remote sensing

Article Spatial-Temporal Characteristics of Glacier Velocity in the Central Karakoram Revealed with 1999–2003 Landsat-7 ETM+ Pan Images

Yongling Sun 1,2, Liming Jiang 1,2,*, Lin Liu 3, Yafei Sun 1,2 and Hansheng Wang 1 1 State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, 340 XuDong Rd, Wuhan 430077, China; [email protected] (Y.S.); [email protected] (Y.S.); [email protected] (H.W.) 2 University of Chinese Academy of Sciences, 19A Yuquan Rd., Beijing 100049, China 3 MOE Key Laboratory of Fundamental Physical Quantities Measurement, Hubei Key Laboratory of Gravitation and Quantum Physics, Institute of Geophysics, School of Physics, Huazhong University of Science and Technology, 1037 LuoYu Rd., Wuhan 430074, China; [email protected] * Correspondence: [email protected]; Tel.: +86-027-8677-8612

Received: 26 July 2017; Accepted: 15 October 2017; Published: 19 October 2017

Abstract: The situation of stable and slightly advancing glaciers in the Karakoram is called the “Karakoram anomaly”. Glacier surface velocity is one of the key parameters of glacier dynamics and mass balance, however, the response of glacier motion to this regional anomaly is not fully understood. Here, we characterize the spatial-temporal variations in glacier velocity over the Central Karakoram from 1999–2003. The inter-annual glacier velocity fields were retrieved using a cross-correlation-based algorithm applied to four Landsat-7 Enhanced Thematic Mapper Plus (ETM+) panchromatic image pairs. We find that most of the glaciers on the southern slope flowed faster than those on the northern slope, which might be attributed to the differences in glacier sizes. Furthermore, ice motion observations over four years reveal that most of the glaciers were quasi-stable or experienced small fluctuations of flow velocity during our study period. We identify a new surging event for the South Skamri Glacier in the study period by investigating the glacier frontal changes and the longer-term time series of surface velocities between 1996 and 2006. From the transverse velocity profiles of seven typical glaciers, we infer that basal sliding is the predominant motion mechanism of the middle and upper glaciers, whereas internal deformation dominates closest to the glacier terminus.

Keywords: Central Karakoram; glacier velocity; spatial-temporal variations; local morphometric effects; motion mechanism; Landsat-7 ETM+ panchromatic image

1. Introduction Field observations and geodetic measurements suggest that glaciers in the Karakoram Range are either stable or have been expanding since 1990 and present positive or less negative mass changes [1–8]. This situation is called the “Karakoram anomaly” [7]. As one of the most glaciated regions outside of the polar regions [9], the Central Karakoram has experienced a slight gain in glacier mass at the beginning of the 21st century [4,10]. The snow and glacial melt in this region is an important source of water for the Indus River in Pakistan [11,12] and the Yarkant River in China [13]. Glacier dynamics in the Central Karakoram have a significant influence on the hydrological cycle, especially hydrological planning and water resource management. As one of the crucial factors in mass balance and glacier dynamics [14], the glacier surface velocity provides an indicator of glacier recession or advance/surge [15–17] and has been used to interpret glacier dynamics [18]. Therefore, the spatial-temporal characteristics of the glacier velocity in this region are essential to improve our understanding of glacier dynamics and the glacier responses to climate change and influences on regional water sources.

Remote Sens. 2017, 9, 1064; doi:10.3390/rs9101064 www.mdpi.com/journal/remotesensing Remote Sens. 2017, 9, 1064 2 of 20

Several studies have investigated the temporal variations of glacier velocity in the Central Karakoram and most of them focused on individual glaciers. Quincey et al. [19] showed that the velocity of the Baltoro Glacier increased from 1992–2005 and then decreased from 2005–2008 as derived from SAR images of ERS and ENVISAT ASAR; however, they did not provide the velocity results from 1999–2003. Mayer et al. [15] discovered that the North Gasherbrum Glacier advanced from 2003–2007 through an analysis of the annual average glacier velocities obtained from Landsat images. Scherler et al. [20] presented surface velocity results of the Biafo Glacier from ASTER images acquired from 2000–2009 and reported that the highest velocity occurred in 2005. Paul [16] used animated sequences of orthorectified satellite images, which were acquired from 1990–2015, to demonstrate glacial dynamics in the Central Karakoram. This provided an insight into the dynamic glacier behaviour on a different way (although this may be a less quantitative approach). Recently, Heid et al. [21] used two pairs of Landsat Enhanced Thematic Mapper Plus (ETM+) images (2000/2001) and Landsat Thematic Mapper (TM) images (2009/2010) to map the ice flow velocities of the Central Karakoram and found a slight increase in glacier velocity between the two epochs of observation. Dehecq et al. [22] derived the glacier velocity field for the year 2000 over the entire Karakoram by employing all of the available archived Landsat 5 and 7 images to improve the percentage of successful measurements and reduce the uncertainty of the velocity field. Overall, inter-annual variations of glacier velocity in the Central Karakoram are still limited, especially at the beginning of the 21st century. The extremely rugged topography of the Central Karakoram, which is home to more than sixty peaks that are more than 7000 m above sea level (a.s.l.), has the complex effects of the southern and northern slopes on glacier dynamics [14]. Copland et al. [23] noted that there were insignificant differences in the ice velocities of glaciers on the southern and northern slopes in the period from 2006–2007. The velocity maps in Heid et al. [21], Rankl et al. [1] and Dehecq et al. [22] showed higher velocities in glaciers on the southern slope; however, they did not quantitatively estimate the velocity difference between the southern and northern glaciers. A comparison of the velocity results reported in Copland et al. [23], Mayer et al. [15], and Quincey et al. [19] indicate that the maximum ice rates of the Skamri Glacier (250 m/year) and the North Gasherbrum Glacier (401 m/year) located on the northern slope were higher than that of the Baltoro Glacier (190 m/year) on the southern slope in the same period of 2006–2007. Consequently, the velocity variations between glaciers on the southern and northern slopes and their potential reasons are not yet fully understood and further studies are required. The main aim of this study is to characterize the spatial-temporal variability of glacier surface velocity over the Central Karakoram in the period from 1999–2003. The inter-annual glacier velocity results are achieved by using a cross-correlation algorithm with four pairs of Landsat-7 Enhanced Thematic Mapper Plus panchromatic images (Table1). These results are put into a broader context of glacial dynamics in the Central Karakoram and thus enable a narrowing of the knowledge gaps with regard to glacier velocity variations in this region at the beginning of the 21st century. We also aim to analyse the ice flow velocity of major glaciers on the southern and northern slopes as a function of multiple topographic factors (size, median elevation, slope and aspect), and further estimate the influence of debris cover. Finally, the ice motion mechanisms of seven typical glaciers are discussed based on the velocity distribution patterns along the glacier transverse profiles.

2. Study Area The study area (from 35◦N to 36.5◦N and from 75.5◦E to 77.5◦E) is situated in the Central Karakoram and covers a glaciated region of more than 5800 km2, which is estimated based on the glacier area from the Randolph Glacier Inventory (RGI v5.0) [24]. The Central Karakoram is an area with significant topography and extreme relief that is home to more than sixty peaks that are higher than 7000 m a.s.l. [25]. Four of these peaks exceed 8000 m a.s.l. including K2 (the second highest peak in the world at 8611 m a.s.l.), Gasherbrum I (8068 m a.s.l.), Broad Peak (8047 m a.s.l.), and Gasherbrum II (8035 m a.s.l.). This region is also one of the most heavily glaciated parts of the world outside of Remote Sens. 2017, 9, 1064 3 of 20

than 7000 m a.s.l. [25]. Four of these peaks exceed 8000 m a.s.l. including K2 (the second highest peak

Remotein the Sens. world2017, 9at, 1064 8611 m a.s.l.), Gasherbrum I (8068 m a.s.l.), Broad Peak (8047 m a.s.l.),3 ofand 20 Gasherbrum II (8035 m a.s.l.). This region is also one of the most heavily glaciated parts of the world outside of the polar regions and contains more than 30 glaciers over 20 km in length, some of which theare polarthe longest regions mountain and contains glaciers more on Earth than 30outside glaciers of the over Polar 20 kmRegions, in length, e.g., Siachen some of Glacier which (76 are km) the longestand the mountainBiafo Glacier glaciers (68 km) on Earth rank outsideas the longes of thet Polar and Regions,second longest e.g., Siachen glaciers Glacier in the (76 Karakoram km) and the[26,27]. Biafo Generally, Glacier (68 the km) glaciers rank asof thethe longestCentral andKarakoram second longestare divided glaciers into in two the groups Karakoram by the [26 main,27]. Generally,ridge (see theFigure glaciers 1). One of the group Central is located Karakoram on the are southern divided intoslope two and groups includ byes the the main Biafo, ridge Baltoro, (see FigureSiachen,1). Kaberi, One group and is Panmah located onglaciers. the southern The other slope gr andoup includeson the northern the Biafo, slope Baltoro, mainly Siachen, contains Kaberi, the andBraldu, Panmah Skamri, glaciers. Sarpo Laggo, The other North group Gasherbrum, on the northern Urbak, slopeSingkhu, mainly and containsKyagar glaciers. the Braldu, The name Skamri, of Sarpoeach glacier Laggo, is North based Gasherbrum, on RGI v5.0 [24] Urbak, and Singkhu, partly on and Paul Kyagar [16]. As glaciers. the hillslope-erosion The name of each rates glacier usually is basedincrease on with RGI v5.0hillslope [24] andangle, partly high on and Paul deeply [16]. Asincised the hillslope-erosion mountain relief ratesresults usually in thick increase debris with that hillslopecovers most angle, of the high glaciers and deeply in this incised region mountain [28]. relief results in thick debris that covers most of the glaciersThe in regional this region climate [28]. of the Central Karakoram is influenced by the varying dominance of westerliesThe regional and the climate Indian of monsoon the Central [5,25] Karakoram and has is influenceda considerable by the microclimatic varying dominance variability of westerliesinfluenced and by thethe Indianhigh mountain monsoon terrain [5,25] and [26,29]. has a In considerable the northwest, microclimatic two-thirds variability of the high-altitude influenced bysnowfall the high in mountainthe Central terrain Karakoram [26,29]. Inin the winter northwest, is contributed two-thirds by of westerlies. the high-altitude However, snowfall in the in thesoutheast, Central 80% Karakoram of the summer in winter precipitation is contributed is byprovided westerlies. by the However, Indian mo in thensoon southeast, [1,5,25]. 80% Since of thethe summer1990s, relative precipitation stability is or provided even an by advance the Indian for monsoonmany glaciers [1,5,25 has]. Since been the observed 1990s, relativeacross the stability Central or evenKarakoram an advance [1,2,4,7], for manywhich glaciers may have has beenbeen observedinduced acrossby strengthening the Central Karakoramwesterlies [2,30]. [1,2,4, 7Recently,], which mayseveral have studies been focused induced on by the strengthening velocities of westerlies the glacier [2 in,30 the]. Recently, Central Karakoram several studies and focusedreported on that the a velocitiesvelocity change of the glacierwas associated in the Central with Karakoramclimate change, and reportede.g., Quincey that aet velocity al. [19] changeand Scherler was associated et al. [20] withall reported climate change,that the e.g., peak Quincey velocities et al. in [ 192005] and of ScherlerBaltoro etGlacier al. [20 and] all reportedBiafo Glacier that theare peak related velocities to the inprecipitation. 2005 of Baltoro Glacier and Biafo Glacier are related to the precipitation.

FigureFigure 1.1. AnAn overviewoverview ofof thethe studystudy area.area. TheThe backgroundbackground isis thethe LandsatLandsat ETM+ETM+ imageimage acquiredacquired onon 2121 JulyJuly 20012001.. TheThe blueblue lineline marksmarks thethe YarkantYarkant River,River, andand thethe blueblue dasheddashed lineline marksmarks thethe boundaryboundary betweenbetween glaciersglaciers onon thethe southernsouthern andand northernnorthern slopes.slopes. The green trianglestriangles representrepresent fourfour peakspeaks (exceeding(exceeding 80008000 mm a.s.l.)a.s.l.) from north toto south:south: K2 K2,, Broad Peak, Gasherbrum II and Gasherbrum I. TheThe yellow points mark mark the the glaciers glaciers mentioned mentioned in in context. context. Abbreviations: Abbreviations: Br = BrBraldu = Braldu Glacier, Glacier, Sk = SkSkamri = Skamri Glacier, Glacier SL ,=SL Sarpo = Sarpo Laggo Laggo Glacier, Glacier, NG NG= No =rth North Gasherbrum Gasherbrum Glacier, Glacier, Ur Ur= Urbak = Urbak Glacier, Glacier, Sh Sh= Singkhu = Singkhu Glacier, Glacier, Ky Ky = =Kyagar Kyagar Glacier, Glacier, Si Si = =Siachen Siachen Glacier, Glacier, Ka Ka = = Kaberi Kaberi Glacier, Bt == BaltoroBaltoro Glacier,Glacier, PaPa == PanmahPanmah Glacier,Glacier, BiBi == BiafoBiafo Glacier.Glacier.

Remote Sens. 2017, 9, 1064 4 of 20

3. Data and Methods

3.1. Landsat-7 ETM+ Datasets In this study, we utilized Landsat-7 ETM+ images acquired from 1999–2003 to estimate the glacier surface velocity over the Central Karakoram. The Enhanced Thematic Mapper Plus is one of the main instruments on board the Landsat-7 satellite, which was successfully launched on 15 April 1999. The ETM+ image has a relatively short revisit of 16 days and a large coverage scene (185 km × 170 km). Additionally, the Landsat-7 ETM+ sensor offers several enhancements over the Landsat-4, 5 TM sensor including improved geometric accuracy, reduced noise, reliable calibration, the addition of a panchromatic band (15 m), and increased spectral information content. The ETM+ images are a potential source of remotely sensed data for monitoring mountain glacier dynamics. Table1 lists the Landsat-7 ETM+ images over the study area acquired between 1999 and 2003. The four pairs of panchromatic images used in this study were in the same path/row (148/35) to minimize geometric distortions. Additionally, the time interval of the four image pairs was almost one year to reduce the seasonal effects and the influence of the sun altitude. Thus, the similar glacier surface condition between the two acquisitions could improve the accuracy of the glacier velocity estimation. We tried to select the images acquired in summer with the lowest percentage cloud cover. However, the ETM+ scan line corrector failed in May 2003 [31], and resulted in no suitable images available in the summer for 2003. Therefore, the image pair used for 2002/2003 was acquired in spring. The Landsat ETM+ images were downloaded from the United States Geological Survey (USGS) EarthExplorer [32].

Table 1. Landsat ETM+ image pairs used in this study.

Sensor Date Image t1 Date Image t2 Path/Row Landsat ETM+ 16 July 1999 16 June 2000 148/35 Landsat ETM+ 16 June 2000 21 July 2001 148/35 Landsat ETM+ 21 July 2001 9 August 2002 148/35 Landsat ETM+ 19 April 2002 21 March 2003 148/35

3.2. Methodology and Data Processing The glacier surface displacement measurements were calculated using the cross-correlation algorithm in the frequency domain proposed by Leprince et al. [33] which has already shown good results for similar glacier applications [20,34,35]. This method relies on the identification of surface features across two time-separated satellite scenes. It consists of three key processing steps: (i) orthorectification; (ii) co-registration; and (iii) correlation. The Landsat image was provided at Level 1T (L1T), which was already orthorectified by USGS; consequently, we did not conduct the orthorectification for our data processing [35,36]. The images were co-registered in ENVI software, and the horizontal displacements were calculated with the COSI-Corr software package [37]. The principle of the cross-correlation algorithm in the frequency domain can be found in Leprince et al. [33]. In this paper, horizontal displacements along the East/West (E/W) and North/South (N/S) directions were derived from this cross-correlation algorithm using a 64 × 64 (pixels) window as the initial search window and a 32 × 32 (pixels) window as the final window. The optimal window size was obtained after extensive testing. Patches with the largest window size were correlated first, and the correlation process was conducted iteratively until the minimum window size was reached or until the correlation failed. Moreover, to avoid the occurrence of a correlation bias, a frequency mask was applied to only select parts of the cross-spectrum where the phase information was valid. The mask parameter was set to 0.9 and five robustness iterations were applied. Finally, the results with a ground resolution of 30 m (a sliding step of two pixels for the correlation window) obtained by the correlation process included 2D horizontal glacier motion fields along the E/W and N/S directions, and the Signal-to-Noise Ratio (SNR) images corresponding to the measurements. A Non-Local Means Remote Sens. 2017, 9, 1064 5 of 20

Filter was applied to reduce noise, which is implemented in the COSI-Corr software package. In this study, the noise parameter of the filter was set to 1.6 times of the estimated standard deviation, the search area set to 21 × 21 pixels, and the patch size set to 5 × 5 pixels. The 2D glacier velocities were derived by dividing the horizontal ground displacements along the E/W and N/S directions by the time interval between the two images.

3.3. Accuracy Estimation The main error sources of the cross-correlation algorithm included (1) the image quality (i.e., snow cover, cloud cover and melting glaciers); and (2) the image registration errors. Due to a lack of in situ measurements of the glacier velocity, it was difficult to directly assess the results of the cross-correlation algorithm. Considering the stable properties of off-glacier areas that should not be displaced, the displacements of the off-glacier area have been widely used to evaluate the cross-correlation performance. Here, we used a method similar to the one adopted by the cited references [38,39]. By assuming that the off-glacier area was stable (namely, no displacement), the error of the displacements in the off-glacier area is:

p 2 2 eo f f = SE + MED (1)

eo f f is the error of displacement, MED is the mean displacement, and SE is the standard error of the mean displacement, as the computation formula below:

STDV SE = q (2) Ne f f

STDV is the standard deviation of the mean displacement in the off-glacier area; Ne f f represents the number of independent measurements and its computation formula is:

N × PS N = total (3) e f f 2D where PS represents the pixel size and D is the distance of spatial autocorrelation. According to experience, D is approximately 20 times as much as PS [38].

4. Results

4.1. Spatial Pattern of the Glacier Velocity Figure2 illustrates the average results of the glacier velocity fields over the Central Karakoram derived from four annual pairs (Table1). For better visualization, the results were masked by the glacier inventory of RGI 5.0 [24] and were overlaid on the hillshade image of SRTM DEM [32]. The average annual velocities show the main characteristics of glacier flow dynamics. In general, the large-sized glaciers flowed faster than the small glaciers, and there was a general increase in velocity from the glacier terminus to the upper area. The maximum velocity reached approximately 200 m/year at the Biafo glacier and the South Skamri Glacier from 1999–2003. The uncertainty of the velocity results based on the off-glacier statistics was approximately ±7 m/year in the four epochs of observation, which is less than one-half a pixel. This is comparable with those errors reported in the scientific literature [19,20] and demonstrates the reliability of the results in this study. Remote Sens. 2017, 9, 1064 6 of 20 Remote Sens. 2017, 9, 1064 6 of 20

Figure 2.2.Average Average annual annual velocity velocity field field across across the Central the KarakoramCentral Karakoram from 1999–2003. from 1999–2003. The background The backgroundis the hillshade is the image hillshade of SRTM image DEM. of SRTM The blue DEM. dashed The blue line marksdashed the line boundary marks the between boundary the betweensouthern the and southern northern and slopes’ northern glaciers. slopes’ Abbreviations: glaciers. Abbreviations: Br = Braldu Br = Glacier, Braldu SkGlacier, = Skamri Sk = Glacier,Skamri Glacier,SL = Sarpo SL Laggo= Sarpo Glacier, Laggo NG Glacier, = North NG Gasherbrum = North Gasherbrum Glacier, Ur = UrbakGlacier, Glacier, Ur = ShUrbak = Singkhu Glacier, Glacier, Sh = SingkhuKy = Kyagar Glacier, Glacier, Ky =Si Kyagar = Siachen Glacie Glacier,r, Si = Siachen Ka = Kaberi Glacier, Glacier, Ka = Kabe Bt =ri Baltoro Glacier, Glacier, Bt = Baltoro Pa = PanmahGlacier, PaGlacier, = Panmah Bi = Biafo Glacier, Glacier. Bi = Biafo Glacier.

For most of the studied glaciers, the results of ice flow flow velocity in the ablation areas are more reliable thanthan those those in in the the accumulation accumulation areas areas where where the trackable the trackable features features are lacking are and lacking the variability and the variabilityof the snow of extent the snow is high extent (as shownis high in (as Figure shown1). in To Figure detect the1). To spatial detect variations the spatial in icevariations flow velocity in ice flowbetween velocity glaciers between on the southernglaciers andon the northern southern slopes, and we northern analysed theslopes, annual we averageanalysed velocities the annual from average1999–2003 velocities along the from glacier 1999–2003 centerlines along over the ablation glacier areascenterlines of twelve over major ablation glaciers areas with of antwelve area major larger glaciersthan 80 kmwith2 (Tablean area2). larger As the than glaciers 80 km are2 (Table different 2). As in size,the glaciers we calculated are different the average in size, velocities we calculated that thelimited average to the velocities lower 30% that of thelimited glacier to the length lower along 30% the of centerline the glacier for length each glacier, along inthe which centerline there arefor eachrelatively glacier, reliable in which velocity there data are forrelatively most glaciers. reliable Tablevelocity2 suggests data for that most the glaciers. average Table velocities 2 suggests of the thatsouthern-slope the average glaciers velocities were of much the southern-slope higher than those glaciers of the were northern-slope much higher glaciers, than exceptthose forof the northern-slopeSingkhu Glacier. glaciers, The average except velocity for ofthe the Singkh five southern-slopeu Glacier. The glaciers average was 77.42velocity m/year, of the whereas five southern-slopeit was 29.92 m/year glaciers for was the seven77.42 glaciersm/year, onwhereas the northern it was 29.92 slope. m/year This difference for the seven could glaciers be attributed on the northernto the different slope. topographicThis difference influences could be between attribut theed to southern the different and northern topographic slopes. influences The relationship between thebetween southern the glacierand northern velocity slopes. and these The factors relationship are discussed between in detailthe glacier in Section velocity 5.3. and these factors are discussed in detail in Section 5.3.

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Table 2. Average centerline velocities (limited to the lower 30% of the glacier length along the centerline) of major glaciers over the Central Karakoram from 1999–2003, and average annual velocity during 2007–2011 reported by Rankl et al. [1]. NS denotes that the glaciers locate on the northern slope and SS denotes that the glaciers locate on the southern slope. s = surge-type glacier, n = normal glacier. The locations of the numbered glaciers are marked in Figure2.

Length of Centerline 1999–2000 2000–2001 2001–2002 2002–2003 Diff. between 1999–2003 2007–2011 No. Glacier Type (km)/Altitude of the Velocity Velocity Velocity Velocity Max. and Min. Average Velocity Average Velocity Start (m) (m/yr) (m/yr) (m/yr) (m/yr) (m/yr) (m/yr) (m/yr) 1 Braldu (NS) s 10.79/3963 16.17 13.44 12.06 17.19 5.13 14.72 No data 2 Skamri (NS) s 12.35/3995 36.60 51.43 35.61 43.92 15.82 41.89 58.34 3 Sarpo Laggo(NS) s 8.97/4289 13.02 19.36 7.42 13.22 11.94 13.26 No data 4 North Gasherbrum (NS) s 7.75/4299 21.67 25.93 25.97 34.75 13.08 27.08 71.89 5 Urbak (NS) s 8.12/4468 11.70 10.20 3.78 23.80 20.02 12.37 5.73 6 Singkhu (NS) s 8.07/4519 91.24 79.61 72.48 78.37 18.76 80.43 83.14 7 Kyagar (NS) s 6.44/4891 24.52 23.13 10.88 20.32 13.64 19.71 13.66 8 Siachen (SS) n 22.89/3698 94.48 89.10 89.67 90.99 5.38 91.06 90.97 9 Kaberi (SS) n 12.24/3218 67.37 66.06 66.46 68.92 2.86 67.20 51.84 10 Baltoro (SS) n 19.03/3525 55.06 49.13 46.98 48.95 8.08 50.03 75.23 11 Panmah (SS) n 9.99/3593 55.15 60.35 55.99 47.29 13.06 54.70 No data 12 Biafo (SS) n 20.34/3184 138.97 123.04 119.50 114.90 24.07 124.10 92.53 Remote Sens. 2017, 9, 1064 8 of 20

4.2. Temporal Changes of Glacier Velocity during 1999–2003 Figure S1 in the Supplementary Material illustrates the annual velocity results of the glaciers in the Central Karakoram for four epochs from 1999–2003. Overall, the variations in ice velocities from 1999–2003 are not obvious for most of the studied glaciers including surge-type and non-surge-type glaciers. Table2 quantitatively reveals the fluctuations of ice motion for 12 major glaciers on the southern and northern slopes. Using a similar method that was proposed in [28] to identify whether glacier front was stable or not, we estimate fluctuation status of flow velocity of the 12 major glaciers with the difference between the maximum and minimum of annual velocities. Since the uncertainties in the derived glacier velocities are ~7.0 m/year (see Section 4.1), the uncertainty of difference between the maximum and minimum of annual velocity for a glacier is less than 10 m/year according to the linear error propagation law, which we thus take as a lower bound to discriminate stable-flow glacier from fluctuant glacier in velocity. According to Table2, the ice motion of Braldu, Siachen, Kaberi and Baltoro glaciers are quasi-stable, while the other glaciers (except for Urbak and Baifo glaciers) experienced slight fluctuations during the study period. It is worth noting that there are several surge-type glaciers in this area [1,15,40] (particularly on the northern slope), which undergo non-steady flow with alternations between active phases and the quiescent phase. Most of the inventoried surge-type glaciers in the study area (e.g., the Braldu, Skamri, Sarpo Laggo, North Gasherbrum, Urbak and Kyagar glaciers) experienced relatively small rates of motion (Supplementary Figure S1; Table2); however, the Singkhu glacier flowed fast compared with the other surge-type glaciers. The surge dynamics of such surge-type glaciers will be discussed in detail in Section 5.2. We focus on Baltoro Glacier and the Siachen Glacier in the analysis of inter-annual velocity variations from 1999–2003, as they have relatively complete spatial-coverage of ice velocity. Additionally, the time series of the annual velocity fields of the North Gasherbrum Glacier and Singkhu Glacier are presented in Supplementary Figures S2 and S4, and the annual speed profiles along the centerlines of the two glaciers are presented in Supplementary Figures S3 and S5. The ice motion of the North Gasherbrum Glacier slightly increased from 1999/2000 to 2002/2003, while the Singkhu Glacier experienced velocity decrease during 1999–2003.

4.2.1. Baltoro Glacier Figure3 illustrates the annual velocity fields of the Baltoro Glacier for four years from 1999–2003. Relatively complete coverage of the ice flow fields was retrieved from the glacier trunk and most of its tributaries even though the velocity results in 2002/2003 were obtained from ETM+ images acquired in spring. As shown in Figure4, the speed generally increased with increasing elevation and with a maximum speed (160 m/year) of approximately 45 km from the terminus. A peak surface speed of 140 m/year was found below the area of Concordia (Figure3a), the confluence of the Baltoro South and North tributaries, which is mainly related to the extra ice flow from the two tributaries. However, the velocity quickly decreased as the centerline profile extends up the south tributary. The annual velocities range from 46.98 m/year to 55.06 m/year during 1999–2003 (Table2), indicating that the ice motion of this glacier was small. Figure4 also shows that the velocities are almost independent of the slopes (<10 degree) along the 45-km-long centerline profile from the terminus. Remote Sens. 2017, 9, 1064 9 of 20 Remote Sens. 2017, 9, x FOR PEER REVIEW 9 of 20 Remote Sens. 2017, 9, x FOR PEER REVIEW 9 of 20

Figure 3. Four annual velocity fields of the Baltoro Glacier and its tributaries from 1999–2003. (a) FigureFigure 3. 3. Four annual annual velocity velocity fields fields of ofthethe Balt Baltorooro Glacier Glacier and andits tributaries its tributaries from 1999–2003. from 1999–2003. (a) 1999–2000; (b) 2000–2001; (c) 2001–2002; and (d) 2002–2003. The background is the hillshade image (a1999–2000;) 1999–2000; (b) (2000–2001;b) 2000–2001; (c) 2001–2002; (c) 2001–2002; and (d and) 2002–2003. (d) 2002–2003. The background The background is the hillshade is the hillshadeimage of SRTM DEM. AA’ is the centerline of the glacier and T1, T2, T3, and T4 are the transverse profile’s imageof SRTM of SRTM DEM. DEM.AA’ is AA’the centerline is the centerline of the glacier of the and glacier T1, T2, and T3, T1, and T2, T4 T3, are andthe transverse T4 are the profile’s transverse locations. profile’slocations. locations.

Figure 4. Annual speed profiles along the centerline (AA’) of the Baltoro Glacier. The centerline Figure 4. Annual speed profiles along the centerline (AA’) of the Baltoro Glacier. The centerline Figurelocation 4. Annual is indicated speed by profiles the dashed along theline centerline in Figure (AA’) 3a. The of the gray Baltoro dashed Glacier. line marks The centerline the end-point location location is indicated by the dashed line in Figure 3a. The gray dashed line marks the end-point is indicatedlocation of by the the centerline dashed lineprofile in Figurefor average3a. The velocity gray dashedestimation line in marks Table 2. the end-point location of the centerlinelocation of profile the centerline for average profile velocity for av estimationerage velocity in Tableestimation2. in Table 2. 4.2.2. Siachen Glacier 4.2.2. Siachen Glacier 4.2.2. SiachenFigure Glacier5 shows temporal variations in the annual velocity of the Siachen Glacier from 1999– Figure 5 shows temporal variations in the annual velocity of the Siachen Glacier from 1999– 2003. Compared with the Baltoro Glacier, the insufficient trackable features resulted in less 2003.Figure Compared5 shows with temporal the Baltoro variations Glacier, in the the annual insufficient velocity oftrackable the Siachen features Glacier resulted from 1999–2003.in less complete coverage of the velocity fields for the Siachen Glacier, especially in its accumulation Comparedcomplete withcoverage the Baltoro of the Glacier,velocity the fields insufficient for the trackableSiachen Glacier, features resultedespecially in in less its complete accumulation coverage region. Similar to the Baltoro Glacier, the ice motion of Siachen Glacier underwent small ofregion. the velocity Similar fields to forthe the Baltoro Siachen Glacier, Glacier, the especially ice motion in its accumulationof Siachen Glacier region. Similarunderwent to the small Baltoro fluctuations from 1999–2003 with no obvious changes in multi-temporal velocities. This temporal Glacier,fluctuations the ice from motion 1999–2003 of Siachen with no Glacier obvious underwent changes in small multi-temporal fluctuations velocities. from 1999–2003 This temporal with no pattern was also present in the annual velocity profiles along the glacier centerline (BB’) (Figure 6). obviouspattern changeswas also inpresent multi-temporal in the annual velocities. velocity Thisprofiles temporal along the pattern glacier was centerline also present (BB’) in(Figure the annual 6). Figure 6 shows that the glacier motion rates are generally dependent on elevation but two peaks are velocityFigure 6 profiles shows that along the the glacier glacier motion centerline rates are (BB’) generally (Figure dependent6). Figure 6on shows elevation that but the two glacier peaks motion are found at approximately 8.7 km and 17.5 km from the terminus with speeds of approximately 140 ratesfound are at generally approximately dependent 8.7 km on and elevation 17.5 km butfrom two the peaks terminus are foundwith speeds at approximately of approximately 8.7 km 140 and m/year and 105 m/year. These might result from the change of ice flow direction and the narrow m/year and 105 m/year. These might result from the change of ice flow direction and the narrow 17.5width km fromof the the glacier. terminus With with a further speeds increase of approximately of elevation, 140 the m/year speed decreased and 105 m/year.and dropped These to might its resultwidth from of the the glacier. change With of icea further flow directionincrease of and elevation, the narrow the speed width decreased of the glacier. and dropped With ato further its

Remote Sens. 2017, 9, 1064 10 of 20

Remote Sens. 2017, 9, x FOR PEER REVIEW 10 of 20 increaseRemote of Sens. elevation, 2017, 9, x theFOR speedPEER REVIEW decreased and dropped to its minimum (70 m/year) at approximately10 of 20 minimum (70 m/year) at approximately 24 km from the terminus. The maximum velocity that 24 km from the terminus. The maximum velocity that located at 40 km is mainly attributed to that locatedminimum at 40 km(70 m/year)is mainly at attributedapproximately to that 24 manykm fr omtributary the terminus. glaciers Thefeed maximum this area, velocityespecially that the many tributary glaciers feed this area, especially the Teram Shehr and Lolofond glaciers (Figure5a). Teramlocated Shehr at 40and km Lolofondis mainly attributedglaciers (Figure to that many5a). As tributary shown glaciers in Figure feed 6,this the area, slopes especially along the the As shown in Figure6, the slopes along the centerline are small (<8 degrees), and there is no obvious centerlineTeram areShehr small and (<8 Lolofond degrees), glaciers and there (Figure is no 5a). obvious As shown correlation in Figure between 6, the the slopes slope along (along the the correlation between the slope (along the 42-km-long centerline profile from the terminus) and the 42-km-longcenterline centerline are small profile(<8 degrees), from the and terminus) there is no and obvious the speed correlation of the Siachenbetween Glacier. the slope (along the speed42-km-long of the Siachen centerline Glacier. profile from the terminus) and the speed of the Siachen Glacier.

Figure 5. Four annual velocity fields of the Siachen Glacier and its tributaries from 1999–2003. FigureFigure 5. 5. FourFour annualannual velocity fields fields of of the the Siac Siachenhen Glacier Glacier and and its its tributaries tributaries from from 1999–2003. 1999–2003. (a) 1999–2000; (b) 2000–2001; (c) 2001–2002; and (d) 2002–2003. The background is the hillshade (a) 1999–2000; (b) 2000–2001; (c) 2001–2002; and (d) 2002–2003. The background is the hillshade (a) 1999–2000;image of SRTM (b) 2000–2001; DEM. BB’ is ( cthe) 2001–2002; centerline of and the (glacier,d) 2002–2003. and T1, T2, The T3, backgroundand T4 are the is transverse the hillshade imageimageprofile’s of of SRTM SRTM locations. DEM. DEM. BB’BB’ isis thethe centerlinecenterline of of the the glacier, glacier, and and T1, T1, T2, T2, T3, T3, and and T4 T4 are are the the transverse transverse profile’sprofile’s locations. locations.

Figure 6. Annual speed profiles along the centerline (BB’) of the Siachen Glacier. The centerline location is indicated by the dashed line in Figure 5a. The gray dashed line marks the end-point FigureFigurelocation 6. 6.Annual Annual of the speed centerline speed profiles profiles profile along alongfor theaver the centerlineage centerline velocity (BB’) estimation (BB’) of the of Siacheninthe Table Siachen Glacier.2. Glacier. The centerlineThe centerline location islocation indicated is byindicated the dashed by the line dashed in Figure line5a. in The Figure gray 5a. dashed The gray line marksdashed the line end-point marks the location end-point of the

centerlinelocation of profile the centerline for average profile velocity for aver estimationage velocity in Tableestimation2. in Table 2.

Remote Sens. 2017, 9, 1064 11 of 20

5. Discussion

5.1. A Comparison with Previous Studies To analyse the temporal changes, we compared our velocity results with previous studies. A quantitative comparison analysis for major glaciers in the study area was performed between our velocity results (1999–2003) and the velocity measurements (2007–2011) provided by Dr. Rankl, who compiled surface velocities mosaic for a Karakoram-wide coverage derived with the SAR dataset between 2007 and 2011 [1]. As shown in Table2, in general, the average annual velocities of most glaciers for the two epochs have small fluctuations or slight increases. Note that the average velocity of the North Gasherbrum Glacier during 2007–2011 is up to ~71 m/year and is 2.5 times as high as the velocity of ~27 m/year during 1999–2003. This significant velocity difference might be due to the different surge phases of the glacier for the two epochs, and more details will be discussed in Section 5.2. Moreover, the velocity for the Biafo Glacier decreased from ~124 m/year in 1999/2003 to ~92 m/year in 2007/2011, and these variation patterns are comparable with those averaged velocity variations during the period from 2001–2009 in Scherler et al. [20]. This indicates that the large velocity fluctuations for the Biafo Glacier, as reported in Scherler et al. [20], might occur during a longer period from 1999 to 2011. Regarding the well-investigated Baltoro Glacier, the average velocity in 2007/2011 was ~50% higher than that in 1999/2003 as shown in Table2, whereas Quincey et al. [ 19] reported that the velocity in 2006/2008 was ~15–20% greater than that in the early 2000s. This suggests that the Baltoro Glacier might speed up slightly from 2008 to 2011. In addition, the annual velocity profiles along the centerline of the Baltoro Glacier (Figure4) are generally in agreement with the previous study in Quincey et al. [19], which reported a clear variability of ice flow across the entire glacier. In particular, flow profiles for the lowermost 10 km of the glacier terminus show a strong consistency between our results (from 1999–2003) and the previous results (2003–2008) reported by Quincey et al. [19]. However, the greatest differences between the two results were in the area around Concordia (the Concordia location is marked in Figure3), where the 1999–2003 annual velocities of ~140 m/year increased to ~160 m/year from 2003–2005 (see Quincey et al. [19], Figure4). Such velocity variations imply that the speed-up of the glacier around Concordia and as reported in Quincey et al. [19] might start in 1999. The velocity fluctuations for the Siachen Glacier are insignificant from 1996–2011 based on the comparison of our results with those published in the literature. Kumar et al. [27] derived the velocities of the Siachen Glacier from two pairs of ERS images using the Differential Interferometry SAR (D-InSAR) method. They found that the maximum velocity of the glacier was 43.00 cm/d (equating to 156.00 m/year) in 1996 at about 40 km from the terminus (Figure6a). This is comparable to the velocity at the same area (~150 m/year, Figure6a) in our study. Rankl et al. [ 1] recorded a velocity of approximately 0.47 m/d (equating to 171.55 m/year) in the middle and upper areas of the Siachen Glacier from 2007–2011, through an SAR offset tracking algorithm. Moreover, the velocity results derived from the D-InSAR algorithm on the upstream area of the Siachen Glacier are better than the downstream area due to low coherence [27]. In contrast, our results from the cross-correlation algorithm show a more complete coverage of ice velocity in the downstream area with lots of null values occurring on the upstream area covered by snow. Therefore, if there are contemporary SAR and optical images available, a combination of the two methods may obtain a more complete ice flow field of the Siachen Glacier.

5.2. Surge-Type Glacier Dynamics There are many surge-type glaciers in the Central Karakoram [1,15,16,23,40–43] including Braldu Glacier, Skamri Glacier, North Gasherbrum Glacier, Sarpo Laggo Glacier, Singkhu Glacier, Urbak Glacier, and Kyagar Glacier. Most of these surge-type glaciers experienced relatively small motion rates, which implies that these glaciers might be in their quiescent phases during our observation Remote Sens. 2017, 9, 1064 12 of 20 period.Remote Sens. Due 2017 to, incomplete9, x FOR PEER velocity REVIEW fields of small surge-type glaciers, we focused our discussion12 of on 20 the North Gasherbrum, Singkhu and Skamri glaciers. TheThe NorthNorth GasherbrumGasherbrum GlacierGlacier waswas identifiedidentified asas aa surge-typesurge-type glacierglacier byby KotlyakovKotlyakov [[44].44]. QuinceyQuincey etet al. al. [41 [41]] reported reported that that a glacier a glacier surge surge was initiatedwas initiated in the in autumn the autumn of 2005 of with 2005 a peak with velocity a peak ofvelocity 0.5 km/yr of 0.5 (equating km/yr (equating to 500 m/year) to 500 m/year) in the summerin the summer of 2006, of which2006, which agreed agreed with withMayer Mayer et al. et [15 al.] who[15] who found found that that this this glacier glacier advanced advanced from from 2003–2007. 2003–2007. As As shown shown in in Table Table2, 2, the the velocities velocities of of the the NorthNorth GasherbrumGasherbrum GlacierGlacier werewere relatively relatively small small and and increased increased from from 21.67 21.67 m/year m/year (1999/2000) (1999/2000) toto 34.7534.75 m/year m/year (2002/2003).(2002/2003). Compared Compared with Quincey et al. [[41]41] and Mayer etet al.al. [[15],15], itit cancan bebe inferredinferred thatthat the the North North Gasherbrum Gasherbrum GlacierGlacier mightmight havehave experienced experienced aa quiescentquiescent phasephase overover thethe periodperiod fromfrom 1999–2003.1999–2003. Moreover,Moreover, thethe significantsignificant velocityvelocity differencedifference betweenbetween ourour resultresult andand RanklRankl etet al.al. [[1]1] isis mainlymainly attributed attributed to to the the fact fact that that the Norththe North Gasherbrum Gasherbrum Glacier Glacier surged duringsurged 2007–2011.during 2007–2011. In addition, In theaddition, Singkhu the Glacier Singkhu exhibited Glacier relatively exhibited high relatively velocities high during velocities the studyduring period. the study Due period. to the limited Due to velocitythe limited results velocity of this results glacier of inthis the glacier published in the literature, published we literature, analysed we the analysed annual the speeds annual along speeds the centerlinealong the of centerline the Singkhu of Glacierthe Singkhu during Glacier 1996–2006 during from 1996–2006 the Landsat from TM andthe ETM+Landsat images TM and (Figure ETM+7). Theimages result (Figure indicates 7). The that result the Singkhu indicates Glacier that the might Singkhu have Glacier undergone might a quiescenthave undergone phase duringa quiescent the studyphase period during due the to study no significant period du surgee to signal.no significant The velocity surge of signal. the Singkhu The velocity Glacier fromof the 2007–2011 Singkhu (TableGlacier2) isfrom comparable 2007–2011 with (Table our results,2) is comparable which indicate with our that results, this glacier which was indicate also in that its quiescent this glacier phase was fromalso in 2007 its toquiescent 2011. phase from 2007 to 2011.

FigureFigure 7.7. (a(a)) SurfaceSurface velocityvelocity overover thethe SingkhuSingkhu GlacierGlacier (2001–2002)(2001–2002) andand EE’EE’ isis thethe centerlinecenterline ofof thethe SingkhuSingkhu Glacier; Glacier; (b ()b Annual) Annual speed speed profiles profiles along along the the centerline centerline (EE’) (EE’) of the of Singkhu the Singkhu Glacier Glacier from 1996from to1996 2006. to 2006.

The Skamri Glacier was reported as a surge-type glacier by Copland et al. [23], which surged The Skamri Glacier was reported as a surge-type glacier by Copland et al. [23], which surged prior to 1978, while the South Skamri Glacier, which is a tributary of the Skamri Glacier (Figure 8), prior to 1978, while the South Skamri Glacier, which is a tributary of the Skamri Glacier (Figure8), surged in 1990 and again in 2007 [23,42]. The centerline speed profiles (Figure 8b,c) show temporal surged in 1990 and again in 2007 [23,42]. The centerline speed profiles (Figure8b,c) show temporal variation patterns of the surface flow of the two glaciers during 1996–2006 that were revealed from variation patterns of the surface flow of the two glaciers during 1996–2006 that were revealed from the the Landsat TM and ETM+ images. During 1999–2003, the South Skamri Glacier had high surface Landsat TM and ETM+ images. During 1999–2003, the South Skamri Glacier had high surface velocities velocities (Figure 8b), which are comparable to those during the period from 2006–2007 in Copland (Figure8b), which are comparable to those during the period from 2006–2007 in Copland et al. [23] et al. [23] and 2007–2008 in Jiang et al. [42]. Moreover, the South Skamri Glacier advanced about 240 and 2007–2008 in Jiang et al. [42]. Moreover, the South Skamri Glacier advanced about 240 m from m from 1999 to 2003 (Figure 9). These indicate that this glacier might have experienced an active 1999 to 2003 (Figure9). These indicate that this glacier might have experienced an active phase from phase from 1999–2003. However, the velocities of the Skamri Glacier (Figure 8c) suggest that this 1999–2003. However, the velocities of the Skamri Glacier (Figure8c) suggest that this glacier might glacier might have undergone its quiescent phase during the observation period. Additionally, the have undergone its quiescent phase during the observation period. Additionally, the velocities of the velocities of the Skamri Glacier were relatively lower than those of the South Skamri Glacier, which Skamri Glacier were relatively lower than those of the South Skamri Glacier, which implies that the implies that the South Skamri Glacier was the dominant flow in the Skamri Basin from 1999–2003. South Skamri Glacier was the dominant flow in the Skamri Basin from 1999–2003. As surge-type glaciers dominate the glacier sample in the northern slope, we might expect to As surge-type glaciers dominate the glacier sample in the northern slope, we might expect to see see lower surface velocities in the northern slope than that of the southern slope, which is lower surface velocities in the northern slope than that of the southern slope, which is dominated by dominated by fast-flowing and non-surging glaciers. That is to say, since surge behavior is not regional, we wouldn’t expect to see more than a small number of glaciers surging at any given time, so the dominant behavior of the surge-type glaciers sample would be quiescence, which tends to be slower than even for non-surging glaciers. Due to the strong impact of the surge cycle on flow

Remote Sens. 2017, 9, 1064 13 of 20 fast-flowing and non-surging glaciers. That is to say, since surge behavior is not regional, we wouldn’t expect to see more than a small number of glaciers surging at any given time, so the dominant behaviorRemoteRemote Sens. Sens. of 2017 2017 the, ,9 9 surge-type, ,x x FOR FOR PEER PEER glaciersREVIEW REVIEW sample would be quiescence, which tends to be slower than1313 even of of 20 20 for non-surging glaciers. Due to the strong impact of the surge cycle on flow velocities, we intend to investigatevelocities,velocities, we glacier-specificwe intendintend toto investigateinvestigate details of their glacier-specificglacier-specific dynamic behavior detailsdetails in of aof more theirtheir extended, dynamicdynamic separate behaviorbehavior study inin aa before moremore drawingextended,extended, further separateseparate conclusions studystudy beforebefore about thedrawingdrawing causes offurthefurthe the spatialrr conclusionsconclusions differences aboutabout in ice thethe velocity causescauses between ofof thethe glaciers spatialspatial ondifferencesdifferences the southern in in ice ice and velocity velocity northern between between slopes glaciers glaciers in this on region.on the the southern southern and and northern northern slopes slopes in in this this region. region.

FigureFigure 8.8. ( (aa)) Surface Surface velocity velocity over over the the Skamri Skamri Glacier Glacier (2000–2001) (2000–2001) and and FF’ FF’ and and GG’ GG’ are are thethe centerlinescenterlines ofof thethe SouthSouth South SkamriSkamri Skamri GlacierGlacier Glacier andand and Skamri Skamri Skamri Glacier; Glacier; Glacier; ( b( (b)b Annual)) Annual Annual speed speed speed profiles profiles profiles along along along the the the centerline centerline centerline (FF’) (FF’) (FF’) of theofof thethe South SouthSouth Skamri SkamriSkamri Glacier GlacierGlacier from fromfrom 1996 1996 to1996 2006; toto 2006; and2006; ( c and)and Annual ((cc)) AnnualAnnual speed profiles speedspeed profilesprofiles along the alongalong centerline thethe centerlinecenterline (GG’) of the(GG’)(GG’) Skamri of of the the Glacier Skamri Skamri from Glacier Glacier 1996 from from to 2006. 1996 1996 to to 2006. 2006.

FigureFigure 9.9. TheThe frontalfrontal changeschanges ofof thethe SouthSouth SkamriSkamri GlGlacieracier fromfrom 19991999 toto 2003,2003, thethe backgroundbackground isis thethe Figure 9. The frontal changes of the South Skamri Glacier from 1999 to 2003, the background is the Landsat ETM+ image acquired on 16 July 1999. Landsat ETM+ image acquir acquireded on 16 July 1999. 5.3. Interpretation of the Velocity Differences between Glaciers on the Southern and Northern Slopes 5.3. Interpretation of the Velocity Differences betweenbetween GlaciersGlaciers onon thethe SouthernSouthern andand NorthernNorthern SlopesSlopes As presented in Table 2, most of the glaciers on the southern slope flowed faster than the As presentedpresented in in Table Table2, most2, most of the of glaciersthe glaciers on the on southern the southern slope flowed slope fasterflowed than faster the northernthan the northern slope glaciers during the observation period. To investigate the possible factors for slopenorthern glaciers slope during glaciers the during observation the observation period. To investigateperiod. To theinvestigate possible factorsthe possible for differences factors for in differences in ice speed between glaciers on the southern and northern slopes, we evaluated the icedifferences speed between in ice speed glaciers between on the glaciers southern on and the northernsouthern slopes, and northern we evaluated slopes, the we dependence evaluated the of dependence of the average annual speeds of 24 glaciers over the study area on the topographic thedependence average annualof the speedsaverage of annual 24 glaciers speeds over of the 24 study glaciers area over on thethe topographic study area factorson the (area,topographic length, factors (area, length, mean width, median elevation, mean slope and mean aspect) and debris cover. meanfactors width, (area, length, median mean elevation, width, mean median slope elevation, and mean mean aspect) slope andand debrismean aspect) cover. and We excludeddebris cover. the We excluded the Skamri Glacier in our analysis since the South Skamri glacier was actively surging SkamriWe excluded Glacier the in Skamri our analysis Glacier since in our the analysis South Skamri since the glacier South was Skamri actively glacier surging was duringactively the surging study during the study period. A linear regression analysis was employed to calculate the correlation period.during Athe linear study regression period. A analysis linear wasregression employed anal toysis calculate was employed the correlation to calculate coefficient the (R)correlation between coefficient (R) between the glacier velocity and each topographic factor. Surface velocity parameters thecoefficient glacier (R) velocity between and the each glacier topographic velocity and factor. each Surface topographic velocity factor. parameters Surface werevelocity obtained parameters from were obtained from average velocity results of 1999–2002 that limited to the lower 30% of the averagewere obtained velocity from results average of 1999–2002 velocity thatresults limited of 1999–2002 to the lower that 30% limited of the to glacier the lower length 30% along of the glacierglacier lengthlength alongalong thethe centerlinecenterline forfor eacheach glacier,glacier, inin whichwhich therethere areare relativelyrelatively reliablereliable velocityvelocity datadata forfor mostmost glaciers.glaciers. TheThe area,area, length,length, medianmedian elevation,elevation, meanmean slopeslope andand meanmean aspectaspect ofof eacheach glacierglacier areare derivedderived fromfrom thethe RGIRGI v5.0v5.0 [24].[24]. FigureFigure 1010 indicatesindicates thethe relationshipsrelationships (the(the 95%95% confidenceconfidence interval)interval) between between the the observed observed average average velocity velocity and and the the morphometric morphometric indices. indices.

Remote Sens. 2017, 9, 1064 14 of 20 centerline for each glacier, in which there are relatively reliable velocity data for most glaciers. The area, length, median elevation, mean slope and mean aspect of each glacier are derived from the RGI v5.0 [24]. Figure 10 indicates the relationships (the 95% confidence interval) between the observed Remote Sens. 2017, 9, x FOR PEER REVIEW 14 of 20 average velocity and the morphometric indices.

Figure 10. Correlations between glacier speed and individual topographic factors and glaciological Figure 10. Correlations between glacier speed and individual topographic factors and glaciological characteristics: (a) area; (b) length; (c) median elevation; (d) mean slope of a glacier; (e) debris cover; characteristics:and (f) mean (a) aspect area; (ofb )a length;glacier. It (c )should median be noted elevation; that the (d )area mean and slope length of in a ( glacier;a,b) are plotted (e) debris on a cover; and (flogarithmic) mean aspect axis. ofThe a glacier.blue points It should represent be sout notedhern that slope the glaciers; area and the length red points in (a ,brepresent) are plotted on a logarithmicnorthern slope axis. glaciers. The blueR is the points linear represent correlation southern coefficient slope and p glaciers; is the significance the red pointslevel ofrepresent the northernrespective slope relation. glaciers. R is the linear correlation coefficient and p is the significance level of the respective relation. Overall, a moderate positive correlation was found between the glacier area and the glacier speed (R = 0.51, p = 0.01 < 0.05, Figure 10a) and a similar correlation for the glacier length (R = 0.54, p Overall, a moderate positive correlation was found between the glacier area and the glacier speed = 0.007 < 0.05, Figure 10b). The mean width of the glacier (glacier area/glacier length) is weakly (R = 0.51,correlatedp = 0.01 with < 0.05, glacier Figure speed 10a) (R and = 0.47, a similar p = 0.021 correlation < 0.05, forSupplementary the glacier length Figure (R S6).= 0.54, However,p = 0.007 the < 0.05, Figuremedian 10b). The elevation mean (R width = −0.28, of p the= 0.19 glacier > 0.05, (glacierFigure 10c) area/glacier is not a significant length) control is weakly on glacier correlated velocity. with glacierThe speed mean (R slope= 0.47, (R =p −=0.46, 0.021 p = <0.026 0.05, < 0.05, Supplementary Figure 10d) is Figure weakly S6). anti-correlated However, glacier the median velocity. elevation It (R = −should0.28, p be= 0.19noted > that 0.05, areas Figure (lengths) 10c) isof not most a significantof the glaciers control on the on southern glacier slope velocity. are much The meanlarger slope (R = −(longer)0.46, p =than 0.026 those < 0.05,on the Figure northern 10d) slope is weakly as show anti-correlatedn in Figures 10a (Figure glacier 10b). velocity. Therefore, It should different be noted that areasglacier (lengths) size (areas of mostand lengths) of the glaciers may be onone the of southernthe primary slope reasons are muchfor spatial larger differences (longer) in than ice those velocity between glaciers on the southern and northern slopes in this region. on the northern slope as shown in Figure 10a (Figure 10b). Therefore, different glacier size (areas and As a key component of a glacier system, debris cover might also affect glacial dynamics [28]. lengths)Debris-covered may be one glaciers of the primarywere widely reasons found for in th spatiale Himalaya-Karakoram differences in ice mountain velocity range between with glaciersmore on the southernthan 20% and debris-cover northern [28]. slopes Several in this studies region. have reported that debris cover had a protective effect on Asthe a ablation key component rates [4,28,45] of a and glacier the velocity system, of debris debris-covered cover might ice was also lower affect than glacial clean dynamicsice in the [28]. Debris-coveredEverest region glaciers and Bhutan were widely [5,46]. Scherler found inet theal. [2 Himalaya-Karakoram8] presented spatially variable mountain impacts range of debris with more than 20%cover debris-cover on glacier dynamics [28]. Several in the studiesGreater Himalaya: have reported quasi-stagnant that debris glaciers cover (surface had a velocity protective less effect on thethan ablation 2.5 m/year) rates [have4,28 ,high45] and debris the cover velocity (>40% of on debris-covered average) and stable ice wasglacier lower fronts than in the clean Central ice in the Himalaya, whereas there is no uniform pattern of the distribution of stagnant debris-covered Everest region and Bhutan [5,46]. Scherler et al. [28] presented spatially variable impacts of debris glaciers in the Karakoram. In this study, similar with the results presented by Scherler et al. [28], we coverfind onglacier that the dynamicspercentage of in debris the Greater cover is not Himalaya: significantly quasi-stagnant correlated with glaciers glacier velocity (surface (R = velocity 0.12, p less than 2.5= 0.585 m/year) > 0.05) have as shown high debrisin Figure cover 10e in (>40% which on the average) percentage and of stabledebris cover glacier for fronts 24 glaciers in the was Central Himalaya,generated whereas from there Landsat is no ETM+ uniform band4/band5-ratio pattern of the distributionimages (see ofSupplementary stagnant debris-covered Figure S7). These glaciers in the Karakoram.regional differences In this study, imply similar a complex with response the results of glacier presented dynamics by Scherlerto debris etcover al. [that28], is we probably find that the percentagedependent of debris on coverdifferent is nottopographic significantly conditions correlated and withhighlights glacier the velocity need (forR =a 0.12,comprehensivep = 0.585 > 0.05) as showninvestigation in Figure in 10thee near in which future. the percentage of debris cover for 24 glaciers was generated from Additionally, the aspect of a mountain glacier affects the solar radiation received by the glacier Landsat ETM+ band4/band5-ratio images (see Supplementary Figure S7). These regional differences surface and is a potential influencing factor in glacier motion, particularly for a glacier system implylocated a complex on both response the southern of glacier and dynamics northern slopes. to debris Figure cover 10f that illustrates is probably that the dependent mean aspects on different of topographic conditions and highlights the need for a comprehensive investigation in the near future.

Remote Sens. 2017, 9, 1064 15 of 20

Additionally, the aspect of a mountain glacier affects the solar radiation received by the glacier surface and is a potential influencing factor in glacier motion, particularly for a glacier system located on both the southern and northern slopes. Figure 10f illustrates that the mean aspects of glaciers on the northern slope are mainly towards the northwest to northeast, whereas the glaciers on the southern slope are mainly southwest to southeast-facing, except for four glaciers. Moreover, most of the southern-slope glaciers had higher flow velocities (Figure 10f) as they received more direct solar radiance [47]. This implies a stronger melting over the glacier surface, which should produce more meltwater flowing to the glacier bedrock through crevasses and moulins [48]. The meltwater can increase water pressure at the bedrock interface and reduce basal friction, which promotes basal sliding and thus increases the ice flow [49–52]. We just provided a qualitative explanation in this study, and there was no quantitative analysis performed due to that we have no solar radiation data of each glacier. In addition, the multiple linear regression (MLR) analysis was also employed to examine the relationship between the glacier speed and two or more morphometric factors by fitting a linear equation to observed data. The area, length and mean width all reflect the size of a glacier, and the area is related to the length and mean width. Therefore, the area was selected to involve in the MLR analysis. Table3 revealed the effects of variable morphometric on the glacier speed which are generally consistent with those revealed by a linear regression analysis (Figure 10). The result shows a significant correlation between glacier speed and the area at the 95% confidence interval (p = 0.019 < 0.05, Table3). It should be noted that the mean slope of the glacier is not significantly correlated with the glacier velocity in the MLR analysis, while it is weakly anti-correlated (Figure 10d) with the glacier velocity in a linear regression analysis. This is mainly related to that the mean slope has a very small effect on ice flow when considering multiple factors. The median elevation, debris cover and mean aspect are not significantly correlated with the glacier speed. The morphological analysis we have performed in this study has not revealed a dependence of averaged flow velocities on any topographic parameter apart from glacier size. It should be noted that both the linear regression analysis and the MLR analysis are limited to these set of observations over studied glaciers during 1999–2003 and can’t be generalized for the entire Karakoram. More such analysis is required in the future to establish a clear relationship between glacier velocity and factors in the Karakoram.

Table 3. Summary of the multiple linear regression analysis in the SPSS software package (T-value (t) and p-value are two parameters obtained from the t-test).

Variable Parameter Estimate Standard Error t p-Value area 0.61 0.024 2.576 0.019 median elevation −0.041 0.035 −1.146 0.267 mean slope −1.878 2.877 −0.653 0.522 debris cover 0.071 0.043 1.643 0.118 mean aspect −1.226 1.388 −0.883 0.389

5.4. Glacier Motion Mechanism Basal sliding and internal deformation are two major types of glacier motion mechanisms [23]. Generally, the surface velocity variations along glacier transverse profiles have been used to infer dominant motion mechanisms [19,23,42]. When the velocities are high and relatively constant in the glacier centre but rapidly reduced close to the glacier margins, the glacier motion mechanisms are of the basal sliding type. If the ice flow regime is dominated by internal deformation, surface velocities are generally low and gradually increase from the glacier margins to the centre in a parabolic pattern. The transverse profiles of average annual velocities of the Baltoro and Siachen glaciers are presented in Figures 11 and 12, respectively. From a visual inspection, it is evident that the speeds increased from the margins to the centre. The speed transverse profiles T1, T2, and T3 across the upper and middle ablation zones of the two glaciers (see Figures4 and6) all exhibited not only a rapid increase away from both glacier margins but were high and relatively constant in the glacier Remote Sens. 2017, 9, 1064 16 of 20 centres as shown in Figures 11a–c and 12a–c . This indicates that basal sliding is the predominant flow Remote Sens. 2017, 9, x FOR PEER REVIEW 16 of 20 mechanismRemote Sens. in these2017, 9, upperx FOR PEER and REVIEW middle areas; however, the transverse profile T4 located on the16 of glacier20 snout (see Figures4 and6) shows quite a different flow pattern from those in T1–T3, where speeds are glacier snout (see Figures 4 and 6) shows quite a different flow pattern from those in T1–T3, where glacier snout (see Figures 4 and 6) shows quite a different flow pattern from those in T1–T3, where relativelyspeeds low are and relatively gradually low roseand gradually to a peak rose in the to centrea peak of in the the glacier. centre of Such the velocityglacier. Such patterns velocity suggest speeds are relatively low and gradually rose to a peak in the centre of the glacier. Such velocity that internalpatterns deformationsuggest that internal dominates deformation close to thedominates terminus close rather to the than terminus basal rather sliding. than Similar basal sliding. discussions patterns suggest that internal deformation dominates close to the terminus rather than basal sliding. wereSimilar reported discussions by Copland were etreported al. [23 ]by and Copland Quincey et al. et [23] al. and [19]. Quincey Figures et 11 al. and[19]. 12Figures also show11 and that 12 the Similar discussions were reported by Copland et al. [23] and Quincey et al. [19]. Figures 11 and 12 velocitiesalso show of the that middle the velocities and upper of the zones middle of and the upper two glaciers zones of are the largertwo glaciers than theare downstreamlarger than the zone. also show that the velocities of the middle and upper zones of the two glaciers are larger than the This isdownstream mainly due zone. to the This larger is mainly mass accumulationdue to the larger at themass middle accumulation and upper at the zones middle than and downstream upper downstream zone. This is mainly due to the larger mass accumulation at the middle and upper zones than downstream area [26]. area [zones26]. than downstream area [26].

Figure 11. Transverse speed profiles of the Baltoro Glacier. Profile locations are depicted in Figure 3a. FigureFigure 11. Transverse 11. Transverse speed speed profiles profiles of of the the Baltoro Baltoro Glacier. Profile Profile locations locations are are depicted depicted in Figure in Figure 3a. 3a. (a–d) The comparison of annual surface velocity change along the transverse profiles of T1, T2, T3, (a–d)( Thea–d) comparison The comparison of annual of annual surface surface velocity velocity change change along along the the transverse transverse profilesprofiles ofof T1, T2, T3, and and T4, respectively. T4, respectively.and T4, respectively.

Figure 12. Transverse speed profiles of the Siachen Glacier. Profile locations are depicted in Figure Figure 12. Transverse speed profiles of the Siachen Glacier. Profile locations are depicted in Figure Figure5a. 12. (a–Transversed) The comparison speed profilesof annual of surface the Siachen velocity Glacier. change Profile along the locations transverse are depictedprofile T1, in T2, Figure T3, 5a. 5a. (a–d) The comparison of annual surface velocity change along the transverse profile T1, T2, T3, (a–d)and The T4, comparison respectively. of annual surface velocity change along the transverse profile T1, T2, T3, and and T4, respectively. T4, respectively.

Remote Sens. 2017, 9, 1064 17 of 20

We also analysed the transverse profiles of the average annual velocities of the Biafo Glacier (Supplementary Figure S8), Panmah Glacier and its tributary Choktoi Glacier (Supplementary Figure S9), Skamri Glacier (Supplementary Figure S10), North Gasherbrum Glacier (Supplementary Figure S11) and Singkhu Glacier (Supplementary Figure S12), which suggest that motion mechanisms of these glaciers are similar with the Baltoro and Siachen glaciers. Therefore, it could be inferred that basal sliding might be the predominant motion mechanism across the middle and upper regions for most nonsurge-type and surge-type glaciers in the Central Karakoram, whereas internal deformation is dominant close to the glacier terminus. Additionally, the active surge-type glacier (southern Skamri glacier) might be dominated by the similar ice-motion mechanisms with nonsurge-type glaciers, which is in agreement with results in Copland et al. [23] and Jiang et al. [42], however, this predominant motion mechanism of quiescent surge-type glaciers have been not reported in the Central Karakoram in previous literatures.

6. Conclusions In this study, the spatial and temporal variability in the surface velocity during 1999–2003 over the Central Karakoram was characterized by an analysis of the inter-annual glacier velocity fields, which extends the period of measurements of glacier velocities in this region. The velocity fields were retrieved using a cross-correlation based approach with four pairs of Landsat ETM+ panchromatic images. The results indicated that the large-sized glaciers flowed faster than the smaller glaciers and the highest surface velocity reached up to approximately 200 m/year and occurred on the Biafo glacier and the South Skamri Glacier. The average annual velocity revealed that most glaciers on the southern slope flowed faster than those of the northern slope. This difference might be attributed to the different glacier sizes on the southern and northern slopes. In addition, the observed differences in flow velocities between the northern and southern glaciers might be dominated by the phase of the surge cycle, rather than morphometric factors. Determining these causes requires investigation of velocities over longer time periods than we have here, and we intend to investigate this further in future studies. Moreover, a new surging event for the South Skamri Glacier was identified in the study period by investigating the glacier frontal changes and the longer-term time series of the surface velocities between 1996 and 2006. Finally, from the transverse profiles of the glacier velocity, we inferred that the motion regime of the middle and upper regions of both glaciers was dominated by basal sliding, whereas internal deformation was a more dominant motion mechanism closer to the glacier terminus.

Supplementary Materials: The following are available online at www.mdpi.com/2072-4292/9/10/1064/s1. Figure S1. Four annual velocity fields over the central Karakoram from 1999 to 2003. a: 1999–2000; b: 2000–2001; c: 2001-2002; d: 2002–2003.The blue dashed line marks the boundary between the southern and northern slopes glaciers. Abbreviations denote: Br = Braldu Glacier, Sk = Skamri Glacier, SL = Sarpo Laggo Glacier, NG = North Gasherbrum Glacier, Ur = Urbak Glacier, Sh = Singkhu Glacier, Ky = Kyagar Glacier, Si = Siachen Glacier, Ka = Kaberi Glacier, Bt = Baltoro Glacier, Pa = Panmah Glacier, Bi = Biafo Glacier; Figure S2. Four annual velocity fields of the North Gasherbrum Glacier from 1999–2003. (a) 1999–2000; (b) 2000–2001; (c) 2001–2002; and (d) 2002–2003. The background is the hillshade image of SRTM DEM. CC’ is the centerline of the glacier; Figure S3. Annual speed profiles along the centerline (CC’) of the North Gasherbrum Glacier. The centerline location is indicated by the dashed line in Figure S2a; Figure S4. Four annual velocity fields of the Singkhu Glacier from 1999–2003. (a) 1999–2000; (b) 2000–2001; (c) 2001–2002; and (d) 2002–2003. The background is the hillshade image of SRTM DEM. DD’ is the centerline of the glacier; Figure S5. Annual speed profiles along the centerline (DD’) of the Singkhu Glacier. The centerline location is indicated by the dashed line in Figure S4a; Figure S6. Correlation between the glacier speed and the mean width. R is the linear correlation coefficient and p is the significance level of the respective relation; Figure S7. Distribution of debris cover derived from Landsat ETM+ band 4/band5-ratio images. The blue dashed line marks the boundary between the southern and northern slopes glaciers. Abbreviations: Br = Braldu Glacier, Sk = Skamri Glacier, SL = Sarpo Laggo Glacier, NG = North Gasherbrum Glacier, Ur = Urbak Glacier, Sh = Singkhu Glacier, Ky = Kyagar Glacier, Si = Siachen Glacier, Ka = Kaberi Glacier, Bt = Baltoro Glacier, Pa = Panmah Glacier, Bi = Biafo Glacier; Figure S8. (a) Surface velocity over the Biafo Glacier (1999–2000); (b–d) Comparison of annual surface speed change along transverse profiles T1, T2 and T3. Profile locations are depicted in (a). Date format is month/day/year; Figure S9. (a) Surface velocity over the Panmah Glacier (2001–2002) which includes two tributary glaciers of Choktoi and Nobande Sobonde; (b–d) Comparison of annual surface speed change along transverse profiles T1, T2 and T3. Profile Remote Sens. 2017, 9, 1064 18 of 20 locations are depicted in (a). Date format is month/day/year; Figure S10. (a) Surface velocity over the Skamri Glacier (2000–2001); (b,c) Comparison of annual surface speed change along transverse profiles T1 and T2. Profile locations are depicted in (a). Date format is month/day/year; Figure S11. (a) Surface velocity over the North Gasherbrum Glacier (2001–2002); (b,c) Comparison of annual surface speed change along transverse profiles T1 and T2. Profile locations are depicted in (a). Date format is month/day/year; Figure S12. (a) Surface velocity over the Singkhu Glacier (2001–2002); (b–d) Comparison of annual surface speed change along transverse profiles T1, T2 and T3. Profile locations are depicted in (a). Date format is month/day/year. Acknowledgments: The work in this study was supported by the National Key R & D Program of China (grant No. 2017YFA0603103), the National Natural Science Foundation of China (grant No. 41431070, 41590854, and 41621091); and the Key Research Program of Frontier Sciences, CAS (grant No. QYZDB-SSW-DQC027 and QYZDJ-SSW-DQC042). The authors would like to express their sincere thanks to the guest editors and three anonymous reviewers for their insightful comments and suggestions. The authors would like to thank the USGS for providing free Landsat ETM+ images and SRTM DEM, and thank Sebastien Leprince and Melanie Rankl for providing the COSI-Corr software package and the velocity data. Author Contributions: Liming Jiang conceived and designed the experiments; Yongling Sun performed the experiments and carried out the data analysis with the significant contributions of Liming Jiang. Lin Liu and Hansheng Wang made substantial contributions to the analysis and interpretation of the results; Yafei Sun focused on statistical computations; and Yongling Sun and Liming Jiang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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