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Climate 10.2 % of the total Sciences Sciences Dynamics Chemistry of the Past ∼ Solid Earth Solid Techniques Geoscientific Methods and and Physics Atmospheric Atmospheric Data Systems Geoscientific Earth System Earth System Measurement Instrumentation Hydrology and Hydrology 4 Ocean Science Ocean Annales Biogeosciences The Cryosphere Natural Hazards cient spatial distribution of and Earth System ffi Model Development Geophysicae ect applied at a regional scale, we ff 25 % of the debris-covered glaciers, , and Q. Liu 2 ect on the ∼ ff ect are apparent, highlighting the impor- ff , S. Liu 2414 Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access Open Access 2413 3

cult to analyze regional debris-cover e ects glacier response to climate change by altering ff ffi Climate Sciences Sciences , K. Fujita Dynamics Chemistry of the Past 1 (Dyurgerov and Meier, 2005) which are a vital source of Solid Earth Solid Techniques Geoscientific Methods and and Physics Advances in Atmospheric Atmospheric 2 Data Systems Geoscientific Geosciences Earth System Earth System Measurement Instrumentation Hydrology and Hydrology Ocean Science Ocean Biogeosciences The Cryosphere Natural Hazards EGU Journal Logos (RGB) and Earth System and Earth Model Development erences in the debris-cover e , Y. Hirabayashi ff 1,2 Please refer to the corresponding final paper in TC if available. This discussion paper is/has been under review for the journal The Cryosphere (TC). Institute of Engineering Innovation, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku,State Tokyo Key Laboratory of Cryospheric Sciences, Cold and Arid RegionsGraduate Environmental School and of Environmental Studies, Nagoya University, Chikusa-ku, Nagoya Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu et al., 2010). Mass changesof of these substantial glaciers importance and for their the response evolution to of climate water change resources are (Immerzeel et al., 2010; tance of debris cover for understandingPlateau glacier and status and other hydrology mountain in ranges both around the the Tibetan world. 1 Introduction The Tibetan Plateau andapproximately surroundings 116 180 km contain massivewater glaciers at the with headwaters of a many total prominent Asian area rivers of (Immerzeel et al., 2010; Kaser on heavily debris-covered glaciers,ablation it area accelerates and ice melting producesresulting on rapid in the wastage similar of Widespread mass debris losses cover between also debris-coveredRegional and facilitates di debris-free the glaciers. development of active terminus regions. Gongga glaciers, maritime glaciersterized in by the a substantial south-eastern reduction Tibetan invanced Plateau, glacier Spaceborne length are and Thermal charac- ice Emissionthermal mass in and property recent Reflection decades. of Ad- Radiometer themantles (ASTER)-derived of debris supraglacial layer debris reveals incover that their to ablation 68 total % zones, glacier in of area whichbalance the the varies model glaciers proportion from accounting of 1.74 have % debris for extensive find to the that 53.0 although %. debris-cover the Using e presence a of surface supraglacial energy-mass debris has a significant insulating e The Tibetan Plateauglaciers, on and which surroundings debrisice cover contain a melting a rates and largedebris spatial thickness number patterns data makes of of it mass debris-covered di loss. Insu Abstract The Cryosphere Discuss., 7, 2413–2453,www.the-cryosphere-discuss.net/7/2413/2013/ 2013 doi:10.5194/tcd-7-2413-2013 © Author(s) 2013. CC Attribution 3.0 License. Spatial debris-cover e maritime glaciers of Mount Gongga, south-eastern Tibetan Plateau Y. Zhang 1 113-8656, Japan 2 Engineering Research Institute,3 Chinese Academy of Sciences, Lanzhou 730000, China 464-8601, Japan 4 610041, China Received: 8 February 2013 – Accepted: 21Correspondence May to: 2013 Y. – Zhang Published: ([email protected]) 6 June 2013 Published by Copernicus Publications on behalf of the European Geosciences Union. 5 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | by altering ff E; Fig. 1) is located on the south- 0 ect accelerates (if debris is thin) or ff 10 ◦ erences in such response are apparent ff –102 0 2416 2415 ect. Such work presents an integrated view of 30 ff ◦ N, 101 0 ect that has influenced mass balance and runo 10 ff ◦ at a large scale (e.g. Lambrecht et al., 2011; Mayer et al., ff –30 0 ects, we systematically discuss the potential influences of debris 20 ect at a regional scale, which is a necessary first step toward un- ff ◦ ff culty in such a study arises from poor knowledge of the large-scale ffi erences in the debris-cover e ff ects of debris cover. remains limited. ff ff In this study, we attempt to determine the spatial distribution of debris cover through- Part of the di 1996). Very few meteorologicaling in stations significant are climatic operating uncertainties, in especially the in the region magnitude (Fig. of 1), precipitation result- at 2 Study area and data 2.1 Study area Mount Gongga (29 eastern margin of the TibetanAsian Plateau, monsoon where in the the climate is summer characterized and by the the westerly East circulation in the winter (Li and Su, Gongga maritime glaciers atgional a di regional scale andthe analyze debris-cover the e potential causesderstanding of the re- response oftheir debris-covered impacts maritime on the glaciers evolution to of climate river discharge change and and water resources. runo out the ablation zones of thePlateau, maritime glaciers which of Mount is Gongga, south-eastern oneglaciers Tibetan of (Shi and the Liu, largest 2000),Using based and on a most field surface and sensitive energy-mass remote-sensing remainingbris balance based measurements. areas model cover and that of its accounts maritime cover for e the and significance its of spatial de- distribution characteristics on the average status of the Mount (e.g. Kayastha et al.,their 2000; application has Nicholoson mainly and relied on Benn,thickness high-quality input and 2006; parameters related thermal Reid to properties and thewhere extent, Brock, of debris 2010), debris cover and but cover. its Consequently,ify spatial an its distribution understanding enhance spatial of or characteristics inhibit and ice to melt rates what to extent mod- melting influences mass balance and cover on the glaciers. Furthermore, several numerical models have been proposed these studies relied on simplified representations of the extent and thickness of debris and e spatial distribution officulties debris of thickness measuring andsupraglacial debris debris properties thickness satellite mapping because and remainingSuzuki of properties in et al., development the on 2007; (e.g. Mihalcea Paul practical glaciers etAlthough et al., dif- and al., several 2008; 2004; recent Racoviteanu methods et investigations al., for glacier have 2009; addressed Casey status et the and al., impact 2012). runo 2011; of Scherler debris et cover al., on 2011; Anderson and Mackintosh, 2012; Immerzeel et al., 2012), consider the debris-cover e both rates and spatial patterns ofof melting. debris Developing cover a in better glacier understanding status ofa and the challenge, hydrology as role at it a will require regionalcover an scale ensemble coupled nevertheless of remains with estimate the a extent large-scale and thickness mass-balance of model debris accounting for the significance suppresses (if it isand thick) ice (Østrem, ice 1959; melting Nakawo and beneath2000). Young, 1981; debris Furthermore, Mattson relative et debris-covered al., to 1993;and glaciers that Kayastha surroundings et are of and al., generally exposed widespread contain snow et in a al., large the 2011; ice Tibetan Benn volume (Paul etto Plateau et al., debris al., 2012). cover 2004; Consequently, can Scherler glacierevolution be response of expected to river climate to forcing discharge exertavailability due a and and significant its water influence impacts resources. on in Realistic the the prediction Tibetan regional-scale of Plateau and future surroundings, water therefore, should from region to regionin the across Tibetan the Plateau plateauet and al., surroundings because 2012). (Fujita, of In 2008; particular, heterogeneousare Fujita considerably the and climate more responses Nuimura, complex forcing of than 2011; those debris-coveredBenn Yao of glaciers debris-free et to glaciers climate (Scherler al., et change 2012), al., 2011; because the debris-cover e Kaser et al., 2010), although systematic di 5 5 25 20 10 15 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2 ). Details of (Song, 1994; Zhang 1 − 200 myr > gridded data of daily precipitation and near- 2418 2417 ◦ measured from aerial photographs acquired in the 1960s 3 http://docs.geogrid.org/TechDocs/ASTERDataBeta C per decade between 1952 and 2009 (Zhang et al., 2012). ◦ 1.9 m water equivalent (w.e.), respectively, for the period 1988–2004 ∼ ne transformation was within 15 m (Zhang et al., 2010). The other two images C and ◦ ffi ne-transformed by referring to a topographical map, and the root mean square error Mount Gongga glaciers have a high ice velocity (Song, 1994; Li and Su, 1996; Zhang Mount Gongga contains 74 monsoonal maritime glaciers with an area of 257.7 km ffi addition, air temperature and precipitationtion data (AWS; observed 3550 at m a.s.l.) an and automatic a weather precipitation sta- gauge (3500 m a.s.l.) installed in the HLG 2.2.2 Meteorological data To drive the detailed surfacevarious energy-mass meteorological balance datasets, model including used dailywind in data speed this for and air study, relative we temperature,2007, humidity used precipitation, and observed observation-based at global GAEORSsurface 0.5 (Fig. temperature 1) from for the the closest period grid 1988– of the region for the period 1951–2007. In vation Grid (GEO Grid; the ASTER Grid system ofmeasures GEO three Grid visible can and beresolution, near found six infrared in shortwave bands Sekiguchi infrared et (VNIR,tion, bands al. 0.4–0.9 and (SWIR, µm) (2008). 1.0–2.5 five at ASTER µm) thermal a atthis infrared 15 a study, m bands we 30 spatial used m (TIR, the spatial 3.0–12 µm) ASTER resolu- VNIR at and a TIR 90 bands. m spatial resolution. In (11.58 a.m. LT), were used forits determining property. There the is spatial noerated distribution cloud and of or debris distributed snow cover as cover(ASTER and GDS) in Level at these 3A01 the images. Earth products Two(ERSDAC, Remote images by 2002; Sensing were Data Fujisada the gen- Analysis et ASTER Center al.,a (ERSDAS) Ground in 2005). Japan Data The geography System locationof the of a these images was were generated and distributed by the ASTER Grid system of the Global Earth Obser- 2.2.1 ASTER data Four orthorectified images ofReflection the Radiometer Terra (ASTER), Advanced obtained Spaceborne on Thermal 17 December Emission 2008 and and 18 January 2009 2.2 Data et al., 2010), with maximumet velocities al., in 2010). the Most order glaciersretreat of on Mount and Gongga mass have experienced lossZhang considerable since terminus et al., the 2010, early 2012; 20th Pan et century al., (Su 2012). et al., 1992; Liu et al., 2010; Glacier (Fig. 1),of have total a ice length volume (Tablecover greater 1). in than Many of their 10 km theseice/snow/rock ablation glaciers and have avalanche zones, together considerable activities a surface account debris weathering occurring consequence for processes of on and 68.4 % structural the the rockfalls (Li steep surrounding and rocky walls Su, 1996). terrain through and frost mixed an increase of 0.13 and a volume of(Pu, 24.65 km 1994). Thesemainly glaciers from belong summer-monsoon to snowfallequilibrium-line (Li the and summer-accumulation altitudes Su, type, 1996; (ELAs)Li gaining Shi and vary mass and Liu, between 2000). Su,Yanzigou Glacier 4800 (YZG) 1996). Glacier, and Five Nanmenguangou 5240 glaciers, m a.s.l. (NMGG) including (Pu, Glacier HLG and 1994; Glacier, Dagongba Mozigou (DGB) (MZG) Glacier, Station (hereafter, GAEORS) located(3000 m near a.s.l.; the Fig. terminus 1)are of 4.1 show Hailuogou that (HLG)(Zhang the Glacier et mean al., annual 2010).region Previous air (Su studies et temperature presented al., and 1992; evidence Liu for precipitation et a al., warming 2010; Zhang trend et in al., this 2010, 2012; Pan et al., 2012), with high elevations. Data from the Gongga Alpine Ecosystem Observation and Research 5 5 20 25 15 10 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ). 5100 m a.s.l. on the 1/SRTM30/ ∼ erent elevations (Zhang ff ciently well with the obser- ffi erence in the length and 0.5 % error ect of various debris thicknesses on ff ff 30 m di 2420 2419 ± erent glaciers in 2009 (Pan et al., 2012). The results of ff http://dds.cr.usgs.gov/srtm/version2 and ASTER data using threshold ratio images, and was + 1 % (Pu, 1994). The glacier outline for 2009 was derived from 4900 m a.s.l. on the eastern slope and ± ∼ 0.5 % to ± ELA data of Mount Gongga glaciers were used to determine the extents of ablation The global gridded data of precipitation and near-surface temperature were bias- The gridded temperature dataset was generated by Hirabayashi et al. (2008) based western slope of Mount Gongga (Su et al., 1992;2.2.5 Pu, 1994). Digital elevation model A DEM was producedThe from aerial accuracy photographs of acquired thea in DEM root 1989 was mean (Zhang validated square et by90 error al., GPS m 2010). of for data 11.2 use on m by glacier-free (Zhang the terrain, et model. yielding al., 2010). The DEM was resampled to the surveys indicate that therein is the about area (Pan etcan be al., found 2012). in Full the details work of of these Pan et data al.zones. and (2012). Glacier their ELAs generation recorded methods inThe the average CGI ELA (Pu, is 1994) range from 4800 to 5240 m a.s.l. Mount Gongga glacier outlinesterminus in changes 1966 of and glaciers.Inventory 2009 The (CGI; were glacier used Pu, outline for 1994),from for estimating was 1966, aerial area interpreted from photographs and and the acquiredand measured Chinese in field by Glacier the stereophotogrammetry investigations 1960s (Pu,varies and from 1994). corrected The by errorLandsat aerial in MSS, photographs glacial TM, extentverified ETM by estimates GPS of surveys the on di CGI et al., 2011), whichice were melt used rates. to analyze the e 2.2.4 Glacier outline and ELA data 2008 using 27 ablation stakes drilled into the glacier at di ness measurements were made atof more than HLG 300 sites Glacier across in the entire 2009, ablation and zone ice ablation data were collected during the summer of which indicates that the bias-correctedvations data correspond at su GAEORS toet be al., used 2012). as input for the2.2.3 energy-mass balance model Debris (Zhang thickness and ablation data The first inglaciers situ was measurements carried out ofand in debris 11 1982 sites thickness at on 23 and Xiaogongba sites (XGB) ablation on Glacier (Li HLG on and Glacier, Mount Su, 19 1996). Gongga sites Systematic on debris DGB thick- Glacier corrected on a daily timescaleestablished by between Zhang the et observations al. from (2012)the GAEORS using period and linear 1988–1997. the regression Also, global equations the ability griddedthe of data the surface over bias-corrected energy-mass global gridded balance data to model drive was verified in the HLG catchment (Fig. 1), was created by collectingonly and analyzing long-term rain-gauge continental-scale observationnetwork high-resolution data across of daily Asia, daily product an rain-gaugeareas that in data contains the for a Middle Asiaperature dense East including and (Yatagai et precipitation the al., datasets Himalayaswork and 2009). and their of Full mountainous generation details Hirabayashi methods et ofinformation can al. the of be global (2008) the found gridded and grid in tem- YatagaiRadar cell the et Topography Mission was al. ( derived (2009), from respectively. the The 30 altitude arc s elevation data of the Shuttle image acquisition days. on monthly temperature valuesfor of the the period 1951–2002 Climate (CRU; Researchvan Mitchell den Unit and Dool Jones, version (2008) 2005) TS for and the 2.1 the period dataset dataset 2003–2007, of whereas Fan the and daily precipitation dataset catchment (Fig. 1) were used for analyzing the meteorological conditions during the 5 5 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (1) ) of the 1 − K 1 − ), defined as debris thickness di- 1 − KW 2 ; m R 2422 2421 are the thickness (m) and the thermal conductivity (Wm λ and h , λ h As investigations on HLG Glacier (Fig. 1) showed (Zhang et al., 2011), ASTER- The air temperature and precipitation time series were interpolated for each elevation = thickness on the Mount Gongga glaciers. ASTER-derived thermal resistances correspondsurveyed well debris with thicknesses, which spatialthickness can patterns reflect of of large-scale the ground- variation debrisfor in debris cover the thickness and extent over and support largetance areas. the as To use assess the the of proxyof validity thermal for this of debris resistance applying region, thermal thickness as wenesses resis- distribution a compared on on proxy DGB along-glacier other and patternswhich debris-covered XGB indicates of glaciers glaciers that ground-surveyed their (Fig. debristhe variations 1) ASTER-derived thick- agree and thermal reasonably resistance ASTER-derived well as thermal (Fig. the proxy 3). resistances, for Therefore, the we spatial use distribution of debris Mount Gongga glaciers mentioned above (Fig.is 2a). 90 The m pixel and size thermal of ASTER resistance TIR was image calculated using thisderived same thermal resolution. resistances correlatedthicknesses over reasonably the well entire with ablation ground-surveyed zone, debris and across- and along-glacier patterns of cation of ASTER acquisition,energy with balance. the The assumption approach ofstudies used negligible (Suzuki in et turbulent this al., fluxes study 2007;was in is retrieved Zhang the from identical et five al., to TIR 2011). bands thaton of The used the ASTER average in and glaciers, brightness was previous temperature andreflectance used as the at the broadband surface the temperature albedo topcorresponding was of surface estimated downward the directly radiation atmospherewe from fluxes calculated in from the the VNIR NCEP/NCAR spectral thermal bands reanalysis resistances of data of ASTER. debris Combined layers over with the ablation zones of the scale. The thermal resistance offrom the the debris surface layer temperature inthe of this net study the radiation was debris estimated therefore layer calculated fromvironmental derived ASTER Prediction from VNIR (NCEP)/US ASTER bands National and TIR Centerreanalysis US for bands Atmospheric National data and Research Centers (NCAR) (Kalnay for En- et al., 1996), which correspond to the nearest time and lo- conductivity of the debris layer are especially time-consuming and unrealistic at a large where debris layer, respectively. Note that field determinations of the thickness and thermal 3.2 Thermal resistance of a debrisThe layer thermal resistance of avided debris by layer the ( thermal conductivityis of expressed the as debris layer (Nakawo and Young, 1981, 1982), R altitude with a constantincreasing lapse altitude rate with (Table a 2),of precipitation and gradient elevation precipitation (% increase; increased of Tableobserved linearly precipitation 2) from with GAEORS increase from was per not the interpolated meter surface and snout (Zhang assumed constant to et across al., the the 2012). glacier top of the glacier. Wind speed obtained from the DEM basedwas on determined the glacier based outlines. on The theof extent DEM of ELAs, and each which glacier ablation outlines we zone glacier combined obtained with from was the the measured distribution CGI fromablation (Pu, zones the 1994). of DEM, In Mount and addition, Gonggathe the central glaciers the flowline aspect was mean (lowermost measured of 1–2 slope along each km, the of depending on profiles the glacier that ice size). follow band surface according in to its the mean elevation. The air temperature decreased with increasing 3.1 Glacier discretization and data preprocessing Glaciers on Mount Gongga50 m were based divided into on a the set DEM of and elevation glacier bands outlines. at The intervals area of of each elevation band was 3 Methods 5 5 25 15 20 10 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | G (4) (3) Q is the 0) sur- 0 and α ≤ L is the sur- R Q S , T S Q ; Fig. 2a), glaciers R ) is calculated from are energy available f M L Q erent periods in the HLG ff is the surface temperature I and T i (positive sign) is refreezing, 0) and debris-free ( ρ F , M R R > Q C), and ◦ ect, and computed the area-averaged C (Zhang et al., 2011). ◦ ff 0 G 2424 2423 = Q 0 G + Q and glacier ablation in di L . 2 ff + Q − L + Q S + Q , (2) S + F Q R Lu are the downward short-wave radiation flux, downward long- + R + Lu Lu V + R E R is the surface albedo (Table 2). All terms are taken to be positive Ld + + are ablation and evaporation (mass loss terms are defined nega- α R I f T Ld L V + and − i R ) is therefore calculated from S R E S T M Ld /ρ + R Q ) = R M ) at any location on the glacier is calculated as S is the conductive heat flux into the debris layer, respectively, and α , 0 G Q and R B 0 G ) S − 0 Q f + R Q α L i (1 = − Ca M = /ρ The various components of the energy balance were calculated using climatic and For the debris-covered ice, the only heat flux considered to reach the glacier ice Accumulation is modelled from the precipitation value by using a simple temperature = (1 Q M M topographic data mentioned above.the Absorbed short-wave surface radiation albedo is and calculated downward from short-wave radiation, which is derived from daily  where albedo of debris-covered surface, whichface is temperature estimated of using the ASTER debris-covered data. of surface the ( debris-ice interface, which was set to 0 is the conductive heatwithin flux the with debris simplifying layer assumption andday the of (Kraus, constant 1975; a Nakawo heat linear and flux Young, temperaturefor stored 1981; melting profile in Zhang ( et the al., debris 2011). layer The from energy day available to Q where wave radiation flux andare upward the long-wave net radiation sensible flux, andrespectively, respectively, latent and heat fluxes and conductivetoward heat the flux surface into in the units glacier of ice, Wm the interface between the snow layering and during glacier winter ice and (Fujita during and shorterspatial Ageta, cooling distribution 2000). events Refreez- is of also the considered.on According thermal Mount to resistance Gongga the are of classified thefaces. as For debris debris-covered the ( cover debris-free ( ice, the energy available for melting ( the conduction of heat into the snow layer and glacier ice and the presence of water at and rain is assumedthreshold for temperature a (Table transition 2). zone The ranging refreezing from amount 1 is K calculated above by and considering 1 K below the B where Ca (positiveQ sign) istively), accumulation respectively. All (snow), terms arefor in melting, units density of of ice m w.e. (Table 2) and latent heatthreshold of to fusion determine (Table 2), whether precipitation respectively. falls as rain from snow. A mixture of snow available for melting fromand energy the exchange atmosphere, between and accumulation. thethe The debris-covered/free subsurface second surface after is meltwater tolates treat percolates the processes surface in mass occurring the balance in for underlyingbalance each ( layers. elevation band The at model a calcu- time step of one day. Mass vals of 50 m altitudemodel for that the can periodmass 1998–2007 consider balance using using the the a obtained debris-cover surfaceenergy-mass area-altitude energy-mass balance e distribution model balance of used each glacier. inet The this al. surface study (2012) is and identicalmodel is to results described that to developed only the by observed briefly Zhang runocatchment here. (Fig. The 1), model one was of2012), verified the which glacierized by consists catchments comparing of on Mount two Gongga coupled (Zhang components. et The al., first is to compute the energy No systematic direct measurementand of glacier only mass isolated balanceDGB, exists measurements for XGB of Mount and Gongga, melt HLGThe glaciers rates have mass on been made debris-covered balances (Li ablation of and zones Su, Mount 1996; of Zhang Gongga et glaciers al., 2011). were therefore estimated at inter- 3.3 Surface energy-mass balance model 5 5 25 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (5) 10 km ect” for > ff ) divided M ), is expressed 0 ect on the ice- ff M ) and are averaged over , accounting for as much 3 2 32 km ) and ( accounts for as much as 78.3 % of ∼ 4 ect” for simplicity), whereas a ratio 2 ff 5 km > 2426 2425 1.0 means that the presence of debris enhances > are respectively calculated using Eqs. ( 0 0 M M M = and We measured changes in glacier area at the terminus from the glacier outlines of 1.0 means that debris inhibits the ice melting (hereafter termed “insulating e ume of debris-covered glaciers with anthe area total glacier volume of Mount Gongga. Among the five glaciers with length These glaciers represent 68.0Mount % Gongga. and 93.4 The % proportion1.74 of % of the to total debris 53.0 %. glacieras cover The 13.5 number to % total of and the debris-covered the area area totallation total of glacier zone is glacier area below of 3800 area m the a.s.l. variesthe region (Fig. elevation (Fig. 1b), from band 1b). but between Debris the 3800 covers areacovered and most of area, 4800 of m debris-covered and a.s.l. the surface only represents ab- across 7 82 %debris % of of cover the the predominantly debris total occurs cover debris- occurs in above the 4800 tongues m a.s.l. of In large particular, valley glaciers, and the vol- The spatial distribution ofthe ASTER-derived ablation thermal resistance zones ofGongga of are the classified the debris as layer Mount debris-covereddistribution and over Gongga debris-free of surfaces the glaciers based thermal on ison resistance the Mount spatial of shown Gongga, the 50 in debris glaciers Fig. are cover covered (Fig. 2a. with 2a). a The Among varying 74 thickness Mount of glaciers debris (Fig. 2a). 1966 (Pu, 1994) and 2009lated mean (Pan annual et terminus al., advance 2012). or Combined retreat with rates glacier for widths the we period 1966–2009. calcu- 4 Results 4.1 Spatial pattern of debris thermal properties the period 1952–2007. A meltthe ratio ice melting (hereafter< termed “accelerating e simplicity). A melt ratioequals of that 1.0 of indicates bare that ice. the ice melt rate beneath the debris layer M 3.4 Melt ratio and glacier terminusTo changes systematically andmelting comprehensively pattern, assess a theby melt the debris-cover melt ratio rate e (MR), under theas defined assumption of as no the debris in sub-debris the same melt pixel ( rateMR ( tion of the conductive heat flux,of which the snow is layer obtained and/or by glacier calculatingis ice. the The calculated temperature surface numerically temperature profile by for iteration debris-covered surface profile with within simplifying the assumptions debris that layer the isis temperature linear the and same the as conductive that heat at flux the at the ice debris surface surface (Zhang et al., 2011, 2012). changes with snow compaction).Stefan–Boltzmann Upward law, and long-wave downward radiation long-wave radiation isature, is relative obtained calculated humidity from using and air the the temper- ratioof of the downward short-wave atmosphere radiation to usingmidity that an at before the 1988 empirical top is scheme calculated (Fujitaof based on precipitation and the and Ageta, relationship relative between 2000). humiditylatent the Relative established turbulent monthly hu- heat means by fluxes Zhang are etdebris-free obtained al. surface by is (2012). bulk obtained Sensible methods. to and The satisfy all surface heat temperature balance for equations by iterative calcula- monthly means of precipitationation and at transmissivity the and topthe the magnitude of incoming of short-wave the absorption radi- of atmosphereFujita short-wave (2007), (Zhang radiation, by is et which calculated al.,scattering using the the 2012). in albedo method an is Surface of ice calculated albedo, plate using controlling having a thickness simplified related concept to of the multiple surface snow density (which transmissivity estimated from daily precipitation using the relationship between the 5 5 20 25 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 20.1 % ∼ 27 % of the ∼ 40.8 % of the ∼ 0.03 m is widely < 0.09 m. Of the in situ ∼ ects of debris cover in ff , and we found that 1 − 50.3 % have a debris thickness of less 17 % of the ablation zone experiences KW ∼ ∼ 2 0.4 m, and the debris cover is thin in the 1.0, has undergone inhibited melting, and m ∼ < 2 − 2428 2427 10 × 10 km and all of these have debris cover (Fig. 2). Their > 1.0, has experienced accelerated melting, ect of debris cover. About 44 % of the ablation zone on HLG ff ect on ice-melting pattern > ff ects on Mount Gongga glaciers ff 25 % have a thickness of less than 0.03 m. Debris thicknesses and ∼ erent glaciers all reveal the same variation trend of debris thickness, 3600 m a.s.l., from where its thickness increases to more than 1.0 m ff ∼ ect of debris cover in their ablation zones (Table 1). ff According to in situ ablation measurements beneath various debris thicknesses on No systematic direct measurement of debris thickness exists for Mount Gongga Figure 2a shows that relatively high values of thermal resistance are generally found 49 % of the total ablation area has the melt ratio equal to 1.0 (Fig. 2b). Five glaciers 6.4 % debris cover. HLG, XGB and DGBdebris glaciers, melt we rates found and that its debris small thickness change principally results controls in sub- a marked change in the ice melt rate. We observed onlysignificant one accelerating of e theGlacier five experiences glaciers, accelerated melting, HLG and inhibited Glacier, melting which (Table 1). isdebris YZG characterized Glacier, cover by the (Table largest 1),the glacier ablation where in zone the accelerating are region, and almostinsulating has e the insulating 11.7 same. % e The remaining three glaciers show significant where the melt ratiototal ablation is area, where the∼ melt ratio is in the region havedebris-covered lengths proportions of total glacier area vary from 1.74 % to 20.1 % (Table 1). homogeneous spatial distribution; in particular,distributed the on thin debris-covered debris glaciers cover on Mount Gongga. 4.2 Debris-cover e 4.2.1 Debris-cover e The spatial distribution of theis melt depicted ratio in in Fig. the 2b. ablation About zones 10.2 of % Mount of Gongga the glaciers total ablation area of debris-covered glaciers, sistances on HLG Glacierstance (Zhang for et 0.03 al., m debris 2011;total layer Fig. is debris-covered 5). 1.34 area The on calculatedsmaller thermal Mount than resis- it. Gongga As glaciers notedASTER-derived has above, thermal debris a thickness resistances thermal revealed of by resistance debris the value spatial layers distribution (Fig. of 2a) shows a significant in- ground-surveyed debris thicknesses and corresponding ASTER-derived thermal re- bris thickness and thermal resistance, which is established using more than 300 than 0.1 m and ASTER-derived thermal resistances along transverseacross profiles, which representative were areas constructed ofablation HLG zone) Glacier (Zhang (terminus, et al., centraltion 2011), and in indicate upper the space, parts considerably especially inhomogeneous ofvaries at distribu- the from the a few profiles centimeters near to the more terminus than 1.0 where m. glaciers the with debris the thickness exceptionmal of resistance HLG, DGB for and 0.03 m XGB glaciers. debris We layer calculated based the ther- on the relationship between the de- near the glacierGlacier terminus. (Fig. According 3c), to we3100 in m find a.s.l., situ that where the surveys itsaltitude debris of mean range cover of thickness debris 3100–3600 is is m a.s.l., thickness thick wheredebris on in its thickness mean measurements the HLG thickness on is altitude the glacier, range of 2900– debris cover in theand is field, thin i.e. and theground-surveyed patchy debris debris in thicknesses the cover (Fig. upper is(Fig. 3) reaches thick 2a) and of and thermal on the ablation resistances di continuousa zones. of at downglacier Spatial debris the patterns increase, layers termini of tematic but debris considerably thickness spatial measurements variabilityHLG on exists Glacier Mount at and Gongga eachthe these have been site. icefall indicates made at Sys- that only debris on emerges on the glacier surface below of the total glacier∼ area, and HLG Glacier, the longest glacier onat Mount the Gongga, termini has ofablation glaciers zones. and Such lower a values trend occur corresponds mainly well in the with upper the reaches observed of spatial the pattern of in the region (Table 1), NMGG Glacier (Fig. 1) has the largest debris cover, 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 0.63 m − erences in the ff 20 % experience inhibited mass > ect on the trend of greater negative ff 0.54 m) is significantly larger than that ∼ 25 % of the debris-covered glaciers where ¨ ab et al., 2012), where regionally averaged ∼ ¨ a 2430 2429 erence also revealed by the spatial distribution ff ect (Table 1). As shown in Fig. 2a, debris-covered ff ect of debris cover. Overall, although the presence of (Fig. 6), and 90 % of the glaciers were retreating. As ff erent from that of debris-free glaciers. This phenomenon 1 ff − erent from the up-glacier decrease in the ablation gradient ect on glacier terminus changes ect on mass balance ff 20 %, compared to that of the no-debris assumption, 25 % of ff ff > 0.16 m) (Fig. 3), a di ∼ ect on the ice-melting pattern are apparent on these glaciers (Fig. 2b), ff 45.0 and 0 myr − To comprehensively assess the impact of debris cover on the mass balance of the Between 1966 and 2009,between both retreating and stable glacier termini existed with rates supraglacial debris has amass significant balance insulating on e thecovered debris-covered proportions glaciers, especiallythe on the debris-covered glaciers glaciers withloss, on and debris- Mount regionally averaged Gongga massdebris-free have loss glaciers, of experienced which debris-covered is accelerated glaciers contrary is mass to similar expectations. to that4.2.3 of Debris-cover e intensive negative mass balancebalance trend is not on statistically Mount di was Gongga, also and found their onthinning averaged Himalaya rates mass glaciers on (K debris-covered icewidely are assumed similar insulating to those e of debris-free ice despite the whole region, we recalculatedof the no mean debris annual cover on massunder the balance the debris-covered under glaciers. plausibly A the real comparisondicates assumption surface of that the condition mass mass with loss balances the those is debris-covered of accelerated proportion on the is no-debris lesscreasing than assumption debris-covered 20 proportion in- % relative and to thatAlthough mass the glaciers loss no-debris-assumption with is case inhibited (Fig. debris-coveredloss with 5). proportions compared in- to that of the no-debris assumption, debris-covered glaciers show an and elevation) but aintosh, relationship 2012). In does general, exist negativecovered proportion with mass at balance debris the is same cover elevation, intensifiedaspect as (Anderson and with well surface and increasing as slope at debris- Mack- (Fig. the same 4). area, and at the same exists between mass balance sensitivity and topographic characteristics (aspect, slope related to mass balance relativemaritime to glaciers debris in cover. This the phenomenon Southern was also Alps found of on New Zealand, where no clear relationship pect, elevation, debris cover2011; or Anderson a and Mackintosh, combinationwith 2012). of Pan topographic many et factors factors al. (glaciernot (e.g. (2012) find size, discussed strong aspect glacier Scherler correlations changes and et betweenrelationships glacier slope) al., between retreat on mean rates Mount and annualpect topographic Gongga and mass factors. surface and The balances slope did andWe are find illustrated glacier that in elevation, these Fig. area, factorscovered 4 exert as- glaciers. along a with Nevertheless, significant proportions influence Fig. of on 4 the debris indicates mass cover. balance that of these debris- factors are more weakly balance for the periodw.e. As 1998–2007 recent with investigations showedet the al., (Liu mean 2012), et temperature annual al., riseespecially mass 2010; has the Zhang balance contributed et summer to of al., glacier temperaturetal shrinkage 2010, increase. control on 2012; on Climatic Mount Pan glacier conditions Gongga, mass1994), balance are but variation the this (Oerlemans fundamen- control and Fortuin, is 1992; influenced Paterson, by other factors, including glacier size, slope, as- debris-cover e making it completelyon di debris-free glaciers. Suchspatial altered variability spatial in patterns the of regional melting ablation regime. will lead4.2.2 to significant Debris-cover e According to the calculation, most of the Mount Gongga glaciers have a negative mass on HLG Glacier ( of thermal resistance (Fig.posite 2a), trends with in the the consequenceglaciers debris-cover e that on the Mount two Gonggathickness glaciers have and show an widespread op- apparently thin inhomogeneous debris distribution cover. of Consequently, debris spatial di The mean debris thickness on DGB Glacier ( 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) on 1 exhib- − ◦ ect. The . ff 30 1 − > 45 myr ∼ − 8.0 myr − ect the spatial pattern ff had smaller retreat rates ◦ 0.1 m. Suzuki et al. (2007) 20 > < erence of retreat rates between ff ect by surrounding mountains and the pres- , the mean retreat rate of Mount Gongga ff 1 2432 2431 − ect the thermal resistance of the debris layer , and 29 % of the debris-free glaciers retreated ff 1 − erent periods in the HLG catchment (Zhang et al., ff 10.0 m yr > , is greater than that of debris-free glaciers, 25.0 and 0 m yr 1 − − 14.0 myr − and glacier ablation for di ff three debris-covered and fourwas debris-free made glaciers. of In model the parameters calculation, that an are assumption constant in both space and time (Table 2). 5.2 Mass balance modeling The mass balance reconstructionability for to the simulate Mount massa Gongga balance surface glaciers and energy-mass was sub-debris balancemodel based model ice was validated that on melt at accounts the rate theruno for catchment at scale the a by debris-cover comparing large e the2012), scale simulations one using to observed of the glacierized catchments of Mount Gongga (Fig. 1), which contains of thermal resistance. As discussed above,resistance we presents believe a that the reasonable ASTER-derived viewMount thermal of Gongga the glaciers, spatial which distribution possibly of providescovered important debris glaciers insight thickness into on without studying in debris- properties of situ debris measurements layer at of a the large scale. extent, thickness and thermal after the image acquisition datefrom is of ASTER vital data. importance In for thister calculating study, season thermal image of resistance acquisition this was region,of near where May–October the 75–90 % and middle of of the the theSu, mean annual dry 1996; annual precipitation win- falls air Zhang in temperature et theat is al., months AWS high 2010). and (Su An at et analysis thewere al., of precipitation 1992; characterized the gauge Li meteorological by (Fig. and conditions highprecipitation. 1) observed The temperatures, shows debris that cloud-free surface the skies, was image therefore and acquisition considered a days to prolonged be dry lack in of the calculation rological data, and suggested that thisof uncertainty the is unlikely thermal to a resistanceby of surrounding the mountains debris layer. may(Suzuki Although not et the al., a shading 2007), ofZhang et the solar al., insolation presence 2012). of Hence, knowledge water of in the the meteorological conditions debris at, does before and (Suzuki et al., 2007; turbulent heat fluxes in the surface energy-balance calculation using observed meteo- Glacier of this region,evaluated the especially uncertainty at in the the debris approach layer described of above caused by neglecting the using ASTER images comeenergy-balance calculation, from the shading neglecting e theence of turbulent water. The heat approach used fluxes inis in this usually study the the is dominant based surface on heatthe the source fact turbulent that on the heat debris-covered net glaciers radiation fluxesand and to Gardner, the 1991; contribution the Kayastha of total et2006; al., energy Suzuki 2000; balance et Takeuchi et is al., al.,the 2007), normally 2000; same which Nicholson small characteristic can and (e.g. of be Benn, Mattson neglected. the Zhang energy et balance al. at (2011; the Fig. debris-covered 8) surface found on HLG 5 Discussion 5.1 Uncertainties in thermal resistances The main sources of uncertainty in calculating the thermal resistance of the debris layer ited larger retreat rates,(Fig. and 6). those In withtreat mean contrast, rates slopes debris-covered when the glaciers debris-coveredslopes proportion with had is gentle relatively relatively large, slopes smaller andatively retreat those exhibited small with rates larger (Fig. steep when re- 6).debris-free the We and debris-covered proportion observed debris-covered glaciers, is aglaciers, i.e. rel- significant the di mean retreat rate of debris-covered with rates between significantly at retreatglaciers rates (hereafter termed “significantshowed retreat”), significant while 51 retreat %Mount with of Gongga. debris-covered some We glaciers observed of that the debris-free highest glaciers with retreat mean rates slopes ( illustrated in Fig. 6, no uniform change occurred in the termini of debris-free glaciers 5 5 25 15 20 10 20 25 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ). This erence 1 ff − 8.0 myr − ect on the average status of ff 10.2 % of the total ablation zone ∼ ect erent periods (Fig. 8a). The overall ff ff 25 % of the debris-covered glaciers. at a large scale, they only accounted s and supraglacial ponds in the abla- ∼ ff ff 2434 2433 erences in the debris-cover e ff 0.001). This finding is in agreement with previous studies (Rana ) is greater than that of debris-free glaciers ( 1 erences in the proportion of debris cover are apparent from region − erences in the debris-cover e ff p < ff ect of debris cover in other regions is to reduce mass loss from the cient between thermal resistances in 2008 and those in 2009 is 0.72 ff ected by the ice movement and deposition of supraglacial debris due ffi ff erent from many eastern and central Himalayan debris-covered glaciers ff 14.0 myr − ect of thick debris cover using a multiplicative reduction factor (e.g. Anderson ff In addition, we observed that the mean terminus retreat rate of debris-covered finding is di homogeneous distribution accelerate the meltingand on produce a more negativeFurthermore, mass a balance widely on distributed surfaceco-existing on debris-covered the Mount ice, Gongga bare glaciers,tion ice, composed zone, ice of makes cli disproportionately large2002). contributions Consequently, regionally to averaged ablation mass (Sakai lossilar et of to al., debris-covered that glaciers 2000, of is debris-free sim- glaciers on Mount Gongga,glaciers which is ( contrary to expectations. 2011; Mayer et al., 2011) (Tablesidered 3). debris In thickness contrast, and our its investigationthermal spatial comprehensively con- characteristics resistance revealed of by the thefound ASTER-derived debris apparently spatial layer di onthe the Mount glaciers, Gongga with maritime the glaciers. consequence In that particular, we the presence of debris and its in- to region (Table 3). In situness measurements on of debris-covered glaciers debris of thicknesson other indicate regions that Mount (Table debris 3) Gongga thick- is glaciers. generallypact Although thicker of than previous debris that investigations cover have onfor addressed the glacier e the status and im- runo and Mackintosh, 2012; Immerzeellated et from the al., thickness 2012) measurements onthickness/elevation or a distributions regional few on glaciers debris with the an thickness neighbouring assumption extrapo- of glaciers similar (e.g. Lambrecht et al., As earlier investigations showed (Suet et al., al., 2011; 1985; Mayer Zhang et al., etthe 2011; al., significant Scherler 2006, et e 2007; al., Lambrecht 2011;glacier, Anderson although and di Mackintosh, 2012), 5.3 Regional di annual terminus retreat ratesance over calculated the by period the 1966–2009 surface energy-massretreat indicates balance of that model the captured the the mass glaciers trenda bal- of (Fig. reasonable rapid 8b). view Asglaciers, of noted which above, the possibly we provide general a believedebris useful condition that cover context and the of for its discussing model spatial the the distribution presents at potential mass on a impacts the of large balance average status scale. of of debris-covered Mount glaciers Gongga rates beneath various debris thicknessesaim during of di this studyspatial was variations for not any to given yearthe simulate but spatial as rather distribution to accurately of systematically debris as evaluateA cover the possible on comparison impacts the the of of average mass status mean of balance annual Mount or Gongga mass glaciers. balances over the period 1998–2007 and mean exists in thermal resistancescorrelation derived coe from the(significance independent level data (Fig. 7).et The al., overall 1997; Nakawothermal and resistance Rana, of 1999; the Suzukicovered debris et glaciers. layer al., can Despite 2007), usually which thisconfirmed be suggested that uncertainty, regarded that the the as model the constant model reconstructed(Zhang on well results et debris- the al., for long-term 2012). the mass In balance HLG particular, (1952–2009) catchment the model was able to reliably estimate the ice melt glaciers were considered aspixel may constant, be although a theto the thermal high resistance ice velocity atet of Mount a al., Gongga 2010). specific glaciers (Song, Comparisonin 1994; of Li 2009 and ASTER-derived at Su, thermal 1996; the Zhang resistances same in pixels 2008 on with Mount those Gongga glaciers indicates that little di Also, thermal resistances of debris layers in the ablation zones of the Mount Gongga 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , ◦ at ff 20 %, it > promote the erent periods ◦ ff 8 < ects using a surface energy-mass ff 10.2 % of the total ablation area and ∼ (Zhang et al., 2010). In contrast, most Hi- 2436 2435 1 − erences from region to region, with the consequence ff 0.03 m are widely distributed on the glaciers. Against the < ect on the trend of greater negative mass balance on the debris- ff ers an opportunity to study a monsoonal maritime glacier system ff ¨ ohl, 2008), lowering the glacier surface (Sakai and Fujita, 2010) and back- 25 % of the debris-covered glaciers on Mount Gongga. The consequence is ∼ Debris-covered glaciers are common in the Tibetan Plateau and surroundings, as These approaches possibly provide important insight into studying the average status from 2 % torates 36 and % spatial (Table patterns 3),shows of the apparent mass impact systematic loss is di ofthat considerably the the significant inclusion debris in of each covera debris region large on and cover scale both in isbased the estimates of techniques ice-melting of vital for glacier importance. mapping mass Despitesatellite the imagery numerous balance distribution and simplifications, and estimating of our runo thebalance supraglacial physically- debris-cover approach debris e can thickness systematically using influence assess on the spatial significance of patterns debris of cover ice and melting its and mass balance at a regional scale. the terminus retreat rates are faster than those of thewell debris-free as glaciers. in many otherZhang mountain et ranges al., aroundAlthough the 2007, world the 2012; (Nicholson percentages Anderson and of Benn, and debris 2006; cover Mackintosh, of 2012; the Brenning studied et debris-covered glaciers al., vary 2012). accelerates the trend ofproduces faster a ice more melting negativerise, on on mass balance, whichthat is regionally averaged primarily mass balance caused offerent debris-covered by from glaciers that temperature is of not debris-free statisticallymass glaciers dif- balance with trend all on glaciers Mountcaused Gongga. exhibiting by Also, an widespread the intensive debris intensely negative cover inhomogeneous insteep ice association surface melting with leads high to ice active velocities terminus and regions relatively of the debris-covered glaciers, of which ity at each site. High-resolutionthin in debris situ thicknesses measurements of ofbackground debris of global thickness warming, indicate although that nificant the presence insulating of e supraglacial debriscovered has glaciers, a especially sig- on the glaciers with debris-covered proportions downglacier increasing trend in debris thickness with significant spatial inhomogene- of supraglacial debrisof in the the total ablation glacier area zones, vary where from the 1.74 % debris-covered to proportions 53.0 %. These glaciers show a general integration and retreat. 6 Conclusions Mount Gongga o with debris-covered andwhere debris-free specific, though glaciers incomplete, in informationmeteorology. is Sixty-eight the available percent for south-eastern both of the Tibetan Mount glaciology Plateau, and Gongga glaciers have extensive mantles wasting, a related topographicblages inversion and processes, numerous can supraglacial yield pondsConsequently, complex high (Benn ice debris and velocities, assem- relatively Evans, steep 2010; surfacemogeneous slopes Benn ice and et the melting intensely al., caused inho- 2012). byof widespread the debris debris-covered cover glaciers lead on to Mount the Gongga, unstable which fronts accelerates their terminus dis- malaya glaciers have low ice velocitieset and al., shallow 2012). termini Note (Scherler that etdevelopment mean al., of surface 2011; slopes Benn stagnant in ice theminus terminus (Scherler regions region et of of most al.,which debris-covered 2011). glaciers facilitate Mean frequent on surface Mount rock fallspointed slopes Gongga and are out in snow greater that the avalanches than the ter- (Liglaciers 8 widespread and can presence Su, of accelerate 1996). supraglacialsurface It glacier debris (R must terminus and be disintegration ponds by on these exposing ice faces at the observations of ice velocities on(Song, HLG, 1994; DGB Li and and XGBare Su, glaciers 1996; largely during Zhang di higher et thanand al., 2011), Su, those the 1996; of ice Shi glaciersgenerally velocities and of in vary Liu, these other from 2000). glaciers regions 28.8 Ice of velocities to in the 205.0 myr the Tibetan ablation Plateau zone (Li of HLG Glacier with stagnant low-gradient terminus regions (Scherler et al., 2011). 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R.: Spatially variable response of Hi- Sakai, A., Nakawo, M., and Fujita, K.: Distribution characteristics and energy balance of Ice Shi, Y. and Liu, S.: Estimation on the response of glaciers in China to the global warming in the Sakai, A. and Fujita, K.: Formation conditions of supraglacialSakai, lakes A., on Takeuchi, N., debris-covered Fujita, glaciers K., and Nakawo, M.: Role of supraglacial ponds in the ablation R 5 5 20 15 10 30 25 20 10 15 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ects. ff ect (%) ff (%) Accelerating Insulating Paterson (1994) Zhang et al. (2012) Cao and Cheng (1994) Zhang et al. (2012) 1 − 1 3 3 − − − 10 km and their debris-cover e C Liu et al. (2009) ◦ ) Debris coverage Debris-cover e > ◦ C km ◦ 2444 2443 ) Length (km) Slope ( 2 Characteristics of five glaciers with length A summary of the values of parameters used to calculate mass balance. Albedo of debris-free iceAlbedo of fresh snowDensity of iceDensity of snow 0.3 0.8 – – 900 415 Zhang et al. (2011) kg m kg m Li and Su (1996) ParameterTemperature lapse rate 4.3 Value Unit Source Precipitation gradientSnow/rain temperature threshold 2.0 18 % (100 m) Glacier Elevation (m a.s.l.) AreaHLG (km DGBMZGYZGNMGG 2990–7556 3660–6684 3600–6886 3680–7556 3460–6540 25.7 21.2 26.8 32.2 16.7 13.1 11.0 11.6 16.0 11.7 10.0 7.3 22.2 13.4 10.5 6.4 16.8 1.74 11.7 20.1 44.0 3.0 2.0 41.0 17.0 17.0 56.0 11.0 50.1 35.6 Table 2. Glacier area, lengthtained and from elevation are theglacier derived DEM. area. from “Debris the coverage” CGI denotes (Pu, the 1994). debris-covered Mean proportion slope of is the ob- total Table 1. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | area- (b) Anderson andtosh (2012) Mackin- Lambrecht et al. (2011) Mayer et al. (2011) Su et al. (1985) Zhang et al. (2006) Zhang et al. (2007) Scherler et al. (2011) This work b b Consider thick debris with a MRF observed thicknesses on few glaciers observed thickness on few glaciers 0 –2.5 m Some large rocks piled up to several meters bris 0–1.4 m Consider spatial distribu- tion of debris thickness ect. MRF is multiplicative reduction factor. ff of a 2446 2445 ect Debris thickness Reference ect in ff ff ect 0–3 m ect Extrapolation from ect Extrapoalation from ect, and ectect Consider the extent of de- Use a MRF (0.15/0.7) Immerzeel et al. (2012) ff ff ff ff ff ff ablation area accelerating in 2 % 10.2 % ofand ablation insulating in area, 40.8 % erences in the debris-cover e ff Map of Mount Gongga glaciers in the south-eastern Tibetan Plateau, and Regional di It is calculated from observedRegional data debris of thicknesses Su are et extrapolated al. from (1985). observed thicknesses on few glaciers with an assumption of similar thickness/elevation distribution on the South Alps, New Zealand 8.0 Insulating e RegionCaucasus Debris (%) Debris-cover e 8.1–23.0 Insulating e Altay 3.7–25.8 Insulating e Tuomur, Tien Shan 7.5–22.0Himalaya and Karakorum Insulating 2.0–36.0 e Insulating e Mount Gongga 13.5 Accelerating e Langtang, Himalaya 19.0 Insulating e a b neighbouring glaciers. Fig. 1. (a) altitude distributions of total glacier (blue),The ablation green area square, (red) star, and and debris-covered cross surfaceThe (grey). denote GAEORS, yellow AWS outline and is precipitation gauge, the respectively. Hailuogou (HLG) catchment. Table 3. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and the (a) glaciers. See Fig. 1 (c) and Hailuogou (HLG) (b) 2448 2447 , Xiaogongba (XGB) (a) in the ablation zones of Mount Gongga glaciers. (b) Comparison of the ground-surveyed debris thicknesses (point) derived from Li and Su Spatial distributions of ASTER-derived thermal resistance of the debris layer Fig. 3. (1996) and Zhang et al.,on (2011) Dagongba and (DGB) ASTER-derived thermal resistances of debris layers (line) melt ratio for glacier locations. Fig. 2. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ◦ , 180 ◦ , 90 ◦ , respectively. Marker- (d) and surface slope (c) 2450 2449 , aspect (b) , area (a) represent the north, east, south and west, respectively. (c) in ◦ Comparison of mean mass balance of debris-covered glaciers calculated with the real Scatter plots of mean mass balance of debris-covered glaciers for the period 1998–2007 Fig. 5. surface condition anddenote that the calculated debris-covered proportion with of total the glacier no-debris area. assumption. Marker-symbol colors Fig. 4. versus glacier elevation symbol colors denote the debris-covered proportion of total glacier area; 0 (360) and 270 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | cient and ffi denote the correlation coe p and r 2452 2451 Comparison of the 2008 and 2009 ASTER-derived thermal resistances at the same Scatter plot of mean annual terminus retreat rates versus mean surface slopes of the Fig. 7. pixels on Mount Gonggasignificance level, glaciers. respectively. The letters Fig. 6. ablation zones of debrisdenote the covered debris-covered and proportion of debris-free total (grey glacier dots) area. glaciers. Marker-symbol colors Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | and mean (a) erent periods ff 2453 . Error bars indicate the standard deviation. (b) Comparisons of observed and modelled ice melt rate in di Fig. 8. annual mass balances over the periodthe 1998–2007 period and 1966–2009 mean annual terminus retreat rates over