lnce model is distriuted cross the glcier surfce using the non-dimensionl elevtion chnge pttern s oserved in The glcier disppers where ice thickness ecomes smller thn zero. This stright-forwrd pproch hs een shown to yield results for 1st century glcier front vritions tht compre well to higher-order 3D ice flow modelling (Huss et l., 1). 3.. Dily meteorologicl series until 1 We generte trnsient time series of future ir temperture nd precipittion from the monthly delt chnges computed y Bosshrd et l. (11) sed on RCM results for three 3-yer periods in the 1st century. We first interpolte the monthly chnges linerly etween the center points of the periods (i.e. 199, 3, 6 nd 8). After 8, chnges re extrpolted with the rte of 6-8. This provides continuous series of monthly men temperture nd precipittion for the period of future modelling (13-1). Assuming no chnge in dily meteorologicl vriility, we rndomly select yers in the period from the Zermtt sttion, nd shift the monthly mens of the mesured meteorologicl vriles to the scenrio results (see lso Huss et l., 8). Applying this method, continuous dily series of temperture nd precipittion for 13-1 re otined tht preserve the chrcteristic meteorologicl vriility of the oserved series used for model clirtion, nd include trends in monthly climtic chnges s prescried y the RCMs. Finlly, 1 model runs for ech scenrio simultion re performed in order to filter vriility originting from the chrcteristics of the rndomly selected yers used for the genertion of the future dily meteorologicl series. We do not consider glol rdition dt given y the RCMs for the future modelling nd use monthly mens of r(t) (Eq. 4)

5 Tle : Importnt prmeters of the melt-ccumultion model for the reference clirtion. Prmeter Vlue unit k 33. W m k 1 1. W m C 1 α ice. α snow,.9 α firn. c prec 4. T thres 1. C dt/dz.6 C m 1 s oserved in the pst for the model runs until 1. Climte models generlly hve limited skill in reproducing chnges in glol rdition nd cloudiness (e.g. Wild nd Schmucki, 11). Furthermore, evlution of RCM-sed glol rdition series for the study region showed neither significnt nor consistent trends etween 19-1 for the summer months Model clirtion nd vlidtion GERM is clirted nd vlidted in multi-ojective pproch over For this period, vrious field mesurements llow us to constrin ll components of the wter lnce in the dringe sin of Findelengletscher. The mesured glcier volume chnge is chosen s the min clirtion dt source. This oservtion ccurtely integrtes the glcier mss udget t multidecdl scle. Correctly cpturing the dynmics of long-term ice storge chnge is most criticl to ssessing future ctchment wter lnce. In ddition, winter snow oservtions re used for clirtion llowing us to unmiguously seprte ccumultion nd melt components. Oserved glcier front vritions, nnul mss lnce mesurements nd monthly/dily runoff re used for model vlidtion. Model prmeters re clirted mnully due to constrints with computtion time nd the need for consistency etween prmeter sets otined for the different experiments (see chpter 3.4). The ctchment precipittion is clirted to mtch sptilly distriuted snow ccumultion mesured in five yers using the fctor c prec (Eq. ). The prmeters of the energy lnce model, k nd k 1, re tuned to the long-term ice volume chnge. The ledo of re glcier ice is set to α ice =. (e.g., Klok et l., 3). T thres =1. C is sed on literture vlues (Rohrer, 1989). The temperture lpse rte dt/dz =.6 C m 1 ws derived from comprison of high-ltitude sttions in the vicinity of Findelengletscher (inset in Fig. 1) s men for the melting seson (June-August). The djusted prmeters of GERM re given in Tle. GERM is initilized with the glcier geometry of 198 nd is run until 1 using meteorologicl dt of the Zermtt sttion. The chnge in glcier length, re nd ice surfce elevtion is ccurtely reproduced over the 3-yer period (Fig. ). The retret of the terminus of Findelengletscher since 198 is cptured, s well s the detchment of the tongue of Adlergletscher from the min glcier rnch. Clculted surfce el Dischrge (m 3 s -1 ) Dischrge (m 3 s -1 ) R =.96 Mesured Simulted Ice melt My June July August Septemer 3 R = Dy of yer Figure 4: Clculted nd mesured dily runoff from the ctchment of Findelengletscher over the melting sesons of () 1988, nd () 3. The Nsh nd Sutcliffe (197)-vlue (R ) is given. Htched res indicte runoff due to re-ice melt. evtion mtches the DEM of 9 within root-men-squre error of 7.1 m over ll grid cells. No sptil is in the error ws detected. The cumultive ice volume loss in the dringe sin of Findelengletscher grees with the glcier volume chnge determined sed on the geodetic method (Frinotti et l., 1), ut is slightly more negtive in comprison to the oserved volume chnge etween nd 1 (Joerg et l., 1, Fig. 3). The ltitudinl distriution of winter ccumultion nd nnul mss lnce is cptured y the model (Fig. 3), ut ltion is overestimted systemticlly for - 1. Monthly summer runoff volumes re reproduced with model efficiency criterion fter Nsh nd Sutcliffe (197) of R =.89 nd the is etween mesured nd simulted runoff is within 4% for ech month (Fig. 3c). Comprison of oserved nd simulted dily runoff hydrogrphs t the guging sttion indicte good greement (Fig. 4). From 198 to 1, Nsh nd Sutcliffe (197)-vlues for dily dischrge rnge etween.76 nd.96, with n verge of.88. This result is stisfying given tht the model hs not een specificlly tuned to the dischrge series ut to the storge chnge components (ccumultion, ice volume loss) tht ensure the closure of the wter lnce (see e.g., Schefli nd Huss, 11) Experiments In order to evlute the integrted uncertinty in future glcier retret nd runoff, set of 1 experiments is defined. Ech experiment ddresses one component of model uncertinty. The description of the relted process is individully

6 Elevtion (m.s.l.) Elevtion (m.s.l.) Os 9 Model 9 Profile 1 Bedrock Profile Distnce (km) Elevtion (m.s.l.) Oserved 198 Oserved 9 Simulted 9 6 Longitudinl profile Distnce (km) Profile Profile c Figure : Vlidtion of clculted glcier retret. Simulted (red) nd oserved (lue) glcier extent for the ltion re of Findelen- nd Adlergletscher fter running the model from 198 to 9. The insets show (,) cross-sections of the glcier tongue, nd (c) longitudinl profile. Cumultive volume chnge (km 3 ) Simulted Oserved (F1) Oserved (J1) Elevtion (m.s.l.) Oserved (B_nn) Simulted (B_nn) Oserved (B_win) Simulted (B_win) Annul lnce Winter lnce Mss lnce (m w.e. -1 ) Simulted monthly dischrge (mio. m 3 ) 1 1 June (Bis = 3.6%) July (Bis = 1.3%) Aug (Bis =.1%) Sept (Bis =.9%) 1 1 Mesured monthly dischrge (mio. m 3 ) c Figure 3: () Clculted cumultive ice volume chnge of Findelengletscher for the period in comprison to oservtions sed on the geodetic method (F1: Frinotti et l., 1; J1: Joerg et l., 1). () Modelled nd mesured nnul mss lnce (verge -1) nd winter lnce (verge, 9-1) for 1 m elevtion nds. (c) Mesured versus simulted monthly runoff volume. The months June to Septemer for re symol-coded. The reltive is is given. 6

7 modified in the model implementtion. By compring the experiment result to reference simultion sed on the model nd the prmeters descried ove, the effect of the considered model modifiction on clculted runoff for the period 13-1 cn e ssessed. Key chrcteristics of ll experiments re compiled in Tle 3 nd re descried in more detil elow. The experiments re seprted into the driving climtologicl input given y the RCMs (Exp. I), nd its tretment in the glcio-hydrologicl model (Exp. II-X). The runoff model uncertinties cn e further divided into the downscling of the RCM dt (Exp. II), the ccurcy nd vilility of glciologicl dt for model clirtion or initiliztion (Exp. III to V), nd the description of glcio-hydrologicl processes (Exp. VI to X) Experiment I: Climte scenrio The clirted model is driven y chnges in ir temperture nd precipittion given y 9 RCMs sed on evlutions within the CH11 project (Bosshrd et l., 11, Tle 1). Differences in runoff etween the model runs re detched from the glcio-hydrologicl model nd depict the priori uncertinty resulting from the use of different RCM outputs. The short-term meteorologicl vriility s oserved in the pst is preserved for the modelling in the 1st century, nd the RCM provides the long-term trends Experiment II: Direct use of RCM dt Climte models run t high temporl resolution nd directly provide time series of meteorologicl vriles tht include clculted internl chnges in short-term vriility nd extreme vlues. By pplying the delt chnge pproch this potentilly importnt informtion on the chrcteristics of future climte is not ccounted for. However, the direct use of climte model results for driving impct models is not trivil nd requires creful dt tretment (Kotlrski et l., 1; Slzmnn et l., 1). The genertion of future meteorologicl time series in hydrologicl studies is thus often sed on the delt chnge pproch (e.g., Frinotti et l., 1; Bvy et l., 13). Experiment II investigtes whether the procedure of downscling RCM dt hs n impct on clculted glcier retret nd runoff. For this experiment we generte future dily meteorologicl time series different from the reference model. Dily ir tempertures directly given y ech RCM for the grid point closest to Findelengletscher re first compred to the Zermtt wether sttion dt, nd monthly is etween modelled nd oserved temperture is computed. Assuming the monthly is to remin constnt in time, continuous series for re generted. Their dily temperture vriility is given y the RCM nd the series hve the sme long-term trends s for the delt chnge pproch. This experiment only ddresses one prt of the uncertinty introduced y the direct use of RCM dt s climte model results on precipittion nd rdition re not used. For these vriles the series re identicl to the reference model run. The ility of the RCMs to yield locl short-term vritions in precipittion nd rdition re not judged s high enough. This is supported y the evlutions of Slzmnn et l. (1) who found tht RCMs often fil to reproduce oserved dily precipittion ptterns in the Alpine region. The prmeter k 1 (see Eq. 3) of the reference model is re-clirted over using the is-corrected dily RCM series s input Experiment III: Clirtion dt In Experiment III, we exclude the primry clirtion source of the glcio-hydrologicl model (volume chnge 198-7), nd clirte the model for shorter period (-1) to ice volume chnge oservtions (Joerg et l., 1). This llows ssessing the impct of limited dt vilility nd short clirtion period on clculted runoff. Only the prmeter k 1 (see Eq. 3) ws re-clirted nd no other chnges were pplied to the model geometry, nd the prmeters of the reference model Experiment IV: Ice volume Mesurements of ice thickness re scrce nd glcier volume estimtes re ssocited with considerle uncertinties. Bsed on different pproches, the uncertinty in glcier volume clcultions without priori knowledge on ice thickness hs een estimted s out ±3% (e.g., Gi et l., 1; Huss nd Frinotti, 1; Linsuer et l., 1). To ssess the impct of this uncertinty source, the oservtion-sed ice thickness distriution of Findelengletscher is scled with fctor f =.7, mimicking 3% ice volume underestimte (Exp. IV-), nd f =1.3, corresponding to n ice volume overestimte (Exp. IV). The model is run with the reference settings Experiment V: Snow ccumultion Severl studies show tht the unmiguous clirtion of hydrologicl models in glcierized sins requires the incorportion of dt on glcier mss lnce, s mesured dischrge lone does not crry sufficient informtion on the source of runoff, i.e. melt or precipittion (Konz nd Seiert, 1; Schefli nd Huss, 11; Myr et l., 13). Experiment V ddresses the impct of lcking informtion on winter snow ccumultion, which is common prolem in mny glciohydrologicl studies (e.g., Verunt et l., 3; Finger et l., 1; Uhlmnn et l., 13). Su-Experiment V- ssumes tht the totl mount of winter snow is equl to the reference simultion, ut tht the sptil distriution of ccumultion is unknown. To this end, D(x, y) (Eq. ) is set to 1 everywhere on the glcier, ssuming tht there is no sptil vriility in snow ccumultion. For su-experiment V-, no informtion on winter snow ccumultion t ll is ssumed to e ville for model clirtion. The model is clirted to dischrge dt only, without vlidting ginst sesonl glcier mss lnce or glcier front vritions. This re-clirted prmeter set yields significntly less snow (c prec =., see Tle for comprison), nd less melt (k 1 =1 W m C 1 ) which compenste for ech other in terms of runoff.

8 Tle 3: Experiments to ssess different uncertinty components in future glcier runoff projections. See text for more detiled descriptions. Exp. Topic Short description Relted studies I Climte scenrio Model driven with results from 9 different RCMs ( delt chnge pproch) Bosshrd et l. (11) II Direct use of RCM dt Model directly driven y downscled dily RCM output Slzmnn et l. (1) III Clirtion dt Model clirtion restricted to short period (-1) Joerg et l. (1) IV Ice volume Mesured ice thickness reduced y 3% Gi et l. (1) Mesured ice thickness incresed y 3% V Snow ccumultion No sptil vriility in snow ccumultion distriution (D(x, y)=1, Eq. ) Sold et l. (13) Model purely clirted on dischrge; snow ccumultion nd melt dt not considered VI Glcier retret Glcier surfce updted in 1-yer steps using the AAR-method Pul et l. (7) VII Melt model Clcultion of snow nd ice melt using distriuted degree-dy model Hock (1999) Clcultion of snow nd ice melt using n enhnced temperture-index model Pellicciotti et l. () VIII Snow ledo prm. Snow ledo constnt t α snow =.7 Mchguth et l. (6) Brock-prmeteriztion for snow ledo chnge Brock et l. () IX Ice surfce ledo Bre-ice ledo reduced to α ice =.1 Oerlemns et l. (9) X Suprglcil deris Prescied deris-coverge dynmiclly thickening nd expnding Anderson () Experiment VI: Glcier retret Some hydrologicl studies in glcierized sins clculte glcier retret sed on mss-conserving prmeteriztions (Huss et l., 8; Weer et l., 1), or physicl ice flow modelling (Immerzeel et l., 1). The glcier modules of most hydrologicl models re however sttic nd do not llow trnsient simultion of the ice melt contriution (e.g., Schefli et l., ; Rössler et l., 1; Bvy et l., 13). In these studies, glcier re is updted in discrete time steps ssuming the glcier to e in equilirium with current climte ccording to the so-clled Accumultion Are Rtio (AAR)-method (Pul et l., 7). Glcier surfce elevtion is constnt. In Experiment VI, glcier re A is updted in 1-yer time intervls sed on the clculted re of the ccumultion zone A cc nd AAR =6% (Pul et l., 7) s A = A cc /AAR. (6) All other model settings correspond to the reference simultion Experiment VII: Melt model Experiment VII investigtes differences in clculted runoff sed on severl widely used melt model formultions driven with the sme input dt. Experiment VII- employs distriuted temperture-index model proposed y Hock (1999). The model is clirted over the period sed on the sme dt s the reference model. Melt M = M(x, y, t) is clculted using the empiricl fctors f M =1.9 mm w.e. d 1 C 1, r snow =.17 nd r ice =.4 mm w.e. m W 1 d 1 C 1, the potentil solr rdition I pot, nd ir temperture T s M = ( f M + r snow/ice I pot ) T T > T M. (7) For tempertures elow threshold T M = C no melting occurs. In ddition, we pply n Enhnced Temperture-Index (ETI) model (Pellicciotti et l., ) which is sed on the degree-dy pproch, ut is closer to the energy lnce formultion of the reference model (Eq. 3). The prmeters f T =.1 mm w.e. d 1 C 1 nd r sw =.6 mm w.e. m W 1 d 1 re fitted to the field dt, nd melt M ove T M = 1 C (Pellicciotti et l., ) is otined s M = f T T + r sw (1 α) G T > T M, (8) where the clcultion of the glol rdition G, nd the ledo α correspond to the reference model (Eqs 4 nd ) Experiment VIII: Snow ledo To test the impct of the snow ledo prmeteriztion on modelled future glcier retret nd runoff, α snow is set to.7 s constnt in time nd spce for Experiment VIII- (see e.g., Mchguth et l., 6). For Experiment VIII-, we use n lterntive snow ledo prmeteriztion fter Brock et l. (). This pproch evlutes α snow s function of the ccumulted dily mximum tempertures T m,cc since the lst snowfll s α snow = α snow, c α log 1 T m,cc, (9) with α snow, the ledo of fresh snow, nd c α =.16 n empiricl constnt (Brock et l., ). For oth supplementry pproches to clculte snow ledo, the prmeter k 1 (Eq. 3) of the reference model is re-clirted over Experiment IX: Ice ledo With ongoing glcier wstge, the drkening of re-ice surfces due to ccumultion of minerl dust, lck cron nd the growth of lge is oserved (e.g., Oerlemns et l., 9). The ssocited decrese in ice surfce ledo represents significnt dditionl forcing term. For the terminus of Vdret d Mortertsch, Switzerlnd, Oerlemns et l. (9) found locl ledo chnge from.3 to.1 over the first decde of the 1st century. Experiment IX ddresses this potentil ledo chnge y drsticlly decresing α ice from. to.1 for the entire future modelling period Experiment X: Deris coverge Suprglcil deris is oserved on considerle numer of lpine glciers nd is the dominnt surfce type in the ltion re of some regions such s High Mountin Asi (e.g., Kysth et l., ; Scherler et l., 11). As soon s deris thickness is lrger thn few centimeters, ice melt is reduced sustntilly (Nicholson nd Benn, 6; Hgg et l., 8; Reid nd Brock, 1). With glcier retret oth thickening of the suprglcil deris lyer nd n expnsion of the deris-covered re is expected (Anderson, ).

9 Although Findelengletscher currently only hs minor deris-coverge, we test the influence of suprglcil deris on future glcier retret nd runoff y rtificilly prescriing deris-covered ice elow n elevtion of 3 m.s.l., i.e. out % of the ltion re. In Experiment X, clculted melt for deris-covered ice is multiplied y reduction fctor f deris =. (Huss et l., 7) corresponding to deris thickness of.1 m (e.g., Reid nd Brock, 1) tht is typicl for Alpine glciers. In the model run until 1, deris cover is treted s dynmic lyer tht progressively thickens to mximum of out.3 m y 1. This is descried y continuous decrese in f deris of. 1. The outwrd propgtion of the deris coverge in time nd spce is simulted ccording to n pproch proposed y Jouvet et l. (11). Feedck effects due to ice cliffs nd ponds tht increse melt in the deris-covered re (Benn et l., 1) re not included; our experiment thus represents n upper ound for the deris-cover effect. 4. Results We perform model runs for the chnges in climte forcing given y ech of the 9 RCMs (Exp. I). For Experiments II- X (Tle 3) the model is run with chnged input dt, different sets of clirted prmeters, or modifictions in the model structure for the medin RCM. The results re compred to the reference simultion. The reference model is sed on the MPI ECHAM REMO regionl climte model, the model setup is descried in Section 3.1, nd the vlues of the most importnt prmeters re given in Tle. Bsed on the reference model we expect significnt mss loss of Findelengletscher over the 1st century (Fig. ). A totl glcier terminus retret of 1. km until 3 is clculted. Between 1 nd 6, n ice volume loss of more thn % is found (Fig. ), resulting in glcier re of less thn km y the end of the 1st century (Fig. 6). The reference model indictes n increse in nnul runoff until out (Fig. 6) due to relese of wter from longterm glcil storge. The smller glcier re cn, however, no longer mintin incresing flow towrds the end of the century. A slight increse in evpotrnspirtion is clculted y GERM. Together with the reduced excess runoff from glcier wstge, this cuses decrese in nnul runoff until 1 (Fig. 6). Strong shifts in the hydrologicl regime re found resulting in significnt decrese in summer runoff, nd pek melt dischrge occurring out 1. months erlier in the seson (Fig. 6c). Forcing the glcio-hydrologicl model with different chnges in ir temperture nd precipittion s prescried y the 9 RCMs leds to considerle spred in the results (Fig. 6). Clculted glcier re loss y 1 vries etween 68% nd 98% reltive to 7. The pek in nnul runoff volume is reched round 4 for RCMs with the most rpid increse in summer tempertures (Tle 1) wheres for the more moderte RCM simultions, nnul dischrge might increse until out 6 (Fig. 6). Differences in modelled glcier re, nnul nd August runoff reltive to the oservtion period men re shown in Figure 7. Wheres the uncertinties in clculted nnul runoff volume due to the choice of the RCM remin within ±% of the reference model throughout the modelling period, August runoff y 1 devites from the reference y 6% (+1%) for the RCM providing mximum (minimum) chnges in ir temperture. This confirms tht considering multi-model GCM-RCM-chins in projections of future runoff is indispensle to render plusile spred in the results due to the intrinsic climte model uncertinty. According to Experiment II, the pproch for generting dily temperture time series from the RCM output for driving the hydrologicl model hs limited impct on the results (Fig. 8). Glcier re nd runoff chnge clculted with the is-corrected dily RCM time series re within ±1% of the results sed on the delt chnge pproch nd no systemtic differences re evident except for spin-up effect in the first decde of the modelling. Considering monthly to sesonl chnges in the men of meteorologicl vriles ( delt chnge pproch) nd dily vriility s oserved in the pst seems to e sufficient for cpturing the governing processes. This conclusion might however not e vlid for pplictions other thn glciers where the short-term vriility or extreme events (lso in terms of precipittion) re more importnt. The clirtion dt used for constrining the hydrologicl model hve considerle effect on clculted glcier chnge nd runoff (Fig. 8). In Experiment III, the rte of modelled glcier mss loss is reduced, nd the expected August runoff in 1 is higher y 17% reltive to the reference model (Fig. 7). This indictes tht runoff projections over the 1st century re sensitive to the vilility of clirtion dt, nd the time period covered y these dt. For n ice thickness underestimte (Exp. IV-), glcier re loss is ccelerted leding to nnul runoff volumes 14% elow the reference model y 7 (Fig. 9). In contrst, the glcier response to the sme chnges in climte is delyed in the cse of n ice thickness overestimte (Exp. IV-). Runoff volumes remin higher thn tody throughout lmost the entire 1st century, nd no wter shortge even during the summer months is expected (Fig. 9). Experiment IV emphsizes the importnce of knowledge out the ice volume presently stored in glcierized sins to correctly model future runoff, nd consequent impcts on the mngement of wter resources of high-mountin ctchments. Experiment V demonstrtes tht informtion on winter snow ccumultion is highly importnt for correctly simulting future chnges in runoff for severl resons. If dt on the quntity of ccumultion re ville ut the sptil snow distriution is not ccounted for (Exp. V-), Findelengletscher retrets considerly fster nd shifts in the hydrologicl regime re ccelerted (Fig. 1). For exmple, decrese in August dischrge y % is expected until 7, wheres the reference model predicts lmost stle runoff ( 3%). In this experiment the model ignores sptil ccumultion ptterns on Findelengletscher tht hve een detected sed on mesurements. A complete lck of snow ccumultion dt (Exp. V-), nd clirtion of the hydrologicl model on mesured dischrge lone hs drstic consequences on projected runoff (Fig. 1). A

10 3 Ice volume (km 3 ) Are (km )=11.3 Volume (km 3 )=.8 6 Are (km )= 3.36 Volume (km 3 )= Are (km )=14.48 Volume (km 3 )= Ice thickness (m) Elevtion (m.s.l.) Distnce (km) 4 6 Figure : Modelled glcier retret nd ice thickness in the hydrologicl sin of Findelengletscher in 3, 6, nd 9 ccording to the medin RCM nd the reference model. The glcier outline of 1 is depicted. Insets show () the evolution of glcier ice volume, nd () ice surfce topogrphy in longitudinl profile (dshed line in first pnel). Are chnge (%) Exp. I REFERENCE mx +1 min Exp. II - +4 Exp. III + + Exp. IV Exp. V Exp. VI -1-6 Exp. VII Exp. VIII Exp. IX Exp. X August runoff chnge (%) Annul runoff chnge (%) Exp. I REFERENCE Exp. I REFERENCE Exp. II -1 + Exp. II Exp. III - +6 Exp. III Exp. IV Exp. IV Exp. V Exp. V Exp. VI -6-8 Exp. VI Exp. VII Exp. VII Exp. VIII Exp. VIII Exp. IX - -3 Exp. IX +1-1 Exp. X - +6 Exp. X -4 c +1 Figure 7: Chnges in () clculted glcier re reltive to 7, () nnul runoff nd (c) August runoff reltive to oservtions (men 196-1) y nd 1. Brs show results for the reference model nd ech experiment (cf. Tle 3). Numers in the rs stte the difference of clculted re/runoff reltive to the reference model result y nd 1, respectively, in percent. For Experiment I, only model runs with mximum/minimum chnges re given. Ornge rs refer to the results of every experiment, nd lue rs to su-experiment, if ville. 1

11 Glcier Are (km ) Annul runoff (1 m ) Annul runoff (1 m ) Glcier Are (km ) Downscling (Exp. II) Dily RCM dt -8% -4% +3% Dily RCM dt +% Alt. clirtion Alt. clirtion Clirtion (Exp. III) -4% -44% +3% +37% AAR-Method -81% -4% Glcier Retret (Exp. VI) +3% AAR-Method -3% Glcier re 7 1 Snow Aledo (Exp. VIII) Glcier re 7 Men runoff Const. ledo Brock prm. Men runoff Const. ledo Brock prm. -4% -3% -% +3% +6% +9% e Glcier re 7 Men runoff Glcier re 7 Decr. ice ledo -4% -63% +3% Decr. ice ledo +% Ice Aledo (Exp. IX) f Glcier re 7 Men runoff Deris cover Men runoff Men runoff Deris cover c Deris Coverge (Exp. X) -4% -46% +3% +36% g Glcier re 7 DD Model ETI Model DD Model ETI Model Men runoff Melt Model (Exp. VII) -4% -67% -49% +3% +19% +33% d Figure 8: Trnsient chnges in glcier re nd nnul runoff clculted for the reference model nd () n lterntive downscling of RCM dt (Exp. II), () shorter clirtion period (Exp. III), (c) different model for glcier retret (Exp. VI), (d) different pproches to compute snow nd ice melt (Exp. VII), (e) lterntive prmeteriztions for snow ledo (Exp. VIII), (f) reduced re-ice ledo (Exp. IX), nd (g) deris-covered glcier tongue. Reltive chnges compred to initil glcier re (7), nd men oserved runoff re given in % for the yer 7. 11

12 Glcier Are (km ) Annul runoff (1 6 m 3-1 ) Dischrge (m 3 s -1 ) s/197s 198s/199s s c MPI_ECHAM_REMO SMHI_ECHAM_RCA KNMI_ECHAM_RACMO ICTP_ECHAM_REGCM ETHZ_HdCM3Q_CLM HC_HdCM3Q_HdRM3Q SMHI_HdCM3Q3_RCA CNRM_ARPEGE_ALADIN SMHI_BCM_RCA Men August runoff April My June July Aug Sept Oct Nov Glcier Are (km ) Annul runoff (1 6 m 3-1 ) Dischrge (m 3 s -1 ) c Are chnge -4% Smll volume (IV-) -6% Lrge volume (IV-) -43% Runoff volume chnge +3% IV- +1% IV- +49% Initil Ice Volume (Exp. IV) Men August runoff Ref. IV- IV- -3% -3% +16% August runoff chnge April My June July Aug Sept Oct Nov Figure 9: Chnges in () glcier re, () nnul runoff, nd (c) runoff hydrogrphs sed on different ssumptions of initil ice volume (Exp. IV). Reltive chnges compred to glcier re in 7, nd men runoff re given in % for 7. Figure 6: Clculted future chnges in () glcier re, () nnul runoff volume, nd (c) runoff hydrogrphs (y 1) sed on climtic conditions prescried y 9 RCMs (Exp. I). The reference model is shown with the solid lck line. Mesured decdl mens of pst nnul runoff re indicted with dots in (), nd the oserved runoff regime (196-1) is shown in (c). 1

13 Glcier Are (km ) Annul runoff (1 6 m 3-1 ) Dischrge (m 3 s -1 ) Are chnge Sn. distri. Sn. quntity -4% -69% -91% Runoff volume chnge +3% V- +17% V- -43% c Snow Distriution (Exp. V) Men August runoff Ref. V- V- -3% -% -7% August runoff chnge April My June July Aug Sept Oct Nov Figure 1: Chnges in () glcier re, () nnul runoff, nd (c) runoff regime clculted sed on different uncertinties in winter ccumultion dt (Exp. V). Reltive chnges compred to initil re (7), nd men runoff re given in % for 7. complete disintegrtion of the glcier is found for 8. No trnsient dischrge increse (s for ll other experiments) is simulted, nd long-term reduction in nnul runoff y more thn % is evident (Fig. 1). Simulted runoff in August is smller y 8% in this experiment t the end of the 1st century compred to the reference model (Fig. 7). The mistkes in setting the ccumultion nd the melt prmeters for this model experiment re ovious; compenstion of too little precipittion y dditionl ice melt remined undetected in the clirtion period s no dt on the components of runoff (winter ccumultion, glcier melt) were considered. Experiment V shows tht including dt on storge chnge components (snow ccumultion, ice melt) for constrining long-term hydrologicl projections is crucil to their ccurcy. The AAR-method to updte ice surfce extent (Exp. VI) is not mss conserving, i.e. ice volume cn e lost without contriuting to dischrge, nd ssumes the glcier lwys to e in equilirium with climte. This leds to underestimted glcier res throughout the 1st century nd nnul runoff vlues systemticlly elow (up to 3%) the reference model (Fig. 7 nd Fig. 8c). According to this experiment, the generl evolution of future runoff chrcteristics in glcierized sins is strongly ffected y the glcier retret clcultion, which is still rudi mentry in mny glcio-hydrologicl models. For Findelengletscher, the degree-dy model (Hock, 1999) leds to slightly fster glcier mss loss compred to the reference model (Exp. VII-). An erlier pek in nnul runoff is found (Fig. 8d). This oservtion indictes higher sensitivity of this model to ir temperture chnge. The ETI-model (Pellicciotti et l., ) results in modertely slower glcier response to prescried climtic conditions. The uncertinties in terms of runoff in August due to the choice of the melt model re in the rnge of ±-1% (Fig. 7). This issue is thus less importnt compred to Experiments III-VI. The prmeteriztion to clculte snow ledo (Exp. VIII) hs reltively smll effect on clculted future runoff (Fig. 8e). Although the differences in snow ledo used in the modelling re considerle, nd strongly lter the energy lnce, errors seem to cncel ech other out, nd the uncertinties in projected nnul runoff remin within ±3% (Fig. 7). Assuming temporlly constnt snow ledo, slightly slower glcier re chnge is found, which is ttriuted to likely snow ledo decrese over the next decdes due to reduced frequencies of fresh snow flls nd wrmer winter tempertures. Drsticlly decresing glcier ledo in the re ice region (Exp. IX) leds to n ccelertion of glcier re loss nd n erlier shift in the hydrologicl regime (Fig. 8f). This crucil prmeter in the energy lnce of ice surfces does however only trigger reltively moderte differences in clculted runoff (round ±7%) compred to the reference model (Fig. 7). The extensive suprglcil deris coverge prescried in Experiment X delys the clculted re loss of Findelengletscher compred to the reference simultion (Fig. 8g). Pek nnul runoff occurs 1 yers lter, nd dischrge in August is 1% higher y 1 (Fig. 7). Deris-covered ice is only found in the ltion re, nd its modelled propgtion in spce is slower thn the rise in equilirium lines over the 1st century. Even the out three-fold thickening ssumed does not completely stop the melting process (e.g., Nicholson nd Benn, 6). The importnce of ccounting for the feedck of suprglcil deris in runoff projections strongly depends on the chrcteristics of the investigted glcier. The effect lies in the rnge of the processes investigted in Experiments VII-IX, nd is thus reltively smll given tht our ssumptions rther represent n upper ound for the potentil impct.. Discussion Uncertinties in runoff projections from high-mountin dringe sins need to e ddressed in n integrtive wy in order to provide resonle error rs for the mngement of wter resources. Bsiclly, uncertinties due to (1) the climte models nd () the pproch to trnslte the chnges in climte into runoff response, i.e. the glcio-hydrologicl impct model, cn e discerned. The impct modeller hs to rely on clculted trends in ir temperture, precipittion nd other vriles provided y the climte modelling community. The spred in these results thus represents the priori uncertinty. The strong differences in clculted high lpine runoff using different RCM results sed on