Status of Major Glaciers of Hunza River Basin, Under Changing Climatic Conditions of Pakistan Over the Period of (1990-2018)

Ice masses and snow of Hunza River Basin (HRB) are an important primary source of fresh water and lifeline for downstream inhabitants. Changing climatic conditions seriously put an impact on these available ice and snow masses. These glaciers may affect downstream population by glacial lake outburst oods (GLOF) and surge events due to climatic variation. So, monitoring of these glaciers and available ice masses are important. This research delivers an approach for selected glaciers of the Hunza river basin. An attempt is made in this study using Landsat (OLI, ETM, ETM+, TM), digital elevation model (DEM), Geographic Information System and Remote Sensing techniques (RS&GIS) techniques. We delineated 27 glaciers within HRB from the period of 1990-2018. These glaciers' total area is about 2589.75 ±86km 2 in 1990 and about 2565.12 ±68km 2 in 2018. Our results revealed that from 2009 to 2015, glacier coverage of HRB advanced with a mean annual advance rate of 2.22±0.1 km 2 a -1 . Conversely, from 1994 to 1999, the strongest reduction in glacier area with a mean rate of - 3.126±0.3km 2 a -1 is recorded. The glaciers of HRB are relatively stable compared to Hindukush, Himalayan and Tibetan Plateau (TP) region of the world. The steep slope glacier's retreat rate is more than that of gentle slope glaciers, and the glaciers below elevation of 5000 m above sea level change signicantly. Based on climate data from 1995-2018, HRB shows a decreasing trend in temperature and increasing precipitation. The glacier area's overall retreat is due to an increase in summer temperature while the glacier advancement is induced possibly by winter and autumn precipitation. glacier


Introduction
Snow and ice masses in the high mountains of Hindukush, Karakorum and Himalaya (HKH) are the primary source of water for the mountainous population's living. These snow and ice masses are likely to be affected by changing climatic conditions of the region, but to what extent is yet unclear (Immerzeel et al., 2010). Changes in temperature and precipitation are expected to affect the melting characteristics of the cryosphere seriously. Hassan et al. (2017) have shown that the snow and ice melt contribution in river discharge is signi cant. In recent ten years, the ice caps and glaciers are retreating worldwide (Oerlemans et al., 2007). Himalayan Glaciers are important natural resources and feed the major rivers of South Asia. In the past 20 years, there is a decrease in the mass of Himalayan glaciers (Yutaka et al., 2001). Glaciers' ablation in the Himalayan region is due to decreased annual precipitation and an increase in average annual temperature (Ren et al.,2006). Glaciers are the good indicators of climate change in mountainous areas; however; mountainous region of HKH is poorly gauged. Most glaciers of the Himalayan region are shrinking (Kadota et al., 2000). IPCC (The Intergovernmental Panel on Climate Change) states that the declining of Himalayan glaciers is faster than any other region of the globe. If the earth's temperature is increased by the current rate, then these glaciers will be diminished by 2035 (IPCC, 2007). This may be due to the presence of Himalayan region near the equator, and a small increase in temperature cause huge impact and loss of fresh ice and glacier mass. Apart from Polar Regions, the Himalayan and Tibetan plateau has the world's most glaciated area (Racoviteanu et al., 2008). There is a signi cant climate trend that in uences glacier behavior (Bocchiola & Diolaiuti, 2013). The Himalayas is considered the third pole of the world (Kehrwald et al., 2008). It comprises ice bodies and glaciers over more than 60,000 km 2 and the largest ice mass source apart from the polar region (Gardelle et al., 2013). The major part of Pakistan's glaciated region is nested within Upper Indus Basin (UIB), which provides a signi cant amount of water for the downstream population and their livelihood. More than 20,000 glaciers in the HKH region, out of which 5000 glaciers are located in the Upper Indus basin (Inman, 2010).
The major sub-Basins of UIB are Hunza, Gilgit, Shigar, Shyok, Shingo and Astore rivers with a number of smaller and larger tributaries. Glaciers in UIB are majorly nourished by winter precipitation; avalanches winds can further add mass to the glacier ice which contributes to the positive mass balance of glacier (Akhtar, 2008). Avalanches may also transport debris load to glacier surface, later on debris cover lead to change the glacier response to the changes in climatic conditions by altering melting rate. Snow and ice melt contribute signi cantly in river discharge of UIB. More than 50% of the water in UIB is contributed by snow and ice melt. More than 7000 glaciers in the Karakorum region but only 15 largest glaciers of the region (Karakorum) cover the 50% of glaciers area (Kehrwald et al., 2008). Since the 20 th century there is a negative mass balance of Karakorum glaciers and mass loss of 5%, however, this mass loss is slowed down in 1970s (Mayewski & Jeschke, 1979). Some glaciers in this region are displayed positive mass balance (Kotlyakov, 1999). According to (Hewitt, 2005), from the 1920s to early 1990s, most of the Karakorum, Himalayan glaciers were also observed that these are retreating except in the 1970s. However, most of the Karakorum glaciers began advancing in the late 1990s (Hewitt, 2005). The glaciers present in Upper Indus Basin are retreating due to the increase in mean air temperature. Karakorum glaciers have unique behavior than rest of world glaciers.
Glaciers around the world are showing negative mass balance except for the glaciers of Central Karakorum region which is termed as "Karakorum Anomaly" these glaciers are either stable or advancing (Dehecq et al., 2019;Hewitt, 2005). Stability and advancement of Karakorum glaciers are also suggesting from snout monitoring in Central Karakorum about 50 % glacier's snout is either stable or advancing (Scherler et al., 2011). Karakorum glaciers' anomalous behavior is might be due to decreasing summer temperature and increasing precipitation trend in the region (Hassan et al., 2017).
The Karakorum glaciers' changes are different from the neighboring mountain ranges like Himalayas and Tangshan, where glacier are in negative mass balance. Climatic condition is one of the major factors which control the glacier uctuation in the Karakorum region (L Iturrizaga, 2011).
The aim and objectives of current study are to use Landsat satellite imagery between 1990 and 2018 to measure temporal and spatial variations of selected glaciers in Hunza river basin, western Karakorum (Fig. 1).

Study Area
Geographically Hunza river basin (HRB) is located in the Karakorum region (western Karakorum) and is a major sub-basin of Upper Indus Basin (UIB) Pakistan (Fig.1). The study area comprises of three districts of Gilgit-Baltistan (Hunza, Nagar and Gilgit). Hunza river basin is present between latitude 35º55′ to 37º06′N and longitude 74º03′ to 75º49′E. According to (Garee et al., 2017), the drainage area of Hunza river Basin is 13,567km², and the total glacier-covered area is 30% to 38% (Hakeem et al., 2014). There are 1384 glaciers, and about 1/3 glaciated area is covered by three major glaciers, namely, Hisper, Batura and Khurdopin (Immerzeel et al., 2010). Elevation of HRB ranges between 1394m to 27885 meters above sea level (m.a.s.l). Glaciated area of Hunza river basin lies between 2280m to 7850m, and the non-glaciated area lies between 1460m and 7570m. 1/3 glaciated area is covered by three major glaciers, namely, Hisper, Batura and Khurdopin (Immerzeel et al., 2010). Common surging glaciers are present in the Karakorum region, and debris-covered glaciers are 32% of the total basin area (Hewitt, 2007). Climatically, the area is alternatively in uenced by mid-latitude winter Westerlies and Asian summer monsoon. The total annual precipitation is 840.8mm, 60 per cent of which is fallen in winters, indicating the main precipitation is brought into the Westerlies and Asian summer monsoon area. The study area is located in the subtropical climate zone and the valleys between elevation range of 2810 m and 3669 m a.s.l. has an average annual temperature of 2.8 to 6.5 o C (Qureshi et al., 2017).

Datasets Glacier Data
We analyzed the glacier changes within seven periods: 1990-1994, 1994-1999, 1999-2004, 2000-2004, and 2004-2009, 2009- Operational land imager (OLI), which are available from the U.S. Geological Survey (http://earthexplorer.usgs.gov/). Alaska Satellite Facility (ASF) Digital elevation model (DEM) having a spatial resolution of 12.5m is downloaded from https://earthdata.nasa.gov/eosdis/daacs/asf. All remote sensing tiles of ablation season (July to August) having minimum cloud cover (<10%) were acquired from 1990-2019 (Table 1). All remote sensing images were preprocessed (image enhancement, co-registration and image stacking, image mosaicking) and were projected into World Geodetic System 1984 (WGS84), Universal Transverse Mercator (UTM) zone 43 projection, using Environment for Visualizing Images (ENVI), Erdas Imagine and ArcMap.  Note: T max -mean maximum temperature, T min -mean minimum temperature, T s -mean summer temperature, T w -mean winter temperature and P-total NDSI measures the relative magnitude of the re ectance difference between visible (green) and shortwave infrared (SWIR). Firstly, we created our glacier inventory then we compared this with Randolph glacier inventory (RGI) version 6.0. After extraction of glaciers, we calculated the slope and Aspect of each glacier using ASF DEM. Finally, area and length of each glacier are calculated using geometry toolbox of ArcMap. All of the work is conducted in a system research institute (ESRI) ArcGIS 10.8 software. Additionally, it is for preprocessing remote sensing data like co-registration, image enhancement, and image mosaicking ENVI software is used. We have also used Google Earth is also used for identi cation of delineation of glacier boundaries.

Aspect and slope of glaciers
Using the approaches used in Manley (2008) and Paul et al. (2009), each glacier aspect is calculated. Using statistical tool of ArcMap tool, the mean gradient is estimated for each glacier, and then used dominant aspect is used to name an aspect of each glacier. Firstly, for slope calculation, using median ltering removed potential outlier altitudes then extracted ASF DEM for each glacier, and the elevation range is calculated based on their outlines.

Climate Trend
Daily climate data taken from three stations of HRB (Khunjerab, Ziarat and Nalter) were processed. For this study 3 climate indices (T max , T min , precipitation) were selected to explore changes in the climate of HRB from 1995-2018 using the 'Rclimdex' package (Zhang and Yang 2004). The RClimDex program uses linear regression for trend calculations (Powell and Keim, 2015). Using the package as mentioned above of RStudio mean monthly and mean annual Temperature and Precipitation is calculated.

Errors and Accuracy Assessment
The error in glacier delineation comes mainly from the operator's experience who identi es and demarcates the glacier boundaries Xiang et al., (2014).
For the current study, the uncertainty is estimated based RS uncertainty approach proposed by Ye et al. (2006) and Li et al. (2015).
the area uncertainty is calculated byes Where, U A is Area uncertainty of glacier area, and U st is linear uncertainty which is given in eq.3. Area uncertainty ranges between 0.004 and 0.01 km 2 .
While the linear uncertainty is described as Where U l is linear uncertainty of glacier terminus, is the original pixel resolution of the satellite image, and is a co-registration error.

Variations in glacier length
The analysis of glacier length from 19990-2018 reveals interesting details (Table 4). Overall, for 27 studied glaciers, there is a decrease of 1.95% of the area. The glaciers show two phases. Firstly, most glaciers retreat until 2004 and then exhibit advancement up to 2015. However, an accelerated retreat in 1994-1995 is observed with peak retreat of about -3.92km variation in the terminus position of glaciers for different periods is depicted in Figure 7. It is also observed that the major changes occurred in low altitude (<5000m) glaciers of HRB. The glacier behavior is shown in gure 6.  (6), followed by south (3), east 2E) southeast (2) and southwest (2) respectively. North facing glaciers occupies about 36.53% of total area and 44.25% of the total length of studied 27 glaciers (Figure 2).
Each glacier slope is calculated separately and classi ed into four classes with a slope of 0-25 0 , 25 0 -30 0 , 30 0 -35 0 , and >35. For steep and very steep gradients the glacier length and coverage both reduced signi cantly (Fig 3). In 1990 17 glaciers were having a slope greater than 25 o and cover an area of 2041.8 ±39km 2 and length of 297.55±17km while 2018 there is 2027.31 ±36km 2 area and length of 289.17±17km respectively. Ten glaciers have a gradient of less than 25 o , but they show less reduction in both coverage and length than steep glaciers as general rule steeper glaciers have high glacier area and terminus loss than gentle slope glaciers. Note: the information about surge history and linkages are taken from the Randolph Glacier Inventory (RGI) 6.0.

Mean monthly climate
In Hunza river basin there are three meteorological stations (Khunjerab, Ziarat and Naltar) operated by WAPDA, the mean monthly climate indices (Tmin, Tmax and precipitation) indicates that temperature and precipitation increase from extreme north Khunjerab towards Nalter which is on the southern side. The monthly mean maximum temperature (Tmax) trend of Khunjerab stations remains below freezing point 7 months a year while the minimum temperature (Tmin) remains below 0°C ranges from 0°C throughout the year. Precipitation of region ranges from 7mm/month to 36mm/month, peak precipitation is observed in August. Data from Ziarat station shows that Tmin ranges from -13°C in January and February to 7°C in July and August. Precipitation of region ranges between 16mm/month in May to 28mm/month in December. Maximum Temperature (Tmax) and minimum temperature (Tmin) at Nalter ranges between -2°C and -9°C in January to 21°C and ten °C in August. Precipitation at Nalter is observed above 30mm/month each month while peak precipitation is recorded in April (97mm) and 31mm month of November respectively. Figure 4 shows the monthly climate of three station of HRB.

Discussion
To evaluate the total glacier area, change in response to climate change and behavior of glaciers of HRB is observed in this study. A complete analysis of glacier area change in response to climate change is di cult. There required time lag for glacier variation in climate change response (Pan et al., 2012). This time lag depends on thickness, size and type of glacier (Mao et al. 2010 andYao et al., 2004)and there is a direct relation of glacier response time to glacier thickness (Jóhannesson et al., 1989). Snow and ice masses in the high mountains of Hindu-Kush Karakorum and Himalaya (HKH) are the primary source of water for the mountainous population. The mountainous population of HKH depends on the upstream snow and ice reserves for seasonal water availability in rivers and streams. These snow and ice masses are likely to be affected by changing climatic conditions of the region, but to what extent is yet unclear (Immerzeel et al., 2010). Temperature is also one of the important factors of the glacier area change. Ablation of glaciers occurs due to an increase in temperature and decreased stability and accumulation of glaciers. Changes in temperature and precipitation are expected to seriously affect the cryosphere's melting characteristics (Milly et al., 2002). According to (Milly et al., 2002), there is a 0.40 o C increase in Pakistan's temperature in the last 40 years. The temperature of HKH has been reported to raise by 1.5°C, which is almost twice/double than the other parts of Pakistan, where the temperature has risen approximately 0.76ºC (Rasul et al., 2012). Glaciers of HKH are losing mass since last thirty-three decades (Mastny, 2000). The mountainous regions face more increase in temperature of 1.5 o C (Yasmeen & Javed, 2018) and mean annual precipitation of the Hunza river basin also shows an increasing trend. According to (K . Hewitt et al., 1989), Karakorum glaciers face three types of weather and two-third of snow accumulation occurs on major Karakorum glaciers in winter. Remaining one third accumulates in winter, which also suggests that monsoon advance over the area in many years (Mayewski & Jeschke, 1979). Negative net mass balance has been reported for the high mountain Asian glaciers between 2000 and 2016; however, some regional anomalies exist (Bocchiola & Diolaiuti, 2012 (Clarke, 2015). Karakorum glaciers are getting more attention because of anomalous behavior (Kääb et al.,2015). Surges on Khurddopin glacier have been documented since the late 1800s, and the most recent surges occur in1979 and 1999 (Copland et al., 2011). The velocity measurements of Karakorum glaciers have been shown that they are moving rapidly, with rates in the ablation zones ranging from 240m -460m per year (Pillewizer, 1957). Generally, Karakorum surging streams are steeper than others which are most reported. Hassanabad and Minapin glacier in Hunza-Nagar valley having the greatest elevation range for mountain surging glaciers, 5.5 km and 5.4 km, respectively, to the lower limit of their last surges (Hewitt, 1998). Our results indicate that overall, there is 0.12% decrease in the total glacier area of 27 glaciers. From 1990From -1998 there is 0.3oC decrease in temperature and 0.5 o C increase from 1998-2018. Precipitation also shows an increasing trend since 1990.

Conclusions
This research demonstrated that it is very good to use multi-temporal satellite images to study glacier area change where observational data records are insu cient. This study analyzed 27 glaciers of the Hunza river basin from 1990-2018 in ve intermediate periods (1994,1999,2004,2009