Article

Early 21st century climatology of snow cover for the western river basins of the Indus River System

Abstract and Figures

In this paper we assess the snow cover and its dynamics for the western river basins of the Indus River System (IRS) and their sub-basins located in Afghanistan, China, India and Pakistan for the period 2001-2012. Moderate Resolution Imaging Spectro-radiometer (MODIS) daily snow products from Terra (MOD) and Aqua (MYD) have been first improved and then analysed on seasonal and annual basis against different topographic parameters (aspect, elevation and slope). Our applied cloud filtering technique has reduced the cloud cover from 37% (MOD) and 43% (MYD) to 7%, thus improving snow cover estimates from 7% (MOD) and 5% (MYD) to 14% for the area of interest (AOI) during the validation period (2004). Our results show a decreasing tendency for the annual average snow cover for the westerlies-influenced basins (Upper Indus Basin, Astore, Hunza, Shigar, Shyok) and an increasing tendency for the monsoon-influenced basins (Jhelum, Kabul, Swat and Gilgit). Regarding the seasonal snow cover, decrease during winter and autumn and increase during spring and summer has been found, which is consistent with the observed cooling and warming trends during the respective seasons. Sub-basins at relatively higher latitude/altitude show higher variability than basins at lower latitude/mid-altitude. Northeastern and northwestern aspects feature larger snow cover. The mean regional snow line altitude (SLA) zones range between 3000 and 5000 m a.s.l. for all basins. Our analysis provides an indication of a decrease in the regional SLA zone, thus indicating a change in the water resources of the studied basins, particularly for the Upper Indus Basin (UIB). Such results are consistent with the observed hydro-climate data, recently collected local perceptions and glacier mass balances for the investigated period. Moreover, our analysis suggests some potential for the seasonal stream flow forecast as a significant negative correlation has been detected for the inter-annual variability of winter snow cover and value of the North Atlantic Oscillation (NAO) index of the previous autumn.
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Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014
www.hydrol-earth-syst-sci.net/18/4077/2014/
doi:10.5194/hess-18-4077-2014
© Author(s) 2014. CC Attribution 3.0 License.
Early 21st century snow cover state over the western river basins of
the Indus River system
S. Hasson
1,2
, V. Lucarini
1,4
, M. R. Khan
3
, M. Petitta
5
, T. Bolch
6,7
, and G. Gioli
2
1
CEN, Centre for Earth System Research and Sustainability, Meteorological Institute, University of Hamburg,
Hamburg, Germany
2
CEN, Centre for Earth System Research and Sustainability, Institute for Geography, University of Hamburg,
Hamburg, Germany
3
Department of Geo-informatics, PMAS-Arid Agriculture University, Rawalpindi, Pakistan
4
Department of Mathematics and Statistics, University of Reading, Reading, UK
5
Institute for Applied Remote Sensing, EURAC, Bolzano/Bozen, Italy
6
Department of Geography, University of Zurich, Zurich, Switzerland
7
Institute for Cartography, Technische Universität Dresden, Dresden, Germany
Correspondence to: S. Hasson (shabeh.hasson@zmaw.de)
Received: 21 August 2013 – Published in Hydrol. Earth Syst. Sci. Discuss.: 4 November 2013
Revised: 30 July 2014 – Accepted: 17 August 2014 – Published: 15 October 2014
Abstract. In this paper we assess the snow cover and its
dynamics for the western river basins of the Indus River
system (IRS) and their sub-basins located in Afghanistan,
China, India and Pakistanfortheperiod2001–2012.First,we
validate the Moderate Resolution Imaging Spectroradiome-
ter (MODIS) daily snow products from Terra (MOD10A1)
and Aqua (MYD10A1) against the Landsat Thematic Map-
per/Enhanced Thematic Mapper plus (TM/ETM+) data set,
and then improve them for clouds by applying a validated
non-spectral cloud removal technique. The improved snow
product has been analysed on a seasonal and annual ba-
sis against different topographic parameters (aspect, eleva-
tion and slope). Our results show a decreasing tendency for
the annual average snow cover for the westerlies-influenced
basins (upper Indus basin (UIB), Astore, Hunza, Shigar
and Shyok) and an increasing tendency for the monsoon-
influenced basins (Jhelum, Kabul, Swat and Gilgit). Seasonal
average snow cover decreases during winter and autumn,
and increases during spring and summer, which is consis-
tent with the observed cooling and warming trends during
the respective seasons. Sub-basins at relatively higher lati-
tudes/altitudes show higher variability than basins at lower
latitudes/middle altitudes. Northeastern and northwestern as-
pects feature greater snow cover. The mean end-of-summer
regional snow line altitude (SLA) zones range from 3000
to 5000ma.s.l. for all basins. Our analysis provides an in-
dication of a descending end-of-summer regional SLA zone
for most of the studied basins, which is significant for the
Shyok and Kabul basins, thus indicating a change in their
water resources. Such results are consistent with the ob-
served hydro-climatic data, recently collected local percep-
tions and glacier mass balances for the investigated period
within the UIB. Moreover, our analysis shows a significant
correlation between winter season snow cover and the North
Atlantic Oscillation (NAO) index of the previous autumn.
Similarly, the inter-annual variability of spring season snow
cover and spring season precipitation explains well the inter-
annual variability of the summer season discharge from most
of the basins. These findings indicate some potential for the
seasonal stream flow forecast in the region, suggesting snow
cover as a possible predictor.
1 Introduction
Snow is an essential part of the climate system, with a large
influence on the hydrological cycle as well as on the atmo-
spheric processes due to its high albedo and low thermal con-
ductivity (Hall and Riggs, 2007). Snow is especially impor-
tant for the hydrological cycle, as large amounts of the water
Published by Copernicus Publications on behalf of the European Geosciences Union.
4078 S. Hasson et al.: Snow cover state over western IRS basins
supplies come from seasonal snowmelt at high latitudes and
in mountainous basins (Barnett et al., 2005). This is partic-
ularly true for the Indus basin, where snowmelt from the
Hindu Kush–Karakoram–Himalaya (HKH) ranges provides
the first water available after a long dry period (October–
March, Immerzeel et al., 2010) that is mainly used for irri-
gation and power generation (Hasson et al., 2013). Besides
its importance, the snowmelt contribution to the Indus flows
is not well known. Immerzeel et al. (2009) have reported the
snowmelt contribution to the upper Indus basin (UIB) flows
to be around 40%. Such an estimate is based on a mod-
elling study and is subject to various uncertainties associ-
ated with the modelling approaches. Nevertheless, in view of
the importance of snowmelt for the Indus basin, an accurate
quantification of snow distribution patterns and their climatic
properties is essential. Additionally, snow cover assessment
is required for the calibration/validation of distributed hydro-
logical models (Konz et al., 2010), and for the seasonal fore-
cast of freshwater supplies.
Consistent with the unprecedented global warming
(IPCC, 2013), the Himalayas have experienced significant
warming during recent decades (Shrestha et al., 1999;
Diodato et al., 2012). Consequently, the annual average
snow cover decreased by 16 % over the entire Himalayas
from 1990 to 2001 (Menon et al., 2010). A similar snow
cover trend has also been observed from 2000 to 2008 (Im-
merzeel et al., 2009), though some regional anomalies do ex-
ist. Similarly, most of the Himalayan glaciers have been re-
treating and losing mass since the end of the Little Ice Age,
where current observations show, on average, an acceleration
of such responses since the mid-1990s (Bolch et al., 2012).
In contrast, the UIB experiences unique signatures of climate
change, featuring cooling temperatures and increasing pre-
cipitation (Fowler and Archer, 2006). Tree-ring-based pre-
cipitation reconstruction further confirms that the last century
was the wettest in the last millennium in this region (Trey-
dte et al., 2006). The Karakoram glaciers within the UIB have
featured irregular behaviour for a long time, showing bal-
anced budgets and several advancing glaciers, especially dur-
ing the last decade (Hewitt, 2005; Bolch et al., 2012; Bham-
bri et al., 2013; Gardelle et al., 2013). Local narratives also
confirm the anomalous features of hydro-climatic changes
within the UIB (Gioli et al., 2014). Under such contrasting
hydro-climatic regimes, the prevailing snow cover state is
largely unknown, leading to uncertainties in the present and
future management of water resources.
The sparse network of short-length high-altitude meteoro-
logical stations within Pakistan makes it hard to assess the
detailed picture of snow cover dynamics on sub-basin and
regional scales. Regional snow surveys are also not possi-
ble in the HKH region, due to its complex terrain and harsh
environment. Furthermore, as snow features a high degree
of variability, it needs mapping at a fine temporal resolu-
tion, unlike glaciers, which require a fine planar resolution.
In this regard, the integration of remote sensing (RS) data
and methods with geographical information system (GIS)
techniques has proved its usefulness in mapping snow cover
in inaccessible areas (Max, 2001; Tong et al., 2009). More-
over, satellite sensors offer a unique opportunity for snow
cover monitoring, supporting the efficient management of the
snow-covered areas (Notarnicola et al., 2011; Thirel et al.,
2012). Some examples are the Moderate Resolution Imaging
Spectroradiometer (MODIS) sensor on-board NASAs Earth
Observing System (EOS) Aqua and Terra satellites (NASA
MODIS), the Advanced Very High Resolution Radiometer
(AVHRR on the NOAA-15 satellite), or the Interactive Mul-
tisensor Snow and Ice Mapping System – IMS, which incor-
porates a variety of satellite images from AVHRR, GOES,
and SSMI (Helfrich et al., 2007; Roy et al., 2010). How-
ever, remote sensing of snow is subject to various limitations,
such as obstruction of snow by dense vegetation, surface het-
erogeneity, spectral similarities between different objects in
mountainous areas, and cloud cover impeding the surface
view (López-Burgos et al., 2013; Hüsler et al., 2014). Di-
etz et al. (2012) provide a detailed review of the common
methods of snow cover mapping from the remotely sensed
data, along with their limitations and advantages. Unfortu-
nately, satellite data have a quite limited ability in retrieving
any direct information about the snow amount, such as snow
depth (SD) and snow water equivalent (SWE). Presently,
available satellite-based SD/SWE observations show large
discrepancies (Dong et al., 2005; Takala et al., 2011), and
their coarse resolution of around 25 km is unsuitable for
mountainous areas. This hinders an adequate analysis of the
snowpack water content in the region.
Recent studies have presented snow cover estimates for
Hunza, Astore and UIB (Immerzeel et al., 2009; Tahir et
al., 2011a; Forsythe et al., 2012). Such estimates are based
on the MODIS 8-day maximum snow cover product, which
is biased positive (Xie et al., 2009) and features cloud data
gaps. As clouds are one of the major problems for optical
remote sensing data, their presence in MODIS snow prod-
ucts prevents adequate snow assessment and introduces un-
certainties in the analysis (Hall et al., 2002). This empha-
sises the need to improve considered snow products by fill-
ing up cloud data gaps prior to their use in the analysis.
Since the MODIS daily snow product features a larger pe-
riod of high snow classification agreement between Terra
and Aqua (Wang et al., 2009), it is highly suitable for fill-
ing cloud data gaps. Furthermore, large-scale snow accu-
mulation/distribution processes are generally controlled by
synoptic-scale meteorological patterns and the large-scale to-
pographical properties (especially elevation and aspect). In-
stead, snow redistribution depends on the local topography
(especially slope) and local meteorological conditions on a
small scale (Kelly et al., 2003). In the present study, we
first validate the hyper-temporal MODIS daily snow prod-
ucts from Terra (MOD10A1) and Aqua (MYD10A1) against
Landsat TM (Thematic Mapper)/ETM+ (Enhanced The-
matic Mapper plus) images, and then improve the validated
Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014 www.hydrol-earth-syst-sci.net/18/4077/2014/
S. Hasson et al.: Snow cover state over western IRS basins 4079
snow product for clouds by applying a validated rigorous
non-spectral cloud removal technique. Selecting practical
study domains in view of melt-runoff modelling and wa-
ter resource management, we then present improved snow
cover estimates for the western river basins (Indus, Jhelum
and Kabul) and their six sub-basins against different topo-
graphic parameters (aspect, elevation and slope). In view of
scarce SD/SWE observations, for the first time we present
a comprehensive picture of precipitation input and its dis-
tribution over the main study region up to an altitude of
4500ma.s.l. from 36 meteorological stations. We also re-
port the precipitation estimates either collected during the
short-term field campaigns or from the stations not studied
here. Furthermore, we provide proxy evidence of a qualita-
tive change in the mass balance of existing glaciers by as-
certaining end-of-summer regional SLA zone tendencies. We
successfully link such findings to recent hydro-climatic sig-
nals as well as to the socio-economic vulnerability observed
over recent decades. In addition, we investigate whether the
North Atlantic Oscillation (NAO) can explain the snow cover
variability over the region, and whether such variability fur-
ther contributes to explaining the variability of melt-season
runoff with a reasonable lead time, in order to explore the
possibility of a runoff forecast well in advance for better
management of water resources in the region.
2 Study area
The study area covers the spatial domain of 30–38
N and
67–84
E and encompasses large part of the HKH ranges. It
comprises of three large trans-boundary river basins, such as,
Jhelum, Kabul and UIB and their six sub-basins, namely As-
tore, Gilgit, Hunza, Swat, Shigar and Shyok (Fig. 1), located
in Afghanistan, China, India and Pakistan. According to the
Indus water treaty (1960), Indus River and two out of its five
eastern tributaries, such as Jhelum and Chenab, are called
the western rivers of the Indus River System (IRS) (Mehta,
1988). Here, we consider only basins of Indus and Jhelum
rivers but include a basin of the Kabul River, which is a west-
ern tributary of the Indus River. The investigated river basins
are located at the boundary between two large-scale circu-
lation modes: the western mid-latitude disturbances and the
south Asian summer monsoon system (Hasson et al., 2014).
The hydrology of the high latitude/altitude (Hunza, Shigar,
Shyok and Astore) sub-basins is dominated by the precipi-
tation regime of the western mid-latitude disturbances dur-
ing the winter and spring seasons (Wake, 1987; Rees and
Collins, 2006; Hewitt, 2011; Hasson et al., 2013). Such high
latitude/altitude sub-basins, located in the rain shadow of
the western Himalayas, receive negligible precipitation from
the summer monsoon (Ali et al., 2009), which is mostly re-
stricted to the lower latitude/altitude (Swat, Jhelum, Kabul
and Gilgit) sub-/basins. Runoff from the study basins primar-
ily depends upon the slow (snow and glacier melt) and a fast
(rainfall) component in the higher and lower altitude sectors,
respectively (Archer, 2003; Ali and De Boer, 2007; Hasson
et al., 2013). It is confined to the summer months (June–
September) and provides almost 80% of the annual surface
water available within Pakistan (Ali et al., 2009). Based on
the hydro-meteorological characteristics of the study region,
Fowler and Archer (2005) have divided it into three major
categories:
High altitude glacier-fed basins with a large percentage
of glacier cover, whose runoff primarily depends upon
the glacier melt and strongly correlates with the concur-
rent summer temperatures. The snow distributionsignif-
icantly affects the timing and magnitude of the glacier
melt runoff from these basins.
Mid-altitude snow-fed basins with lower elevation and
smaller percentage of glacier cover than the glacier-
fed basins, whose runoff primarily depends upon the
snowmelt and strongly correlates with the previous win-
ter season solid precipitation.
Low-altitude foothill rain-fed basins, which mostly re-
ceive precipitation in a liquid form.
Though dominated by the slow runoff component, the hy-
drology of the snow- and glacier-fed basins can further be
differentiated on the basis of their runoff production time
e.g. the peak runoff during June and August, respectively.
Therefore, we have considered the glacier-fed (Hunza, Shi-
gar, and Shyok) and snow-fed (Jhelum, Kabul, Gilgit, As-
tore, and Swat) basins separately for our further investiga-
tions (Fig. 1, Table 1).
The considered basins feature conflicting signals of cli-
mate change. For instance, almost half of the observational
record within the UIB shows a cooling tendency of the
mean annual and seasonal temperatures, except during the
winter season since the 1960s (Fowler and Archer, 2006).
Consequently, the melt season runoff is declining (Khattak
et al., 2011). Furthermore, the diurnal temperature range
is widening in the UIB throughout the year (Fowler and
Archer, 2006), while it has been narrowing worldwide since
1950 (Karl et al., 1993; Easterling et al., 1997). However,
a statistically significant increase for the summer, winter
and annual precipitation has been observed during the sec-
ond half of the 20th century (Archer and Fowler, 2004).
Such hydro-meteorological phenomena along with subse-
quent heterogeneous responses from the existing cryosphere
determine the overall hydrological balance of the study
basins and the water availability downstream in an otherwise
very arid land.
www.hydrol-earth-syst-sci.net/18/4077/2014/ Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014
4080 S. Hasson et al.: Snow cover state over western IRS basins
Figure 1. Study area for snow cover mapping showing three major western river basins, namely Indus, Kabul and Jhelum and their six
sub-basins. Swat is a sub-basin of the Kabul basin, whereas Gilgit, Hunza, Shigar, Shyok and Astore are the sub-basins of UIB. The political
boundaries are tentative.
Table 1. Study basins, their areas and glacier coverage. Note:
glacier areas are derived from the Randolph Glacier Inventory
(Pfeffer et al., 2014).
S. no. Basin at gauging site Area Glacier area
(km
2
) (%)
1 Astore at Doyian 3897 14
2 Gilgit at Gilgit 12652 7
3 Hunza at Dainyor Bridge 13705 28
4 Jhelum at Azad Patan 27291 1
5 Kabul at Nowshera 88676 2
6 Swat at Chakdara 6080 3
7 Shigar at Shigar 6974 30
8 Shyok at Yugo 138836 6
9 UIB at Besham Qila 271359 7
3 Data
3.1 Snow cover data
The MODIS sensors on-board both the Terra and Aqua satel-
lites, passing over the same area in the morning and af-
ternoon, respectively, provide imagery with twice a daily
temporal resolution. A normalised difference snow index
(NDSI) along with the normalised difference vegetation in-
dex (NDVI) is used to detect snow from the MODIS im-
agery through an automated procedure. Snow products are
then produced ranging from a swath level to a composite
global climate modelling grid (CMG) after spatial and tem-
poral transformations. Each higherlevel snow product, there-
fore, assimilates accuracy and errors from its precedingprod-
uct (Riggs and Hall, 2011). In version 5 of the snow products,
a surface temperature filtering has been applied to prevent the
erroneous mapping of the warm surfaces as snow due to their
spectral similarity (Riggs et al., 2006).
We use the MODIS daily snow products of 500m reso-
lution from both Terra and Aqua (MOD- and MYD-10A1)
version 5 (Hall et al., 2006) for the period 2001–2012 for
our analysis. The spatial resolution of 500 m has been con-
sidered highly suitable for estimating the snow cover of the
basins with an area of about 10000km
2
or larger (Hall et al.,
2002); however, it can still be useful for relatively smaller
basins. In order to cover the study area, we have downloaded
snow tiles of h23v5, h24v5 and h25v5 for both MODIS Terra
and Aqua from the online archive of the NASA Distributed
Active Archive Centre (DAAC) located at the National Snow
and Ice Data Centre (NSIDC).
3.2 Satellite images
In view of scarce observations, previous studies have vali-
dated the MODIS snow products over part of our study area
based on a proxy data set. For instance, Tahir et al. (2011a)
have validatedthe 8-daily MODIS Terra snow product for the
Hunza basin using images from Advanced Spaceborne Ther-
mal Emission and Reflection Radiometer (ASTER). Forsythe
Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014 www.hydrol-earth-syst-sci.net/18/4077/2014/
S. Hasson et al.: Snow cover state over western IRS basins 4081
et al. (2012) have validated the 8-daily MODIS Terra snow
product for the Astore basin against the snow cover re-
constructed from temperature records. Therefore, we have
decided to validate the considered MODIS snow products
against the Landsat images. We have downloaded 14 Land-
sat 5 TM and Landsat 7 ETM+ level 1T terrain corrected
images from the Glovis online archive (www.glovis.usgs.
gov). Landsat 7 ETM+ scenes are collected from 2001 to
31 May 2003 because scenes collected after this date feature
largedata gaps due to failure of the scan line corrector (SLC).
However, Landsat 5 TM scenes are collected within the pe-
riod 2001–2012, subject to their availability and clear sky
conditions. Most of the collected images (TM/ETM+) are
either cloud-free or observe cloud cover around 2%. These
images cover most of the maximum snow cover extent of the
study area (Fig. 2), and belong to both accumulation and ab-
lation seasons. Details of the Landsat 5 and 7 sensors and the
images used here are given in the Supplement.
3.3 Digital elevation model (DEM)
The gap-filled Shuttle Radar Topography Mission (SRTM)
digital elevation model (DEM) version 4 (Jarvis et al., 2008)
at 90 m resolution, from the CGIAR Consortium for Spa-
tial Information (http://srtm.csi.cgiar.org/), was used to de-
fine the topography and to delineate the watershed bound-
aries. It was interpolated to the MODIS snow product reso-
lution of 500m using the nearest neighbour method for the
calculation of the required topographic parameters.
3.4 Hydro-meteorological data
For meteorological observations, we have obtained the SWE
and precipitation data from all available sites within the Pak-
istan region of the study area. In 1995, the snow pillows
were installed at various sites in the HKH region of Pak-
istan during the second phase of the Snow and Ice Hy-
drology Project (SIHP) of the Water and Power Develop-
ment Authority (WAPDA), Pakistan through a joint venture
with the Canadian team (SIHP, 1997). Most of the installed
snow pillows, however, have so far faced technical issues
of interfacing with the transmission system as well as un-
expected “jumps” due to possible ice bridging and rupture
effects (SIHP, 1997). Therefore, presently, the SWE observa-
tion from the Deosai (from the 2007–2008 period onwards)
and Shogran (from the 2012–2013 period onwards) sites are
available, offering only 5-year long time series at the Deosai
station (2008–2012) within the analysis period (Fig. 3).
We have obtained the precipitation and temperature data
from 18 high-altitude (between 2200 and 4500ma.s.l.) SIHP
stations within the HKH region for the period 1995–2012.
These high altitude gauges measure both solid and liquid
precipitation in mm water equivalent. Such capability of
these high altitude precipitation gauges has allowed us to ac-
cumulate precipitation for the duration when temperatures
Figure 2. Landsat scenes used for validation of the MODIS Terra
and Aqua products, covering most of the maximum snow cover ex-
tent of the studied basins. Note: each colour indicates a unique lo-
cation (“Path” and “Row”) of the Landsat scene acquisition.
contiguously remained below zero, in order to roughly es-
timate the winter/spring time SWE. However, such esti-
mates may underestimate the actual amount of solid precipi-
tation due to under-catch errors of the precipitation gauges
under strong wind conditions. We have also collected the
long-term precipitation record from 18 stations of relatively
lower elevation (between 600 and 2200ma.s.l.) for the pe-
riod 1961–2010, which are being maintained by the Pakistan
Meteorological Department (PMD), Pakistan. The discharge
data have been obtained from the Surface Water Hydrology
Project (SWHP) of WAPDA, Pakistan for all the nine study
basins. The details of the collected hydro-meteorological
data are given in the Supplement.
3.5 NAO index
Additionally, the station-based seasonal mean
North Atlantic Oscillation (NAO) index (Hurrell,
1995) was downloaded from an online archive
(https://climatedataguide.ucar.edu/sites/default/files/
climate_index_files/nao_station_seasonal.ascii) of the
Climate Analysis Section, National Center for Atmospheric
Research (NCAR), Boulder, USA, to analyse the statistical
relation between snow cover and intensity and the position
of the storm track and the ensuing mid-latitude distur-
bances. The station-based NAO index is calculated from
the normalised sea level pressure differences between Ponta
Delgada, Azores and Stykkisholmur/Reykjavik, Iceland.
4 Methodology
4.1 Processing of MODIS snow products and their
validation
The MODIS Re-projection Tool (MRT) (Dwyer et al., 2001)
jointly with MODIS Snow Tool (MST) (Gurung et al., 2011)
allowed us to:
1. Mosaic the same day tiles h23v05, h24v05, h25v05 for
Aqua and Terra separately;
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4082 S. Hasson et al.: Snow cover state over western IRS basins
Figure 3. Snow water equivalent from snow pillows in the western
Himalayas. (a) Deosai station – 4149ma.s.l.; (b) Shogran station –
2170ma.s.l.
2. Re-project twice-a-daily tiles to the Geographical Coor-
dinate System (GCS);
3. Extract area of interest (AOI) covering all study basins.
The errors in the accuracy of MODIS snow products are
mainly due to the resemblance between snow and cloud
cover. For example, while analysing the MODIS daily snow
product (MOD10A1) for the upper Rio Grande River basin
for the period 2000–2004, Zhou et al. (2005) found a less
than 10 % omission error as a misclassification of snow as
land and vice versa. However, they found a high omission
error of around 50 % as a misclassification of snow or land
as cloud. Therefore, we have validated the considered snow
products over the study region prior to their use in our anal-
ysis. We have adopted the similar methodology of snow ex-
traction from the Landsat images as applied to the MODIS
snowproducts. We have calculated the NDSI and NDVI from
the reflectance of the Landsat TM/ETM+ optical data (bands
2, 3, 4 and 5) and brightness temperatures from the radiance
of thermal data such as band 6 for TM and 6-1 for ETM+
(details are in the Supplement). For the non-vegetated areas,
we have extracted the snow using a threshold of NDSI> 0.4,
while for highly vegetated areas (NDVI> 0.1), a threshold
of 0.28< = NDSI< = 0.4 was applied, provided that the re-
flectance in band 4 was greater than 0.11 and that in band 2
it was greater than 0.10. Mapping of water bodies as snow
was avoided using negative NDVI values. Finally, the resul-
tant snow maps were screened to exclude warm objects de-
tected as snow by applying a brightness temperature thresh-
old of greater than 283K. Snow estimates from the MODIS
snow products and the Landsat TM/ETM+ scenes were then
compared under clear sky conditions. In order to assess the
agreement between the snow estimates of the two data sets,
the mean absolute difference (MAD) was calculated using
Eq. (1).
MAD =
P
Snow
MYD/MOD
Snow
Landsat
n
(1)
4.2 Cloud removal technique and its validation
In order to minimise the cloud data gaps from the val-
idated MODIS snow products, we have applied a rigor-
ous non-spectral cloud removal technique, partially follow-
ing Wang et al. (2009), Gurung et al. (2011) and López-
Burgos et al. (2013). These studies have found the adopted
cloud removal technique remarkably efficient in cloud re-
duction/removal from the MODIS snow products, with few
season-dependent trade-offs, resulting in snow maps in good
agreement with the ground snow observations. However,
these studies suggest numerous cloud removal steps, where
each successive step removes more cloud but is subject to
a high probability of information loss (Gafurov and Bár-
dossy, 2009). We select a few but robust cloud removal steps
and briefly discuss here their functionality and theoretical ac-
curacy.
In the first step, we have merged both MODIS Terra and
Aqua same day images in order to remove short-persisting
clouds. We take the MODIS Terra snow product as base as
it experiences relatively less cloud cover than its counterpart
(Parajka and Blöschl, 2008; Wang et al., 2009). We replace
the cloudy pixels of the MODIS Terra snow images with the
corresponding cloud-free pixels of same day Aqua images.
This step results in a single snow image per day. This step is
most effective and accurate as both the observations are only
few hours apart just 3–4h at the Equator (Gafurov and
Bárdossy, 2009).
As for temporal filling, we replace the present day cloudy
pixel with a corresponding previous day snow pixel. In tem-
poral analysis, if both the previous and next day correspond-
ing cloud-free pixels observe the same class, the present
day corresponding cloudy pixel is replaced by that class.
Here, we assume that the snow cover often persists more
than one day, thus the present-day cloudy pixel can be re-
placed with the corresponding previous day snow pixel. If
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S. Hasson et al.: Snow cover state over western IRS basins 4083
the previous-day snow observation was taken from the Aqua
image as a result of the first step of the cloud removal tech-
nique then it is only few hours apart from the present day
cloudy pixel. During such time, three possibilities of either
rapid snowmelt, snow fall or no change can occur. Possibil-
ity of rapid snowmelt is limited because the present day is
cloudy, blocking incoming solar radiation (Gafurov and Bár-
dossy, 2009), while snow fall or no change possibilities still
favour our hypothesis. Though the MODIS Terra and Aqua
snow products are in good agreement, these products still
have little differences, particularly during the transitional pe-
riod (Wang et al., 2009). These differences are attributed to
change of a pixel from land to snow in case of snow fall and
snowto land in case of rapid melting during the time between
MODIS Terra and Aqua acquisitions (Prajka and Blöschl,
2008). Snow redistribution by a wind blow can also result
in differences among Terra and Aqua acquisitions. Effects of
such sub-daily variations cannot be accounted for in this step
due to the limited availability of sub-daily scaleobservations,
and because our analysis considers only one observation per
day. This information loss even exists under clear sky condi-
tions.
As for spatial filling, we have replaced a particular cloudy
pixel with the dominant class observed among its neighbour-
ing pixels. Such a replacement cannot always be accurate,
but its probability of being accurate is high. A major prob-
lem can be the patchiness of snow, resulting in spurious in-
formation. Therefore, we have considered the spatialwindow
size of 7× 7, in order to decide the new class of the cloudy
pixel based on a reasonable number of neighbouring pixels.
Gurung et al. (2011) have reported this window size as op-
timal for the spatial filling in order to remove/reduce cloud
cover from the MODIS snow products. We first combined
MODIS Terra and Aqua images, and then applied temporal
filling, spatial filling, and temporal analysis. In order to avoid
the influence of the remaining cloud cover, daily snow cover
estimates for each basin with less than 10% cloud cover only
were considered for further analysis.
We have validated our adopted cloud removal technique
before its use to improve the snow products. Since we only
have SWE observations (2008–2012) from the Deosai sta-
tion, we have decided to perform both absolute and relative
validation. For absolute validation, we compare the MODIS
snow products against the Deosai SWE observations (Fig. 3).
The installed snow pillow measures incident snow in units
of 3.18mm water equivalent. Therefore, we have chosen a
threshold of SWE> = 3.18mm to decide the existence of
snow, and a threshold of SWE= 0 for clear land/no snow.
We have provided confusion matrices for the absolute vali-
dation (Table 2), as well as the estimates of overall accuracy
(Eq. 2) and snow miss (Eq. 3) and false alarm (Eq. 4) rates.
Overallaccuracy =
(
a
1
+ b
2
)
(
a
1
+ b
1
+ a
2
+ b
2
)
(2)
Table 2. Confusion matrices for the MODIS snow products against
in situ SWE observations at the Deosai station for the period 2008–
2012.
Observations Snow No snow/
MODIS snow product(s) clear land
Snow (SWE> = 3.18 mm) a1 b1
No snow/clear land (SWE= 0mm) a2 b2
Snowmissrate =
(
b
1
)
(
a
1
+ b
1
+ a
2
+ b
2
)
(3)
Falsealarmrate =
(
a
2
)
(
a
1
+ b
1
+ a
2
+ b
2
)
(4)
a
1
represents the total number of correct snow hits, and b
2
represents the total number of correct land/no-snow hits. The
variable b
1
represents the total number of occurrences when
the MODIS product indicates no snow/clear land, but ground
observationshows the existence of snow. The total number of
situations where the MODIS product indicates the existence
of snow, but the observation suggests no snow/clear land, is
represented by the variable a
2
.
For the relative validation, the year 2004 was taken as
a validation period for a two-fold reason; it was the first
wet year after a long drought (1998/99–2002/03) over the
Indus basin (Levinson and Waple, 2004; Baig and Rasul,
2009), and it experienced the maximum cloud cover condi-
tions. Within the validation period, we have taken ten pairs
of same-day Terra and Aqua snow images that feature large
cloud cover differences, and become nearly cloud free when
the cloud removal procedure is applied. Note that the same-
day Terra and Aqua images are two independent observa-
tions, acquired at different times of the day but processed
with a similar approach (Parajka and Blöschl, 2008). We first
make the Terra snow images cloud free by masking out their
cloud covers, and then apply these Terra cloud masks to the
corresponding same-day Aqua images. At this stage, the rest
of the cloud cover of the Aqua snow images represents the
areas which are cloud free in the corresponding same-day
Terra snow images. We apply all the steps of the cloud re-
moval technique, except the first one, to the new Aqua snow
images to remove their remaining cloud cover. The resultant
snowand cloud cover estimates are then compared to the esti-
mates from the same-day Terra snow images, and MAD was
calculated using Eq. (1). We considered the performance of
our cloud removal technique to be satisfactory if it reduced
the cloud cover to less than 10% of AOI for the validation
period.
4.3 Snow cover estimates and precipitation distribution
We have estimated the snow cover against elevation, slope
and aspect from the improved snow product by dividing
the basin areas into 500m elevation zones, 10-degree slope
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4084 S. Hasson et al.: Snow cover state over western IRS basins
zones and 22.5-degree aspect zones (Figs. 11 and 13). We
have ascertained the mean seasonal snow cover trends for the
four time slices, namely the autumn (September–November),
winter (December–February),spring (March–May) and sum-
mer (June–August) seasons, in addition to mean annual
trends for all the studied basins. It is pertinent to mention
here that the unavailability of SD/SWE from the observa-
tions and the unsuitability of such parameters derived from
remotely sensed data sets have restricted our analysis to snow
cover assessment only. However, we have analysed precip-
itation records from 36 low- and high-altitude stations on
seasonal and annual timescales in order to present a com-
prehensive picture of moisture input to the main parts of
the study region. We also estimate the winter/spring SWE
by accumulating precipitation from the SIHP stations for the
period during which temperatures contiguously remained be-
low zero.
In order to obtain a qualitative mass balance response
of the existing glaciers to the prevailing climatic regime
of the study area, we estimate the end-of-summer regional
SLA zones from height-dependent snow cover estimates,
and ascertain their tendencies. Alternatively, variations in
the accumulation area ratio (AAR), in general, can also be
used in such regards. However, a number of glaciers in our
high-relief study area feature the avalanche-fed accumula-
tion regime (Hewitt, 2011). For such glaciers, AAR can-
not always be related successfully to their mass balances
(e.g. the Dunagiri and Changme–Khangpu glaciers in the Hi-
malayas), especially when a short time series of AAR isanal-
ysed (Dyurgerov et al., 2009). The balanced-budget AAR
for these glaciers, however, is reported to be less than 0.5
(Kulkarni, 1992; Braithwaite and Raper, 2009). In view of
such limitations of AAR for the study area, we have analysed
the end-of-summer regional SLA zone. We also compare the
inter-annual variability of the SLA zone against the median
elevation of basin-hosted glaciers, which is a reasonable to-
pographical data-based proxy for the long-term equilibrium
line altitude (ELA the altitude where net mass gain/loss is
zero) of the glaciers (Braithwaite and Raper, 2009).
4.4 Tele-connections
The hydrology of the studied basins dominates, with mois-
ture inputs from westerly disturbances. The frequency,
strength and track of the westerly disturbances are influenced
by the pressure conditions over the North Atlantic, which
can be explained by the North Atlantic Oscillation (NAO)
index (Hurrell, 1995; Syed et al., 2006). The variation in the
NAO index affects precipitation and temperature over north-
ern Europe (Fowler and Kilsby, 2002) and at the further-
most extent overthe study region.Syed et al. (2006) explain
that during the positive NAO phase, the southern flank of the
Mediterranean storm track becomes intensified over northern
Iran and Afghanistan due to the enhanced low pressure over
Afghanistan and central Asia. Such conditions possibly fea-
ture an additional moisture transport from the Caspian and
Arabian seas resulting in a positive precipitation anomaly
to the study area as well as to a large part of the central
southwestern Asia region. The El Niño–Southern Oscillation
(ENSO) during its warm phase also leads to similar condi-
tions over the study region (Shaman and Tziperman, 2005;
Syed et al., 2006). Here, considering the atmospheric-only
mode of the global index, i.e. NAO, we explore whether such
an index can partly explain the variability of snow cover with
a possible lead time and whether such variability contributes
further to the variability of the melt-season stream flow. Our
analysis is based on studying the simple Pearson correlation
between two time series of equal length (2001–2012). We
have correlated the seasonal NAO index for several seasons
with the winter season snow cover, in order to explore the
effect of changing the lead time.
5 Results
5.1 Validation of MODIS snow products
Our validation results show that MAD between the snow
cover estimates from MODIS and Landsat is around 2% of
the mean investigated area for both Terra and Aqua prod-
ucts (Fig. 4, Table 3). Whereas, the relative difference (dif-
ference in MODIS and Landsat snow cover divided by Land-
sat snow cover) for each individual pair ranges from 0.1 to
25% (Table 3). We have found that MODIS slightly overes-
timates the snow cover during the spring and summer sea-
sons, while it underestimates it during the autumn season,
relative to the Landsat images. Tang et al. (2013) have also
found such an overestimation from the MODIS product for
the Mount Everestregion. The underestimation, however, has
been observed under highly patchy snow conditions, mainly
over the Jhelum basin (Path 149, Row 36), and during the
autumn season (days of year 276 and 290). In fact, the fine
resolution of Landsat TM/ETM+ as compared to MODIS
has allowed the precise detection of such patchy snow cover.
Overall, we have found the MODIS snow cover estimates in
good agreement with the Landsat data under clear sky condi-
tions.
5.2 Validation of cloud removal technique
Our absolute validation shows that under clear sky condi-
tions, the MODIS Terra and Aqua snow products feature
an overall accuracy of 86 % and around 85%, respectively,
against the observations at Deosai station. Both the snow
miss and false alarm rates for the MODIS Terra product are
around 7 %. For the MODIS Aqua product, false alarm rate
is around 6% while the rate of snow miss is around 9%. Rel-
atively higher snow miss rate for the MODIS Aqua product
is due to its larger cloud coverage observed as compared to
the MODIS Terra product – 40% against 36% at Deosai lo-
cation for the period 2008–2012.
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S. Hasson et al.: Snow cover state over western IRS basins 4085
Table 3. Results of the MODIS Terra and Aqua product validation against the Landsat images under clear sky conditions.
S. Landsat scene identifier MODIS Landsat Relative Absolute MODIS Landsat Relative Absolute
no. Terra snow snow diff. diff. Aqua snow snow diff. diff.
(km
2
) (km
2
) (%) (%) (km
2
) (km
2
) (%) (%)
1 LE71490362001097SGS00 15 117 15 535 3 1.3
2 LE71500362001120SGS00 4592 4244 8 1.1
3 LE71480352001138SGS00 13 120 12 909 2 0.6
4 LE71510352001143SGS00 8107 7592 7 1.6
5 LE71490352002100SGS00 20 365 19 225 6 3.5
6 LE71490362002100SGS00 19 344 19 309 0 0.1
7 LE71470352002102SGS00 7225 7755 7 1.6
8 LE71500352002155SGS00 10 116 9931 2 0.6
9 LE71470362002214SGS00 4174 3468 20 2.1
10 LE71490362002276SGS00 1269 1693 25 1.3 944 1073 12 0.4
11 LE71490352003119EDC00 21101 19653 7 4.4 21410 19 681 9 5.2
12 LT51500362009262KHC00 782 830 6 0.1 355 827 57 1.5
13 LT51490362010146KHC00 12582 10431 21 6.6 10845 10 365 5 1.5
14 LT51490362010290KHC00 1174 1506 22 1.0 810 1490 46 2.1
MAD Terra 1.85 MAD Aqua 2.14
Figure 4. Validation results of the MODIS snow products against
Landsat TM/ETM + images. (a) MODIS Terra product against
Landsat scenes; (b) MODIS Aqua product against Landsat scenes.
Note: each colour identifies a unique location (“Path” and “Row”)
of the Landsat scene acquisition, while each different shape identi-
fies a particular season.
Our cloud removal procedure fills up 77 % of the cloud
data gaps in the MODIS Terra product, and around 79% of
the cloud data gaps in the MODIS Aqua product, with an ac-
curacy of 90% against the observations. The procedure also
improves the overall accuracy of the MODIS Terra (86 %)
and Aqua (85%) products to 88%. In individual steps, com-
bining the MODIS Terra and Aqua products fills up 16% of
the total filled data gaps with an accuracy of around 89 %
against the observations (Table 4). The false alarm and snow
miss rates for this step are around 4 and 7%, respectively.
The temporal filling, spatial filling and temporal analysis
steps have filled up around 65, 8 and 14 % of the total filled
data gaps, respectively, with an accuracy of 90% against the
observations. Their false alarm rate is around 4%, while the
snow miss rate is around 6%. The snow miss and false alarm
rates for the filtered MODIS product are mostly observed
during the months of June and November. We have observed
that the snow miss rate is highly sensitive to the applied SWE
threshold in order to decide the presence of snow – if we in-
crease the threshold, the snow miss rate drops considerably.
Table 5a and b shows that clouds from the Aqua images
have been reduced considerably, andthe snow estimates have
consequently been improved, which are now comparable to
the MODIS Terra snow estimates. Figure 5 spatially illus-
trates the validation of the applied technique for day 96 of
the year 2004. The MAD between the MODIS Terra and
Aqua snow estimates is around 0.54%. This may partly be
attributed to the spatial cloud/snow cover differences be-
tween the same-day Terra and Aqua images (as depicted
by Table 5b). It is also partly due to the fact that cloud
has not been completely removed from the Aqua images,
because the first step in combining both same-day images
has been skipped during this validation process. During the
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4086 S. Hasson et al.: Snow cover state over western IRS basins
Figure 5. Spatial illustration of the cloud removal technique valida-
tion over the Aqua snow imagewith respectto theTerra snow image
for day 96 of the year 2004 over the whole AOI (Terra cloud mask
in black, snow cover in blue, and the Aqua cloud cover is shown
in grey). (a) Terra cloud cover, (b) Terra snow image after masking
out the Terra cloud cover, (c) Aqua snow image after masking out
the Terra cloud cover, (d) Aqua snow image after applying all cloud
removal steps, except for combining it with the Terra same-day im-
age.
Figure 6. (a) Cloud coverage from the MODIS Terra (MOD10A1),
Aqua (MYD10A1) and after each step of the cloud removal Tech-
nique for the validation period of year 2004. MOD10A1 and
MYD10A1 are original cloud cover estimates from MODIS Terra
and Aqua, respectively, while steps 1–4 indicate the cloud cover es-
timates after combining both Terra and Aqua products, and after
applying the temporal filling, spatial filling and temporal analysis,
respectively. (b) Same as Fig.6a, but for snow cover estimates.
Figure7. Cloud and snow cover of original MODIS Terra and Aqua
Images and after implementation of five steps for day 144 of year
2004, (a) original MODIS Terra, (b) original MODIS Aqua, (c) af-
ter combining Terra and Aqua, (d) after temporal filling, (e) after
spatial filling, and (f) after temporal analysis.
whole validation period of the year 2004, the applied cloud
removal technique has reduced the cloud cover from 37%
(MOD10A1) and 43% (MYD10A1) to 7%, improving snow
cover estimates from 7 % (MOD10A1) and 5% (MYD10A1)
to 14% for the whole AOI. In the individual steps, combining
the MODIS Terra and Aqua same-day images, the temporal
filling, the spatial filling and the temporal analysis have re-
duced the average cloud cover to 29, 9, 8 and 7 %, and have
improved the snow cover to around 8, 12, 13 and 14 % of the
whole AOI, respectively (Figs. 6a, b and 7).
5.3 Basin-wide snow cover estimates and precipitation
distribution
Note that our analysis was based on the snow cover esti-
mates for the days experiencing cloud cover less than 10 %
of the respective basin areas. Such days accounted for 65–
85% of the total number of days over the whole period for
all basins, except Shigar and Hunza sub-basins. We have ob-
served that winter (30–65%) and spring (60–80%) seasons
have the minimum number of days with less than 10% cloud
cover, whereas for summer and autumn seasons, the num-
ber of such days was always higher (i.e. 60–90%). The Shi-
gar and Hunza (high-latitude/altitude glacierised) sub-basins
have experienced only 45–50% of days of less than 10%
cloud cover for the whole period, whereas such days during
the winter season are about 15% for the Shigar sub-basin and
about 25% for the Hunza sub-basin.
5.3.1 Precipitation distribution
In Fig. 3, we showsnow-pillow-based measurements of SWE
from the Deosai and Shogran stations in the western Hi-
malayas. At the Deosai station, SWE ranges between 400
and 700 mm during the period 2008–2013, while for the
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S. Hasson et al.: Snow cover state over western IRS basins 4087
Table 4. Results of the absolute validation of the cloud removal technique; MODnMYD refers to MOD10A1 and MYD10A1 combined
images, TF refers to temporally filled images, SF refers to spatially filled images, and TA refers to temporally analysed images.
Clear sky conditions Cloudy days Snow miss rate False alarm rate Overall accuracy against
(%) (%) observations (%)
MODIS 895 7.2 6.8 86.0
MYD 946 9.0 6.2 84.8
MODnMYD 750 6.9 6.5 86.7
TF 258 6.3 5.6 88.1
SF 238 6.3 5.6 88.0
TA 203 6.3 5.4 88.2
MODIS cloudy conditions Filled cloudy days Snow miss rate False alarm rate Accuracy of filled data by cloud removal
(%) (%) (%) technique against observations (%)
MODnMYD 16 7.1 4.3 88.6
TF 65 5.9 3.9 90.2
SF 8 5.8 4.0 90.1
TA 14 6.0 3.7 90.3
Table 5. (a) Relative validation: snow cover estimates (%) for AOI, actual and after applying each cloud removal step for the selected dates
within the validation period of the year 2004. Note: MAD is 0.54. (b) Same as (a), but for the cloud cover estimates.
(a)
S. no. DOY Actual MYD snow MYD and MOD
(2004) after snow difference
MOD snow MYD snow masking TF SF TA
1 85 21.3 8.7 9.3 19.9 20.0 20.1 1.2
2 96 12.5 9.2 9.8 12.7 12.8 12.8 0.3
3 97 15.5 7.9 10.4 15.1 15.2 15.2 0.2
4 124 18.4 10.8 12.3 17.8 18.0 18.1 0.3
5 129 7.3 2.4 3.4 7.7 7.7 7.7 0.4
6 212 2.4 0.8 1.0 2.0 2.0 2.1 0.3
7 271 4.6 2.8 3.0 3.9 3.9 4.0 0.7
8 282 6.4 3.3 4.0 6.2 6.2 6.2 0.2
9 290 13.3 7.3 7.1 12.2 12.2 12.3 1.0
10 298 13.8 9.3 9.7 14.5 14.5 14.5 0.7
(b)
S. no. DOY Actual MYD cloud MYD and MOD
(2004) after cloud difference
MOD cloud MYD cloud masking TF SF TA
1 85 15.3 28.9 20.2 0.7 0.4 0.3 0.25
2 96 12.2 30.5 26.5 5.7 4.8 4.8 4.81
3 97 31.7 52.0 36.1 0.3 0.2 0.1 0.07
4 124 17.7 31.0 21.7 1.9 1.5 0.7 0.74
5 129 44.2 54.7 34.8 0.5 0.4 0.2 0.24
6 212 58.2 59.3 37.3 9.0 7.1 1.0 1.03
7 271 16.9 29.7 20.3 0.3 0.2 0.1 0.09
8 282 25.3 38.4 21.8 0.1 0.1 0.0 0.03
9 290 13.5 23.5 16.4 0.6 0.4 0.1 0.07
10 298 8.8 26.0 20.8 0.2 0.1 0.1 0.07
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4088 S. Hasson et al.: Snow cover state over western IRS basins
Figure 8. Seasonal and annual precipitation climatology from low-altitude (below 2200ma.s.l.) PMD stations (1995–2010) and from high-
altitude stations (above 2200 ma.s.l.) WAPDA stations (1995–2012) for: (a) winter, (b) spring, (c) summer, (d) autumn, (e) annual, (f)
climatology of annual precipitation accumulated during the period when temperatures contiguously remained below zero.
Shogran station, the SWE is above 700mm for the years
2012–2013. Figure 8a–f shows that the region receives most
of its moisture during the winter and spring seasons, while
there is relatively little moisture input during the autumn
season. The ratio of winter and spring precipitation to the
total annual precipitation ranges from 43 to 76% for high-
altitude (above 2200 m a.s.l.) stations (WAPDA), and from
26 to 77 % for low-altitude (below 2200 ma.s.l.) stations
(PMD). In the summer season, moisture input to the re-
gion (the Jhelum, Kabul, Swat and Gilgit basins/sub-basins)
is dominated by the south Asian summer monsoonal pre-
cipitation regime. The annual average precipitation ranges
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S. Hasson et al.: Snow cover state over western IRS basins 4089
Table 6. List of nine basins with total areas along with snow coverage (SC) % and trend slopes (2001–2012). Values in bold are statistically
significant; values in italic show a variability greater than or equal to 5%.
S. no. Basin at gauging site Snow cover % Snow cover % trend slope
Mean Mean Avg.
Min± SD Max± SD ± SD DJF MAM JJA SON Ann.
1 Astore at Doyian 2± 1 98± 1 47± 5 0.79 +0.49 +0.76 0.72 0.29
2 Gilgit at Gilgit 3± 1 90± 4 41± 4 0.58 +0.50 +0.77 +0.02 +0.16
3 Hunza at Dainyor Bridge 17± 6 83± 4 49± 3 1.07 +0.09 +0.38 0.30 0.12
4 Jhelum at Azad Patan 0.2± 0.2 77 ± 8 22 ± 2 +0.16 +0.47 +0.33 +0.23 +0.30
5 Kabul at Nowshera 1± 0.3 67 ± 5 18 ± 2 0.05 +0.11 +0.19 0.01 +0.12
6 Swat at Chakdara 1± 0.5 72± 9 27± 3 0.04 +0.39 +0.37 0.09 +0.15
7 Shigar at Shigar 25± 8 90± 3 58± 3 0.76 +0.30 +0.37 0.17 0.02
8 Shyok at Yugo 3± 1 44± 9 14± 2 0.21 0.63 +0.09 0.16 0.17
9 UIB at Besham Qila 4 ± 1 54± 7 21± 2 0.74 0.07 +0.21 0.13 0.15
from below 50 to around 1700mm for the low-altitude sta-
tions, and from around 200 to around 1400mm for high-
altitude stations (Fig. 8e). Within the Karakoram range, we
have found an annual averageprecipitation range from200 to
700mm at the Khunjrab and Naltar stations, respectively. For
the western Himalayas, this range is from 150 to 1400mm
approximately at the Astore and Saif ul Maluk stations, re-
spectively. Within the Hindu Kush range, annual averagepre-
cipitation ranges from below 50 to around 1700 mm at the
Gilgit and Chitral stations, respectively.
Our estimated winter/spring SWE (Fig. 8f) suggests that
the maximum solid moisture input to the region is around
400mm at Bursil and Saiful Maluk stations. For most of
the high altitude stations, estimated SWE is either similar or
higher than their total winter precipitation. The case of higher
accumulated precipitation indicates that the snow accumula-
tion at these locations extends well into the spring season.
We have noted that few stations at lower latitudes, such as,
Shangala, Shogran and Saif ul Maluk observe the opposite
case, indicating smaller amount of incident snow fall (Fig. 8f)
at these locations.
5.3.2 Snow cover
Shigar has the highest annual averagesnowcoverpercentage,
followedby Hunza, Astore and Gilgit (Fig. 9, Table 6). These
sub-basins have shown large year-to-year variation as com-
pared to other lower-latitude/altitude basins. On the other
hand, the Shyok sub-basin has the lowest snow cover per-
centage, mainly due to its large extent at lower latitudes in
the southeast.
The snow cover for the Astore and Gilgit sub-basins of the
UIB ranges from 2 ± 1 to 3± 1 % during the summer sea-
son, and from 98 ± 1 to 90± 4% during the spring season,
respectively. These basins experience relatively low cloud
cover, but high variability in both the accumulation and ab-
lation seasons. The sharp drop in the snow depletion curve
implies that both of the basins have a low glacier melt con-
70
Figure 9. Annual average snow cover for all basins for the period
2001–2012.
tribution, so that their hydrology mainly depends upon the
snowmelt (Fig. 10a–b). This is further clarified by the minute
minimum snow cover (near 0%) and the small glacier cover
reported for these basins.
For the Hunza and Shigar stations, the snow cover ex-
tent ranges from 17 ± 6 to 25± 8% as the minimum during
summer, and from 83± 4 to 90 ± 3% as the maximum dur-
ing spring, respectively. We can observe a relatively smooth
drop of the snow depletion curve during the ablation pe-
riod (Fig. 10c–d). Conversely to what we have observed in
the Astore and Gilgit sub-basins, here the sub-basins fea-
ture relatively low snow cover variability throughout the
year, which may be associated with their particular topo-
graphic characteristics. These basins extend into relatively
high-latitude/altitude zones that feature the hydro-climatic
conditions resulting in perennial snow and permanent ice
cover.
The snow cover for the Jhelum and Kabul basins ranges
from about 1% for both basins during summer to 75± 8 and
67± 5% during the spring season, respectively. These two
basins exhibit high snow cover variability during the accu-
mulation period and low snow cover variability during the
ablation period (Fig. 10e–f). We have observed great sim-
ilarity among the Jhelum and Kabul River basins in terms
of their minimum and maximum snow coverage and snow
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4090 S. Hasson et al.: Snow cover state over western IRS basins
Figure 10. (a–i): Mean snow cover and its variability over the period 2001–2012 for all basins. The blue shaded area shows the spread (mean
minimum and mean maximum), whereas the red line shows the mean of the snow cover percentage over the whole period. Note: (*) indicate
major basins.
depletion patterns (Fig. 10e–f). This observation is further
reinforced by the fact that both river basins provide a sim-
ilar contribution (about 16 %) to the annual average surface
water available in Pakistan (Ali et al., 2009). Swat, a compar-
atively smaller sub-basin of the Kabul basin, receives snow
coverage ranging from 1± 1 to 72± 9%. Its snow accumu-
lation/ablation patterns and other snow cover characteristics
are similar to those of the Kabul and Jhelum basins (Fig. 10i).
The mean annual cycle of the snow cover percentage for
the UIB and its major sub-basin Shyok has been found to
be quite different from that of the other basins. The snow
cover of these basins ranges from 4 ± 1 to 3 ± 1% in the
summer, and from 54± 7 to 44± 9% in the spring season,
respectively (Fig. 10g–h). Both of the basins showed high
snow cover variability in accumulation and ablation seasons.
Such variability was substantially pronounced for the Shyok
sub-basin throughout the year except during winter season
that feature large and persistent cloud cover. A large portion
of the northeastern Shyok sub-basin, characterised by a low
precipitation rate, has neither permanent snow nor a glacier
cover. Instead, the part of the Shyok sub-basin lying in the
Karakoram range features a high concentration of ice (He-
witt, 2007; Bhambri et al., 2013). Hence, its contribution to
the stream flow mainly comes from the glacier melt. For the
Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014 www.hydrol-earth-syst-sci.net/18/4077/2014/
S. Hasson et al.: Snow cover state over western IRS basins 4091
UIB, the minimum snow cover corresponds to the estimates
of its sub-basins Gilgit, Hunza, Shigar, Shyok and UIB itself,
so it exhibits the average effect of the contrasting hydrologi-
cal regimes of its sub-basins.
The analysis of annual data depicted consistency with the
worst drought in Pakistan, which spanned from 1998/99 to
2002/03 and weakened in 2003–2004 due to heavy winter
precipitations (Levinson and Waple, 2004; Baig and Rasul,
2009). Interestingly, our extracted mean annual minimum
snow cover extent for all basins except Astore and Hunza
generally corresponds to the areal extent of the existing
glaciers, with a slight systematic underestimation (Table 1).
Most of the mid-altitude basins, namely Jhelum, Kabul
and UIB including Swat and Shyok sub-basins show a vari-
ability 5 % in their mean maximum snow cover. The As-
tore and Gilgit sub-basins show high variability in their mean
snow cover. Among high altitude basins, only Shigar sub-
basin shows high variability in its mean minimum snow
cover. A large spread is found during the snow accumula-
tion and ablation seasons. Such variability is typically higher
for the snow-fed basins than for the glacier-fed basins. High
variability in the minimum, maximum and the annual aver-
age snow coverage directly affects the melt-water runoff con-
tribution on yearly basis, which may further contribute to the
inter-annual variability of the IRS flows.
5.3.3 Snow cover trends
For the annual average snow cover, we have found a slightly
decreasing trend for UIB and for all the sub-basins, except
Gilgit, and a slightly increasing trend for the rest of the
studied basins, though no trend was statistically significant
(Table 6). The hydrology of the basins showing a decreas-
ing trend is mainly influenced by the westerly disturbances,
while for the basins showing an increasing trend, it is mainly
influenced by the south Asian summer monsoon. On a sea-
sonal timescale, the winter and autumn seasons feature a de-
creasing trend for most of the studied basins. Instead, there
was an increasing trend for the Gilgit basin in the autumn
season and for the Jhelum basin throughout the year. Most
basins show an increasing trend in the rest of seasons, except
for Shyok and UIB, which have shown a decreasing trend for
the spring season. However, a statistically significant trend
was found only for the Jhelum basin in the summer season.
5.3.4 Height dependence of snow cover estimates
We find high seasonal variation of the height-dependent snow
cover for the snow-fed basins as compared to the glacier-fed
basins throughout the year (Fig. 11). The maximum snow
cover for the high-altitude/latitude basins is observed during
winter, while for lower latitude/mid-altitude basins, it occurs
during the spring season (Fig. 11). In most of the basins, a
disproportionately large fraction of the snow cover comes
from high-altitude zones, which include very small surface
12
0
10
20
30
40
0
20
40
60
80
100
0-5 10-15 20-25 30-35 40-45 50-55 60-65 70-75
Snow
Cover (%)
Astore
0
10
20
30
40
50
60
0
20
40
60
80
100
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Snow
Cover (%)
Astore
0
10
20
30
40
50
0
20
40
60
80
100
0-5 10-15 20-25 30-35 40-45 50-55 60-65 70-75
Snow
Cover (%)
Gilgit
0
10
20
30
40
50
60
0
20
40
60
80
100
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Snow
Cover (%)
Gilgit
0
10
20
30
40
0
20
40
60
80
100
0-5 10-15 20-25 30-35 40-45 50-55 60-65 70-75
Snow
Cover (%)
Hunza
0
10
20
30
40
50
60
0
20
40
60
80
100
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Snow
Cover (%)
Hunza
0
10
20
30
40
0
20
40
60
80
100
0-5 10-15 20-25 30-35 40-45 50-55 60-65 70-75
Snow
Cover (%)
Shigar
0
10
20
30
40
50
60
0
20
40
60
80
100
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Snow
Cover (%)
Shigar
0
10
20
30
40
0
20
40
60
80
100
0-5 10-15 20-25 30-35 40-45 50-55 60-65 70-75
Snow
Cover (%)
Jhelum
0
10
20
30
40
50
60
0
20
40
60
80
100
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Snow
Cover (%)
Jhelum
0
10
20
30
40
0
20
40
60
80
100
0-5 10-15 20-25 30-35 40-45 50-55 60-65 70-75
Snow
Cover (%)
Swat
0
10
20
30
40
50
60
0
20
40
60
80
100
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Snow
Cover (%)
Swat
0
10
20
30
40
0
20
40
60
80
100
0-5 10-15 20-25 30-35 40-45 50-55 60-65 70-75
Snow
Cover (%)
Kabul
0
10
20
30
40
50
60
0
20
40
60
80
100
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Snow
Cover (%)
Kabul
0
10
20
30
40
0
20
40
60
80
100
0-5 10-15 20-25 30-35 40-45 50-55 60-65 70-75
Snow
Cover (%)
Shyok
0
10
20
30
40
50
60
0
20
40
60
80
100
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Snow
Cover (%)
Shyok
0
10
20
30
40
0
20
40
60
80
100
0-5 10-15 20-25 30-35 40-45 50-55 60-65 70-75
Snow Cover
(%)
Elevation Zones (100 meters)
UIB
Zone Area (%)
0
10
20
30
40
50
60
0
20
40
60
80
100
0-10 10-20 20-30 30-40 40-50 50-60 60-70
Snow Cover
(%)
Slope (Degrees)
UIB
DJF
MAM
JJA
SON
Figure 11. Snow cover against elevation (left panels) and slope
(right panels) zones along with zone areas.
areas, such as zones above 5000ma.s.l. for the Astore, Swat
and Jhelum basins, above 5500ma.s.l. for the Kabul basin,
above 6000ma.s.l. for the Gilgit, Shyok and UIB basins, and
above 6500 m a.s.l. for the Hunza and Shigar basins. Con-
versely, the Jhelum, Kabul and Swat basins have consider-
able surface areas below 2000m a.s.l., which have a negligi-
ble snow cover percentage (Fig. 11).
We find falling tendencies of the end-of-summer regional
SLA zone for all basins except for the Astore sub-basin.
Such tendencies are statistically significant for the Shyok and
Kabul basins (Table 7). Furthermore, we find that the esti-
mated SLA zones are situated well below the glaciers’ me-
dian elevation within all basins (Fig. 12).
As for the snow cover relationship with slope, our findings
suggest that Gilgit, Hunza, Astore and Shigar basins have
experienced a low snow cover over higher slopes as accu-
mulated snow cannot stay longer at steeper slopes. Instead,
the Jhelum, Swat, Kabul, Shyok and UIB basins have expe-
rienced a low snow cover over lower slopes because their
large surface areas extend to warmer, less mountainous re-
gions (Fig. 11).
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4092 S. Hasson et al.: Snow cover state over western IRS basins
Table 7. Estimated end-of-summer mean regional SLA zone for
each studied basin. Note: statistically significant trends at the 95%
level are shown in bold and italic, while trends significant at 90%
are shown in bold only. Negative values indicate a drop in the SLA
zone, while positive values indicate its rise.
S. no. Basin Mean regional SLA zone trend
SLA zone slopes
1 Astore 4100–4200 17.13
2 Gilgit 3900–4000 9.79
3 Hunza 3400–3500 1.40
4 Jhelum 3600–3700 91.96
5 Kabul 3900–4000 40.21
6 Swat 4500–4600 6.64
7 Shigar 3800–3900 0.70
8 Shyok 4200–4300 9.09
9 UIB 3200–3300 20.63
5.3.5 Aspect-wise snow cover estimates
The quantitative snow cover dependence on aspect differs
from location to location because, e.g., precipitation is im-
pacted by the relationship between the aspect and the pre-
vailing wind direction. Hence, we have calculated north-to-
south (N–S), northwest-to-southeast (NW–SE), northeast-to-
southwest (NE–SW) and west-to-east (W–E) ratios of sea-
sonal snow cover for all the basins (Fig. 13 and Table 8). It
is found that N–S ratios are high during all seasons, with
the maximum during autumn. Such ratios are also higher
than the other aspect ratios during all seasons. Only the Shi-
gar and Shyok sub-basins experience a maximum N–S ra-
tio during the winter season. Similarly, NE–SW ratios are
high for all the basins and during all the seasons, except for
Jhelum and Kabul during spring, and only for Jhelum dur-
ing the summer season. Except for Astore in summer, all
the other basins have either high or similar NW–SE ratios.
The Shyok sub-basin and UIB experience low W–E ratios in
the spring and winter seasons, whereas a similarly low ra-
tio is found for the Hunza sub-basin only during the winter
season. By combining the information contained in Table 8
and Fig. 13, we have derived the fact that the N–S, NE–
SW and NW–SE ratios tend to be higher when the overall
snow cover was lower. It points to the fact that aspect is not a
very strong limiting factor for snow persistence in high, well
snow-fed basins during colder seasons. The snowmelt due to
direct sunlight becomes more relevant when climate condi-
tions are milder and/or snow precipitation is weaker. Overall,
the aspect-wise snow cover analysis shows that the Astore,
Gilgit, Hunza, Shigar, Shyok, Jhelum and UIB basins have
relatively larger basin areas at the northeastern and south-
western aspects, while they experience greater snow cover at
the northeastern to northwestern aspects during all seasons.
12
Elevation (meters) Elevation (meters) Elevation (meters)
2000
3000
4000
5000
6000
7000
8000
9000
2000
2002
2004
2006
2008
2010
2012
Astore
2000
3000
4000
5000
6000
7000
8000
9000
2000
2002
2004
2006
2008
2010
2012
Gilgit
2000
3000
4000
5000
6000
7000
8000
9000
2000
2002
2004
2006
2008
2010
2012
Swat
2000
3000
4000
5000
6000
7000
8000
9000
2000
2002
2004
2006
2008
2010
2012
Hunza
2000
3000
4000
5000
6000
7000
8000
9000
2000
2002
2004
2006
2008
2010
2012
Shigar
2000
3000
4000
5000
6000
7000
8000
9000
2000
2002
2004
2006
2008
2010
2012
*Shyok
2000
3000
4000
5000
6000
7000
8000
9000
2000
2002
2004
2006
2008
2010
2012
*Kabul
2000
3000
4000
5000
6000
7000
8000
9000
2000
2002
2004
2006
2008
2010
2012
Jhelum
2000
3000
4000
5000
6000
7000
8000
9000
2000
2002
2004
2006
2008
2010
2012
UIB
Figure 12. Median elevation of glaciers and inter-annual variabil-
ity of the end-of-summer regional SLA zone for each studied basin.
Note: basins marked with (*) feature statistically significant trends.
The median elevation of glaciers along with the minimum and max-
imum elevations shown in the box plot are time independent.
5.4 Tele-connections
We have found a negative correlation between the autumn
season (SON) NAO index and winter season snow cover for
all the studied basins, which was particularly significant for
UIB (Table 9). On the other hand, there was a significant pos-
itive correlation between the winter season (DJF) NAO index
and the winter season (DJFM) snow cover for the Jhelum
and Kabul basins. This may be due to the fact that local forc-
ing such as topography (Cintia and Uvo, 2003; Bojariu and
Giorgi, 2005), atmospheric aerosols (Cubasch et al., 2001),
and additionally the complex interplay between the mon-
soon and the westerly disturbances during the summer sea-
sons (Archer and Fowler, 2004), can all significantly alter the
NAO effect.
There is a possibility that the strong connection (correla-
tion coefficient greater than or equal to 0.3) found between
a short-length snow cover time series and a large-scale phe-
nomenon showing variability on quasi-biennial and quasi-
decadal timescales (Hurrel and Van Loon, 1997) – may result
from the anomalous behaviour of the used snow product. We
test this hypothesis against the long-term precipitation ob-
servations from PMD meteorological stations. We find, for
the 1961–2000 period, a strong positive correlation of the
Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014 www.hydrol-earth-syst-sci.net/18/4077/2014/
S. Hasson et al.: Snow cover state over western IRS basins 4093
Table 8. Seasonal aspect ratios of snow coverage for all the study basins.
S. no. Basin DJF MAM JJA SON
name
N/S W/E NE/SW NW/SE N/S W/E NE/SW NW/SE N/S W/E NE/SW NW/SE N/S W/E NE/SW NW/SE
1 Astore 1.2 1.0 1.1 1.1 1.2 1.0 1.1 1.1 1.3 1.0 1.1 0.9 1.7 1.0 1.3 1.2
2 Gilgit 1.2 1.0 1.2 1.1 1.3 1.0 1.1 1.1 1.2 1.1 1.1 1.1 1.5 1.1 1.2 1.3
3 Hunza 1.3 0.9 1.3 1.1 1.3 1.0 1.2 1.1 1.4 1.0 1.2 1.1 1.4 1.0 1.3 1.2
4 Jhelum 1.2 1.1 1.0 1.1 1.3 1.1 0.9 1.2 1.5 1.3 0.9 1.4 1.9 1.2 1.1 1.7
5 Kabul 1.2 1.2 1.0 1.2 1.2 1.3 0.9 1.2 1.3 1.3 1.0 1.1 1.5 1.4 1.0 1.5
6 Shigar 1.1 1.0 1.2 1.1 1.1 1.0 1.1 1.1 1.0 1.0 1.1 1.0 1.1 1.0 1.2 1.1
7 Shyok 1.8 0.9 1.5 1.1 1.6 0.9 1.4 1.1 1.3 1.0 1.2 1.1 1.7 1.0 1.4 1.3
8 Swat 1.2 1.0 1.1 1.1 1.2 1.0 1.1 1.1 1.2 1.2 1.0 1.2 1.4 1.1 1.1 1.4
9 UIB 1.5 0.9 1.3 1.1 1.4 0.9 1.2 1.1 1.3 1.0 1.2 1.1 1.6 1.0 1.3 1.4
13
Astore Gilgit Hunza
Jhelum Kabul Shigar
Shyok Swat UIB
0
20
40
60
80
100
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
0
20
40
60
80
100
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
0
20
40
60
80
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
0
20
40
60
80
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
0
10
20
30
40
50
60
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
0
20
40
60
80
100
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
0
5
10
15
20
25
30
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
0
20
40
60
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
DJF MAM
JJA SON
0
10
20
30
40
50
N
NNE
NE
ENE
E
ESE
SE
SSE
S
SSW
SW
WSW
W
WNW
NW
NNW
Figure 13. Aspect-wise distribution of snow cover (in percentage)
during the winter (DJF), spring (MAM), summer (JJA) and autumn
(SON) seasons for the nine study basins.
long-term winter season precipitation with the NAO index,
mainly for the November–January period. In Table 10, we
show only those stations that feature statistically significant
correlations of greater than or equal to 0.3 for at least one of
the seasons.
Our correlation analysis suggests a significant correlation
of the spring season snow cover, spring season long-term pre-
cipitation and estimated winter/spring SWE with the sum-
mer season discharge at nearby or corresponding discharge
stations of mainly the snow-fed basins (Tables 11–13). For
the glacierised basins, we note that the summer season mean
temperatures explain well the variability of summer season
discharge (Table 11).
6 Discussion
Understanding of the statistical properties of snow cover, of
its seasonal and inter-annual variability, and its slow trend
dynamics is crucial for understanding the hydrological sys-
tem and the role of snowmelt runoff within the Indus basin,
and has important consequences for the study of its socio-
ecological systems.
The MODIS instrument provides a good opportunity to as-
sess the snow cover dynamics over the study region, mainly
due to its high temporal resolution. The validation results of
the MODIS snow products are found to be consistent with
the previous studies performed over parts of the study region
(Tahir et al., 2011a; Forsythe et al., 2012) and over the neigh-
bouring regions within the Hindu Kush (Gafurov and Bár-
dossy, 2009) and the western Himalayan (Jain et al., 2008;
Chelamallu et al., 2014) ranges, all suggesting that the use
of the MODIS snow products is effective for the mapping of
snow cover under Himalayan conditions.
Relative and absolute validations of our cloud removal
technique suggest that the applied technique not only re-
moves the clouds but also improves the overall accuracy of
the used MODIS snow products against the observations. We
note that the cloud removal technique is unable to completely
remove the clouds from the snow products. Generally, this
happens when clouds persist longer than the window size
of a temporal filling or the cloud is larger in extent as com-
pared to the window size of the applied spatial filling. Such
conditions are observed over the study region during win-
ter and spring seasons. Therefore, our cloud removal tech-
nique does not perform well during respective seasons, par-
ticularly for the high-latitude/altitude glacierised sub-basins
(Hunza and Shigar) during winter season. Nevertheless, its
overall satisfactory performance encourages its application
to reduce/remove the cloud cover and improve snow cover.
In view of the influence of cloud cover on the analysis (Hall
et al., 2002), estimates of the snow cover-runoff relationship
and of the climate change impacts on melt water runoff
based on the MODIS snow products not improved for clouds
(Immerzeel et al., 2009; Tahir et al., 2011b; Forsythe et al.,
2012) – may be prone to larger uncertainties.
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4094 S. Hasson et al.: Snow cover state over western IRS basins
Table 9. Correlations between the NAO index and the snow cover (SC) of the studied basins. Correlations significant at the 90 % level are
marked in italic only, at the 95% level are marked in bold only, and at the 99% level are marked in bold and italic.
S. no. Basin SCA(DJF)– SCA(DJFM)– SCA(DJF)– SCA(DJF)– SCA(DJF)– SCA(DJF)–
NAO(DJF) NAO(DJF) NAO(ASO) NAO(SON) NAO(OND) NAO(NDJ)
1 Astore at Doyian 0.18 0.13 0.39 0.62 0.52 0.15
2 Gilgit at Gilgit 0.22 0.13 0.19 0.45 0.58 0.26
3 Hunza at Dainyor Bridge 0.10 0.04 0.41 0.66 0.67 0.39
4 Jhelum at Azad Patan 0.54 0.70 0.07 0.18 0.15 0.06
5 Kabul at Nowshera 0.52 0.59 0.30 0.01 0.20 0.11
6 Swat at Chakdara 0.43 0.35 0.03 0.27 0.40 0.15
7 Shigar at Shigar 0.06 0.11 0.28 0.50 0.34 0.32
8 Shyok at Yugo 0.45 0.22 0.59 0.74 0.51 0.06
9 UIB at Besham Qila 0.33 0.25 0.55 0.79 0.67 0.10
Table 10. Correlations between the NAO index and the long-term precipitation from PMD meteorological stations (1961–2000) within the
studied basins. Only those stations which feature a statistically significant correlation of greater than or equal to 0.3 for at least one of the
seasons are presented here. Correlations significant at the 90% level are marked in italic, and at the 95% level are marked in bold.
S. no. Lower-altitude stations Study basin DJF–winter DJF–DJF DJF–ASO DJF–SON DJF–OND DJF–NDJ
(PMD)
1 Astore Astore 0.26 0.27 0.06 0.04 0.13 0.31
2 Garhi Dupatta Jhelum 0.33 0.25 0.04 0.07 0.10 0.35
3 Muzaffarabad Jhelum 0.44 0.40 0.25 0.19 0.11 0.39
4 Skardu UIB 0.32 0.30 0.21 0.16 0.13 0.35
5 Chitral Kabul 0.14 0.20 0.38 0.39 0.06 0.22
6 Jhelum Jhelum 0.32 0.17 0.07 0.08 0.06 0.10
7 Murree Jhelum 0.41 0.31 0.00 0.05 0.11 0.35
Table 11. Correlation coefficients between the long-term seasonal precipitation (P ) or average temperature (Tavg), with the seasonal dis-
charge (Q). Note: values significant at the 95% level are marked in bold.
S. no. Station Basin P (DJF)–Q (MAM) P (MAM)–Q (JJA) Tavg (JJA)–Q (JJA)
1 Muzaffarabd Jhelum 0.78 0.46
2 Azad Pattan Jhelum 0.56 0.73
3 Garhi Dupatta Jhelum 0.37 0.64
4 Balakot Jhelum 0.37 0.64
5 Peshawar Kabul 0.37 0.60
6 Saidu Sharif Kabul 0.64 0.65
7 Drosh Kabul 0.57 0.56
8 Chitral Kabul 0.56 0.63
9 Astore Astore 0.29 0.58
10 Gilgit Gilgit 0.09 0.35
11 Gupis Gilgit 0.28 0.53
12 Skardu Shigar 0.41 0.12 0.48
13 Skardu Shyok 0.04 0.09 0.51
14 Gilgit Hunza 0.16 0.11 0.36
15 Skardu UIB 0.10 0.43
16 Saidu Sharif Swat 0.32 0.35
Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014 www.hydrol-earth-syst-sci.net/18/4077/2014/
S. Hasson et al.: Snow cover state over western IRS basins 4095
Table 12. Correlation coefficients for the estimated SWE (accumulated precipitation during periods when temperatures remained below
zero), with the summer discharge (Q) at the nearby or corresponding gauging stations. Note: values significant at the 95% level are marked
in bold, and at the 90% level in italic.
Basin
name
of the
gauging
stations
Burzil
Deosai
Hushe
Kelash
Khod
Khunjrab
Naltar
Rama
Rattu
Saiful Maluk
Shangla
Shendure
Shogran
Ushkore
Yasin
Zanipass
Ziarat
Astore 0.3 0.6 0.4 0.4
Gilgit 0.2 0.2
Hunza 0.1 0.0 0.2 0.2
Jhelum 0.8 0.3 0.5
Kabul 0.5 0.4 0.1 0.9 0.1
Shigar
Shyok 0.2 0.1 0.4 0.2
Swat 0.1 0.5
UIB 0.2 0.6 0.2 0.1 0.1 0.4 0.1 0.6 0.2
A slight underestimation of the glacier area mapped in the
Randolph Glacier Inventory (RGI Pfeffer et al., 2014) by
the MODIS minimum snow cover, for most of the studied
basins, is mainly due to the inability of the MODIS sen-
sor to detect debris-covered parts of the glaciers (Painter et
al., 2012), which are quite common in the region (Bolch et
al., 2012). Such underestimation is relatively pronounced for
the Hunza sub-basin, which has substantial debris-covered
glacier area. Additionally, discrimination between snow and
ice in the MODIS binary products might be poorly achieved
through automated global algorithm (Hall and Riggs, 2007;
Shea et al., 2013) and may also result in such underestima-
tion. On the other hand, Astore basin shows a large inconsis-
tency between its MODIS minimum snow cover and the total
glacier area. This may be attributed to the fact that the obser-
vation dates of the available glacier data used in RGI and of
the extracted minimum MODIS snow cover do not neces-
sarily coincide, plus the unavoidable issues associated to the
quality of data in a complex terrain (Painter et al., 2012) of
the HKH region. Coarser resolution of the MODIS products
as compared to the data used in RGI may also be responsi-
ble for lower estimates of snow cover as compared to glacial
extent.
We have found that basins under monsoon influence be-
have differently to basins under the westerlies influence.
In the context of increasing winter precipitation (Archer
and Fowler, 2004), decreasing mean annual snow cover for
westerlies-influenced basins indicates (1) enhanced melting
due to the observed warming during winter (Fowler and
Archer 2006; Khattak et al., 2011), and (2) the transforma-
tion of solid precipitation into liquid precipitation (Hasson et
al., 2014). On seasonal timescales, snow cover decreases dur-
ing the winter and autumn seasons, but increases during the
summer and spring seasons, in agreement with Immerzeel et
al. (2009) and with reports of cooling and warming trends
Table 13. Correlation coefficients between the seasonal snow cover
area (SCA) for spring (MAM) and winter (DJF) with the discharge
(Q) of the summer season (JJA). Note: values significant at the 95%
level are marked in bold, and at the 90% level in italic. The Shigar
gauging site is no longer operational, so far.
S. no. Basin SCA(DJF)–Q(MAM) SCA(MAM)–Q(JJA)
1. Atore 0.56 0.70
2. Gilgit 0.18 0.59
3. Hunza 0.42 0.49
4. Shigar
5. Shyok 0.35 0.54
6. UIB 0.01 0.47
7. Jhelum 0.05 0.57
8. Kabul 0.06 0.30
9. Swat 0.52 0.66
during the respective seasons (Fowler and Archer, 2006; Gi-
oli et al., 2014). It is pertinent to mention here that since we
do not know the water content of the snowpack, snow cover
increase or decrease does not necessarily correspond to an
increase or a decrease in the water resources. However, such
variations certainly contribute to the snowmelt runoff vari-
ability on spatio–temporal scales.
Here, we extend the general picture of precipitation dis-
tribution presented in section 5.3 by summarizing reports
from different studies. Winiger et al. (2005) have reported
the snow depths of around 1200 and 1800 mm at Dame
(36
01
0
N, 74
35
0
E at 3670 m a.s.l.) and Diran (36
04
0
N,
74
36
00
E at 4050ma.s.l.) stations, respectively, in Bagrot
Valley 20 km northeast of Gilgit, Karakoram range. They
also reported that, along the Gilgit-Khunjrab transect within
the Hunza basin, precipitation ranges between 600 and
1200mm within the altitude belt of 3500–4500 m a.s.l., of
which 90 % is incident as snow, while it is around 400mm
www.hydrol-earth-syst-sci.net/18/4077/2014/ Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014
4096 S. Hasson et al.: Snow cover state over western IRS basins
below 3000ma.s.l., occurring only 10% as snow. In the cen-
tral Karakoram, Hewitt (2011) has reported the maximum
precipitation at 4800ma.s.l., which occurs entirely in solid
form. At 4840ma.s.l., Batura Investigation Group (1979)
had reported the annual snowfall around 1034mm over the
Batura Glacier lying in the eastern part of the Hunza sub-
basin. For the altitudinal belt of 4800–5800ma.s.l. over the
Biafo and Hispar glaciers (along the transect of Hunza and
Shigar sub-basins), Wake (1987) reported that the snowfall
here generally exceeds 1000mm and at some locations it ex-
ceeds 2000mm water equivalent.
Relative to normal conditions, a fall in the end-of-summer
regional SLA zone features fewer glacier/ice/snow surface
areas exposed to melting at higher elevations, indicating a
two-fold positive change: (1) the glaciers lose relatively less
mass, and (2) the glacier/ice mass increases because of the
remaining accumulated snow. On the other hand, a rise in
the end-of-summer regional SLA zone features increased ex-
posure of snow and snow-free glacier areas to thaw condi-
tions at higher elevations, indicating enhanced melting. In
view of the absence of snowpack water content information,
we do not know how much moisture is accumulated due to
the observed descent of the SLA zone; however, such condi-
tions confirm at least less melting at higher elevations, indi-
cating a positive change in the water resources therein. The
placement of a regional SLA zone well below the median el-
evation of existing glaciers a proxy of ELA (Braithwaite
and Raper, 2009) also confirms an indication of a positive
mass balance of these glaciers. Such a proxy finding is fur-
ther confirmed by Gardelle et al. (2013), who show a possi-
ble slightly positive mass balance of the Karakoram glaciers
for the last decade, by Fowler and Archer (2006), showing
observed cooling of the summer season, and by Khattak et
al. (2011), reporting a subsequent reduction in the summer
season flows.
As meltwater largely contributes to the overall freshwater
availability in Pakistan (Immerzeel et al., 2010; Hasson et al.,
2014), less summer melt resulting from the above-described
situation leads to decreased water availability downstream,
indicating an alarming situation for the water resource man-
agement in the country. The findings based on modelling
studies, that initially the stream flow will increase due to
increased melting and then will abruptly decrease when
glaciers disappear (Rees and Collins, 2006; Akhtar et al.,
2008; Immerzeel et al., 2009; Tahir et al., 2011b), are quite
misleading at present for many reasons. These include the
fact that (1) the region has not yet completely followed the
warming signal of climate change as observed globally or
as projected by the present-day climate models (increase in
temperature), (2) climate models have generally been un-
able to describe such anomalies over the region (Hasson et
al., 2013, 2014), and (3) it is not known when the summer
cooling phenomena will come to an end. Most commonly
adopted scenarios of temperature increase and glacier area
decrease, therefore, presently seem inappropriate for short-
term scenarios (see Fowler and Archer, 2006, Khattak et al.,
2011, and Minora et al., 2013), but may be relevant for the
long-term future. Under such a scenario, we encourage the
modelling community to consider additionally the observed
hydro-climatic scenario, in order to assess the near-future
melt-runoff contribution to the hydrology of the region.
The significant correlation found for snow cover and long-
term precipitation with the NAO index suggests the fore-
cast possibility of these variables with a lead time of one
month up to a season. Archer and Fowler (2004) also found
a strong significant correlation between winter precipitation
and the November-to-January NAO index for a few stations
considered in the study. The strong correlation found be-
tween the stream flow observations and precipitation sug-
gests good potential for providing indicators of runoff vari-
ability during the melt season in the region. Based on such
a relationship, Archer and Fowler (2008) performed a sta-
tistically based seasonal stream flow forecast for the Jhelum
basin using long-term precipitation data. Similarly, the spring
season snow cover shows a strong correlation with the sum-
mer season discharge of the corresponding basins. Forsyth
et al. (2012) also reported a similar connection for the As-
tore basin. Though short in length, seasonal snow cover
shows a similar strength of correlation between NAO and
stream flow as per the long-term precipitation record. For
the south Asian summer monsoonal precipitation, Immerzeel
and Bierkens (2010) showed that spring season snow cover
over the Tibetan Plateau can serve as an important pre-
dictor, when combined with the global indices of ocean–
atmospheric modes such as NAO and ENSO. Therefore, our
findings here suggest the possibility of a statistically based
stream flow forecast for the region in advance – or at least an
opportunity to address the stream flow variability – allowing
snow cover as one of the predictors.
7 Conclusions
This study constitutes an effort to understand the present state
of a snow-cover regime and its dynamics and its temporal
and spatial variability in the region, taking into account dif-
ferent geophysical parameters. The data set time frame is too
short to allow for robust conclusions about the general be-
haviour and long-term changes of snow cover. However, ob-
served tendencies of snow cover and of the SLA zone further
confirm the trends in related variables as reported by differ-
ent studies (Archer and Fowler, 2004; Khattak et al., 2011;
Gardelle et al., 2013; Gioli et al., 2014). The main findings
of our study are summarised here:
The westerlies-influenced basins (UIB, Hunza, Shigar,
Shyok and Astore) show a decreasing snow cover ten-
dency, while the monsoon-influenced basins (Jhelum,
Swat, Kabul and Gilgit) show an increasing snow cover
tendency on an annual timescale. All basins show
decreasing snow cover trends during the winter and
Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014 www.hydrol-earth-syst-sci.net/18/4077/2014/
S. Hasson et al.: Snow cover state over western IRS basins 4097
autumn seasons, except for Gilgit during the autumn
season. The Jhelum basin shows an increasing snow
cover trend throughout the year, while the rest of the
basins show increasing snow cover trends for the spring
and summer seasons (except Shyok and UIB). However,
only the summer snow cover trend for Jhelum was sta-
tistically significant.
High variability found for the snow accumulation and
ablation seasons is relatively more pronounced for the
snow-fed than for the glacier-fed basins, and during
the winter and spring seasons than during the sum-
mer and autumn seasons. An east–west gradient is not
present in terms of snow cover and its variability. How-
ever, sub-basins at higher latitudes/altitudes show more
snow cover variability than basins at relatively lower
latitudes/mid-altitudes.
High seasonal variation with respect to elevation is
observed for the snow-fed basins as compared to the
glacier-fed basins throughout the year. The average re-
gional SLA zone for the Hunza and UIB basins ranged
from 3000 to 3500ma.s.l., for the Gilgit, Shigar, Jhelum
and Kabul basins from 3500 to 4000 m a.s.l., for the As-
tore and Shyok basins from 4000 to 4500ma.s.l., and
for the Swat basin from 4500 to 5000m a.s.l.
The Astore, Gilgit, Hunza, Shigar, Shyok, Jhelum and
UIB basins have comparatively larger areas at the north-
eastern and southwestern aspects, and greater snow
cover at the northeastern aspect. Northern aspects, as
expected, have in general more snow cover than south-
ern aspects. Such discrepancies become larger when
considering warmer seasons or basins at lower altitudes,
where temperature is a strong limiting factor for the
snow persistence.
The Gilgit, Hunza, Astore and Shigar basins have low
snow cover over higher slopes, whereas the Jhelum,
Swat, Kabul, UIB and Shyok basins have low snow
cover over lower slopes.
Under the prevailing climatic conditions, there was an
indication of a positive change in the frozen water re-
sources of the region, particularly for the UIB. This was
evident from the facts of (1) an observed increase in
winter and summer precipitation (Archer and Fowler,
2004), (2) a possible positive mass balance of the central
Karakoram glaciers (Gardelle et al., 2013), (3) decreas-
ing summer flows (Khattak et al., 2011), (4) a falling
end-of-summer regional SLA zone (significant for the
Shyok and Kabul basins), and (5) increasing summer
season snow cover. In contrast, a warming trend during
the winter season and a consequent increase in the win-
ter season flows (Khattak et al., 2011) indicated a possi-
ble seasonal shift in the snow distribution. Such a shift
depends partly on the precipitation input during the ac-
cumulation season and partly on the prevailing seasonal
temperature regimes. The observed trends agreed with
the recently collected local perceptions of the climate
change and variability (Gioli et al., 2014).
A significant correlation of snowcoverand precipitation
with the NAO index, and furthermore with the stream
flow, reveals the possibility of a short-term forecast of
water resources with a lead time from one month up to a
season, suggesting snow cover as one of the predictors.
The Supplement related to this article is available online
at doi:10.5194/hess-18-4077-2014-supplement.
Acknowledgements. The authors acknowledge the National Snow
and Ice Data Center (NSIDC) and the MODIS team for their roles
in making available the MODIS daily (Aqua and Terra) snow
cover products for the period 2001–2012. S. Hasson acknowledges
the support of BMBF, Germany’s CLASH/Climate variability
bundle project and landscape dynamics in southeastern Tibet and
the eastern Himalayas during the late Holocene reconstructed
from tree rings, soils and climate modelling. V. Lucarini ac-
knowledges the support of the FP7/ERC Starting Investigator
grant NAMASTE/Thermodynamics of the Climate System (grant
no. 257106). The support from CliSAP/Cluster of excellence in
the Integrated Climate System Analysis and Prediction is also
acknowledged.
Edited by: H.-J. Hendricks Franssen
References
Akhtar, M., Ahmad, N., and Booij, M. J.: The impact of cli-
mate change on the water resources of Hindukush–Karakorum–
Himalaya region under different glacier coverage scenarios, J.
Hydrol. 355, 148–163, 2008.
Ali, G., Hasson, S., and Khan, A. M.: Climate Change: Implications
and Adaptation of Water Resources in Pakistan, GCISC-RR-13,
Global Change Impact Studies Centre (GCISC), Islamabad, Pak-
istan, 2009.
Ali, K. J. and De Boer, D. H.: Spatial patterns and variation of sus-
pended sediment yield in the upper Indus River basin, northern
Pakistan, J. Hydrol., 334, 368–387, 2007.
Archer, D. R.: Contrasting hydrological regimes in the upper Indus
Basin, J. Hydrol., 274, 19–210, 2003.
Archer, D. R. and Fowler, H. J.: Spatial and temporal variations
in precipitation in the Upper Indus Basin, global teleconnections
and hydrological implications, Hydrol. Earth Syst. Sci., 8, 47–61,
doi:10.5194/hess-8-47-2004, 2004.
Archer, D. R. and Fowler, H. J.: Using meteorological data to fore-
cast seasonal runoff on the River Jhelum, Pakistan, J. Hydrol.,
361, 10–23, 2008.
www.hydrol-earth-syst-sci.net/18/4077/2014/ Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014
4098 S. Hasson et al.: Snow cover state over western IRS basins
Baig, M. H. A. and Rasul, G.: The effect of Eurasian snow cover on
the Monsoon rainfall of Pakistan, Pakistan J. Meteorol., 5, 1–11,
2009.
Barnett, T. P., Adam, J. C., and Lettenmaier, D. P.: Potential impacts
of a warming climate on water availability in snow-dominated
regions, Nature, 438, 303–309, doi:10.1038/nature04141, 2005.
Batura Investigations Group: The Batura Glacier in the Karakoram
Mountains and its variations, Scientia Sinica, 22, 958–974, 1979.
Bhambri, R., Bolch, T., Kawishwar, P., Dobhal, D. P., Srivastava,
D., and Pratap, B.: Heterogeneity in glacier response in the upper
Shyok valley, northeast Karakoram, The Cryosphere, 7, 1385–
1398, doi:10.5194/tc-7-1385-2013, 2013.
Bojariu, R. and Giorgi, F.: The north Atlantic oscillation signal in
a regional climate simulation for the European region, Tellus A,
57, 641–653, 2005.
Bolch, T., Kulkarni, A., Kääb, A., Hugget, C., Paul, F., Cogley, J.
G., Frey, H., Kargel, J. S., Fujita, K., Scheel, M., Bajracharya,
S., and Stoffel, M.: The state and fate of Himalayan glaciers,
Science, 336, 310–314, 2012.
Braithwaite, R. J. and Raper, S. C. B.: Estimating equilibriumline
altitude (ELA) from glacier inventory data, Ann. Glaciol., 50,
127–132, 2009.
Chelamallu, H. P., Venkataraman, G., and Murti, M. V. R.:
Accuracy assessment of MODIS/Terra snow cover product
for parts of Indian Himalayas, Geocarto Int., 29, 592–608,
doi:10.1080/10106049.2013.819041, 2014.
Cintia, B. and Uvo, C.: Analysis and regionalization of Northern
European winter precipitation based on its relationship with the
North Atlantic oscillation, Int J. Climatol., 23, 1185–1194, 2003.
Cubasch, U., Meehl, G. A., Boer, G. J., Stouffer, R. J., Dix, M.,
Noda, A., Senior, C. A., Raper, S., and Yap, K. S.: Projections of
future climate change, in: Climate Change 2001: The Scientific
Basis. Contribution of Working Group I to the Third Assessment
Report of the IntergovernmentalPanel on Climate Change, edited
by: Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., van der
Linden, P., Dai, X., Maskell, K., and Johnson C. I., Cambridge
University Press, Cambridge, United Kingdom and New York,
NY, USA, 525–582, 2001.
Dietz, A., Wohner, C., and Kuenzer, C.: European snow cover
characteristics between 2000 and 2011 derived from improved
MODIS Daily snow cover products, Remote Sens., 4, 2432–
2454, 2012.
Diodato, N., Bellocchi, G., and Tartari, G.: Howdo Himalayan areas
respond to globalwarming?, Int. J. Climatol.,32, 975–982, 2012.
Dong, J., Walker, J. P., and Houser, P. R.: Factors affecting remotely
sensed snow water equivalent uncertainty, Remote Sens. Envi-
ron., 97, 68–82, 2005.
Dwyer, J., Weiss, J., Schmidt, G., Logar, T., Burrel, R.,
Stubbendieck, G., Rishea, J., Misterek, B., Jia, S., and Heuser,
K.: The MODIS Reprojection Tool, in: American Geophysical
Union Spring Meeting, Boston, Massachusetts, USA, 2001.
Dyurgerov, M., Meier, M. F., and Bahr, D. B.,: A new index of
glacier area change: a tool for glacier monitoring, J. Glaciol., 55,
710–716, 2009.
Easterling, D. R., Horton, B., Jones, P. D., Peterson, T. C., Karl,
T. R., Parker, D. E., Salinger, M. J., Razuvayev, V., Plummer,
N., Jamason, P., and Folland, C. K.: Maximum and minimum
temperature trends for the globe, Science, 277, 364–367, 1997.
Forsythe, N., Fowler, H. J., Kilsby, G. C., and Archer, D. R.: Oppor-
tunities from Remote Sensing for Supporting Water Resources
Management in Village/Valley Scale Catchments in the Upper
Indus Basin, Water Resour. Manage., 26, 845–871, 2012.
Fowler, H. J. and Archer, D. R.: Hydro-climatological variability
in the Upper Indus Basin and implications for water resources,
Regional Hydrological Impacts of Climatic Change Impact
Assessment and Decision Making (Proceedings of symposium
S6, Seventh IAHS Scientific Assembly at Foz do Iguaçu, Brazil,
IAHS Publ., 295, 2005.
Fowler, H. J. and Archer, D. R.: Conflicting Signals of Climatic
Change in the Upper Indus Basin, J. Climate, 9, 4276–4293,
2006.
Fowler, H. J. and Kilsby, C. G.: Precipitation and the North Atlantic
Oscillation: A study of climatic variability in Northern England,
Int. J. Climatol., 22, 843–866, 2002.
Gafurov, A. and Bárdossy, A.: Cloud removal methodology from
MODIS snow cover product, Hydrol. Earth Syst. Sci., 13, 1361–
1373, doi:10.5194/hess-13-1361-2009, 2009.
Gardelle, J., Berthier, E., Arnaud, Y., and Kääb, A.: Region–wide
glacier mass balances over the Pamir-Karakoram-Himalaya dur-
ing 1999–2011, The Cryosphere, 7, 1263–1286, doi:10.5194/tc-
7-1263-2013, 2013.
Gioli, G., Khan, T., and Scheffran, J.: Climatic and environmental
change in the Karakoram: Making sense of community percep-
tions and adaptation strategies, Reg. Environ. Change, 14, 1151–
1162, doi:10.1007/s10113-013-0550-3, 2014.
Gurung, D. R., Kulkarni, A. V., Giriraj, A., Aung, K. S., and
Shrestha, B.: Monitoring of seasonal snow cover in Bhutan using
remote sensing technique, Current Sci., 101, 1364–1370, 2011.
Hall, D. K. and Riggs, G. A.: Accuracy assessment of the
MODIS snow products, Hydrol. Process., 21, 1534–1547,
doi:10.1002/hyp.6715, 2007.
Hall, D. K., Riggs, G. A., Salomonson, V. V., DiGirolamo, N. E.,
and Bayr, K. J.: MODIS Snow-Cover Products, Remote Sens.
Environ., 83, 181–194, 2002.
Hall, D. K., Salomonson, V. V., and Riggs, G. A.: MODIS/(Aqua
and Terra) Snow Cover Daily L3 Global 500m Grid, Version 5,
2001–2012, National Snow and Ice Data Center, Boulder, Col-
orado USA, 2006.
Hasson, S., Lucarini, V., and Pascale, S.: Hydrological cycle
over South and Southeast Asian river basins as simulated by
PCMDI/CMIP3 experiments, Earth Syst. Dynam., 4, 199–217,
doi:10.5194/esd-4-199-2013, 2013.
Hasson, S., Lucarini, V., Pascale, S., and Böhner, J.: Seasonality of
the hydrological cycle in major South and Southeast Asian river
basins as simulated by PCMDI/CMIP3 experiments, Earth Syst.
Dynam., 5, 67–87, doi:10.5194/esd-5-67-2014, 2014.
Helfrich, S. R., McNamara, D., Ramsay, B. H., Baldwin, T., and
Kasheta T.: Enhancements to, and forthcoming developments
in the Interactive Multisensor Snow and Ice Mapping System
(IMS), Hydrol. Process., 21, 1576–1586, 2007.
Hewitt, K.: The Karakoram Anomaly? Glacier Expansion and the
“Elevation Effect” Karakoram Himalaya, Mount. Res. Dev., 25,
332–340, 2005.
Hewitt, K.: Tributary glacier surges: An exceptional concentration
at panmah glacier, karakoram 30 himalaya, J. Glaciol., 53, 181–
188, 2007.
Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014 www.hydrol-earth-syst-sci.net/18/4077/2014/
S. Hasson et al.: Snow cover state over western IRS basins 4099
Hewitt, K.: Glacier Change, Concentration, and Elevation Effects
in the Karakoram Himalaya, Upper Indus Basin, MRD, 31, 188–
200, 2011.
Hüsler, F., Jonas, T., Riffler, M., Musial, J. P., and Wunderle, S.:
A satellite-based snow cover climatology (1985–2011) for the
European Alps derived from AVHRR data, The Cryosphere, 8,
73–90, doi:10.5194/tc-8-73-2014, 2014.
Hurrell, J. W.: Decadal trends in the North Atlantic Oscillation:
regional temperatures and precipitation, Science-AAAS-Weekly
Paper Edition, 269, 676–678, 1995.
Hurrell, J. W. and Van Loon, H.: Decadal variations in climate as-
sociated with the North Atlantic Oscillation, Clim. Change, 36„
301–326, 1997.
Immerzeel, W. W. and Bierkens, M. F. P: Seasonal prediction of
monsoon rainfall in threeAsian river basins: the importance of
snow cover on the Tibetan Plateau, Int. J. Climatol., 30, 1835–
1842, 2010.
Immerzeel, W. W., Droogers, P., de Jong, S. M., and Bierkens, M. F.
P.: Large-scale monitoring of snow cover and runoff simulation
in Himalayan river basins using remote sensing, Remote Sens.
Environ., 113, 40–49, 2009.
Immerzeel, W. W., van Beek, L. P. H., and Bierkens, M. F. P.: Cli-
mate change will affect the Asian Water Towers, Science, 328,
1382–1385, doi:10.1126/science.1183188, 2010.
IPCC, 2013: Climate Change 2013: The Physical Science Basis.
Contribution of Working Group I to the Fifth Assessment Report
of the Intergovernmental Panel on Climate Change, edited by:
Stocker, T. F., Qin, D., Plattner, G.-K., Tignor, M., Allen, S. K.,
Boschung, J., Nauels, A., Xia, Y., Bex, V., and Midgley, P. M.,
Cambridge University Press, Cambridge, United Kingdom and
New York, NY, USA, 1535 pp., 2013.
Jain, S. K. Goswami, A., and Saraf, A. K.: Accuracy assessment of
MODIS, NOAA and IRS data in snow cover mapping under Hi-
malayan conditions, Int. J. Remote Sens., 29, 5863–5878, 2008.
Jarvis, A., Reuter, H. I., Nelson, A., and Guevara, E.: Hole-filled
seamless SRTM data V4, International Centre for Tropical Agri-
culture (CIAT), available at: http://srtm.csi.cgiar.org (last access:
3 June 2012), 2008.
Karl, T. R., Knight, R. W., Gallo, K. P., Peterson, T. C., Jones, P. D.,
Kukla, G., Plummer, N., Razuvayev, V., Lindseay, J., and Charl-
son, R. J.: A NewPerspective on Recent Global Warming: Asym-
metric Trendsof Daily Maximum and Minimum Temperature, B.
Am. Meteorol. Soc., 74, 1007–1023, 1993.
Kelly, R. E. J., Chang, A. T. C., Tsang, L., and Foster, J. L.: A
prototype AMSRE global snow area and snow depth algorithm.
IEEE Trans. Geosci. Remote Sens., 41, 230–242, 2003.
Khattak, M. S., Babel, M. S., and Sharif, M.: Hydrometeorological
trends in the upper Indus River Basin in Pakistan, Clim. Res., 46,
103–119, doi:10.3354/cr00957, 2011.
Konz, M., Finger, D., Buergi, C., Normand, S., Immerzeel, W. W.,
Merz, J., Girirah, A., and Burlando, P: Calibration of a dis-
tributedhydrological model for simulationsof remoteglaicerised
Himalayan catchments using MODIS snow cover data, Global
Change: Facing Risks and Threats to Water Resources (Proc. Of
the Sixth World FRIEND Conference, Fez, Morocco, IAHS Pub-
lication 340, 465–473, 2010.
Kulkarni, A. V.: Mass balance of Himalayan glaciers using AAR
and ELA methods, J. Glaciol., 38, 101–104, 1992.
Levinson, D. H. and Waple, A. M.: State of the climate in 2003, B.
Am. Meteorol. Soc., 85, S1–S72, 2004.
López-Burgos, V., Gupta, H. V., and Clark, M.: Reducing cloud
obscuration of MODIS snow cover area products by combining
spatio-temporal techniques with a probability of snow approach,
Hydrol. Earth Syst. Sci., 17, 1809–1823, doi:10.5194/hess-17-
1809-2013, 2013.
Max, K.: Measuring Snow and Glacier Ice Properties from Satellite,
Rev. Geophys., 39, 1–27, 2001.
Mehta, J. S.: The Indus Water Treaty:a Case Study in the Resolution
of an International River Basin Conflict, Nat. Resour. Forum, 12,
69–77, doi:10.1111/j.1477-8947.1988.tb00803.x, 1988.
Menon, S., Koch, D., Beig, G.,Sahu, S., Fasullo, J., and Orlikowski,
D.: Black carbon aerosols and the third polar ice cap, Atmos.
Chem. Phys., 10, 4559–4571, doi:10.5194/acp-10-4559-2010,
2010.
Minora, U., Bocchiola, D., D’Agata, C., Maragno, D., Mayer, C.,
Lambrecht, A., Mosconi, B., Vuillermoz, E., Senese, A., Com-
postella, C., Smiraglia, C., and Diolaiuti, G.: 2001–2010 glacier
changes in the Central Karakoram National Park: a contribution
to evaluate the magnitude and rate of the “Karakoram anomaly”,
The Cryosphere Discuss., 7, 2891–2941, doi:10.5194/tcd-7-
2891-2013, 2013.
Notarnicola, C., Di Rosa, D., and Posa, F.: Cross-comparison of
MODIS and CloudSat data as a tool to validate local cloud cover
masks, Atmosphere, 2, 242–255, 2011.
Painter, T. H., Brodzik, M. J., Racoviteanu, A., and Armstrong,
R.: Automated mapping of Earth’s annual minimum exposed
snow and ice with MODIS, Geophys. Res. Lett., 39, L20501,
doi:10.1029/2012GL053340, 2012.
Parajka, J. and Blösch, G.: Spatio-temporal combination of MODIS
images – potential for snow cover mapping, Water Resour. Res.,
44, W03406, doi:10.1029/2007WR006204, 2008.
Pfeffer, W., Arendt, A. A., Bliss, A., Bolch, T., Cogley, J. G., Gard-
ner, A. S., Hagen, J.-O., Hock,R., Kaser, G., Kienholz, C.,Miles,
E. S., Moholdt, G., Mölg, N., Paul, F., Radi
´
c, V., Rastner, P.,
Raup, B. H., Rich, J., and Sharp, M. J. A. T. R. C.: The Randolph
Glacier Inventory: a globally complete inventory of glaciers, J.
Glaciol., 60, 537–552, 2014
Roy, A., Royer, A., and Turcotte, R.: Improvement of springtime
stream-flow simulations in a boreal environment by incorporat-
ing snow-covered area derived from remote sensing data, J. Hy-
drol., 390, 35–44, 2010.
Rees, H. G. and Collins, D. N.: Regional differences in response
of flow in glacier-fed Himalayan rivers to climatic warming, Hy-
drol. Process., 20, 2157–2169, 2006.
Riggs, G. A. and Hall, D. K.: MODIS snow and ice products,
and their assessment and applications, in: Land Remote Sens-
ing and Global Environment Change, edited by: Ramachan-
dran, B., Justice, C. O., and Abrams, M. J., LLC 2011,
Remote Sensing and Digital Image Processing 11, Chapter
30, Springer Science+Business Media, New York, 681–707,
doi:10.1007/978-1-4419-6749-7, 2011.
Riggs, G. A., Hall, D. K., and Salomonson, V. V.: MODIS Snow
Products User Guide to Collection5, http://modis-snow-ice.gsfc.
nasa.gov/uploads/sug_c5.pdf (last access: 31 October 2013),
2006.
www.hydrol-earth-syst-sci.net/18/4077/2014/ Hydrol. Earth Syst. Sci., 18, 4077–4100, 2014
4100 S. Hasson et al.: Snow cover state over western IRS basins
Shaman, J. and Tziperman, E.: The Effect of ENSO on Ti-
betan Plateau Snow Depth: A Stationary Wave Teleconnection
Mechanism and Implications for the South Asian Monsoons, J.
Climtol., 18, 2067–2079, 2005.
Shea, J. M., Menounos, B., Moore, R. D., and Tennant, C.: An ap-
proach to derive regional snow lines and glacier mass change
from MODIS imagery, western North America, The Cryosphere,
7, 667–680, doi:10.5194/tc-7-667-2013, 2013.
Shrestha, A. B., Wake, C. P., Mayewski, P. A., and Dibb, J. E.:
Maximum temperature trends in the Himalaya and its vicinity:
an analysis based on the temperature records from Nepal for the
period 1971–94, J. Climate, 12, 2775–2786, 1999.
SIHP: Snow and Ice Hydrology, Pakistan Phase-II Final Report to
CIDA, IDRC File No. 88-8009-00 International Development
Research Centre, Ottawa, Ontario, Canada, KIG 3H9, Report No.
IDRC.54, 1997.
Syed, F. S., Giorgi, F., Pal, J. S., and King, M. P.: Effect of re-
mote forcing on the winter precipitation of central southwest
Asia Part 1: Observations, Theor. Appl. Climatol., 86, 147–160,
doi:10.1007/s00704-005-0217-1, 2006.
Tahir, A. A., Chevallier, P., Arnaud, Y., and Ahmad, B.: Snow cover
dynamics and hydrological regime of the Hunza River basin,
Karakoram Range, Northern Pakistan, Hydrol. Earth Syst. Sci.,
15, 2275–2290, doi:10.5194/hess-15-2275-2011, 2011a.
Tahir, A. A., Chevallier, P., Arnaud, Y., Neppel, L., and Ahmad, B.:
Modeling snowmelt-runoff under climate scenarios in the Hunza
River basin, Karakoram Range, Northern Pakistan, J. Hydrol.,
409, 104–117, 2011b.
Takala, M., Luojus, K., Pulliainen, J., Derksen, C., Lemmetyi-
nen, J., Kärnä, J.-P., Koskinen, J., and Bojkov, B.: Estimating
northern hemisphere snow water equivalent for climate research
through assimilation of space-borne radiometer data and ground-
based measurements, Remote Sens. Environ., 115, 3517–3529,
doi:10.1016/j.rse.2011.08.014, 2011.
Tang, B.-H., Shrestha, B., Lia, Z.-L., Liua, G., Ouyangb, H., Gu-
rung, D. R., Giriraj, A., and Aung, K. S.: Determination of snow
cover from MODIS data for the Tibetan Plateau region, Int. J.
Appl. Earth Observ. Geoinform., 21, 356–365, 2013.
Thirel, G., Notarnicola, C., Kalas, M., Zebish, M., Schellenberger,
T., Tezlaff, A., Duguay, M., Mölg, N., Burek, P., and de Roo,
A.: Assessing the quality of a real-time snow cover area prod-
uct for hydorological applications, Remote Sens. Environ., 127,
271–287, 2012.
Tong, J., Déry, S. J., and Jackson, P. L.: Topographic control of
snow distribution in an alpine watershed of western Canada in-
ferred from spatially-filtered MODIS snow products, Hydrol.
Earth Syst. Sci., 13, 319–326, doi:10.5194/hess-13-319-2009,
2009.
Treydte, K. S., Schleser, G. H., Helle, G., Frank, D. C., Winige,
M., Haug, G. H., and Esper, J.: The twentieth century was the
wettest period in northern Pakistan over the past millennium, Na-
ture, 440, 1179–1182, 2006.
Wake, C. P.: Snow accumulation studies in the central Karako-
ram, Proc. Eastern Snow Conf. 44th Annual Meeting Frederic-
ton, Canada, 19–33, 1987.
Wang, X., Xie, H., Liang, T., and Huang, X.: Comparison and val-
idation of MODIS standard and new combination of Terra and
Aqua snow cover products in northern Xinjiang, China, Hydrol.
Process., 429, 419–429, doi:10.1002/hyp.7151, 2009.
Winiger, M., Gumpert, M., and Yamout, H.: Karakorum–