1 | INTRODUCTION
With the combination of western disturbance (WDs), Indian summer monsoon
(ISM), and Tibetan anticyclone principal synoptic systems, the climate
pattern of the Hindukush-Karakoram-Western Himalaya (HKKH) mountain
ranges are different from other adjacent Tibetan Plateau (TP) region,
hence HKKH comprises the most heavily glacierized watershed outside the
poles (Bishop et al., 2010; Farhan, Zhang, Ma, Guo & Ma, 2015; Hewitt,
1998; Palazzi, Hardenberg & Provenzale, 2013; Treydte et al.,
2006). The Indus river emerges
from the HKKH ranges, and the upper Indus basin (UIB) is known as
”Pakistan Water Tower” (Farinotti, 2017; Immerzeel, Ludovicus &
Bierkens, 2010). Comprehensive analysis of the Water Tower Index
indicated that UIB ranked top of the most critical and vulnerable unit
in Asia even globally with abundant water resources (Immerzeel et al.,
2019). More than 40% of freshwater in UIB depends on the seasonal
ice/snow melting, it satisfies
municipal and industrial water usage for more than 237 million people
living in the downstream area. Global warming, along with the heavy
irrigation and densely population are creating ever-greater pressure on
exploiting mountain glacier resources, which makes the Indus aquifer the
most overstressed aquifer worldwide (Lutz, Immerzeel, Shrestha &
Bierkens, 2014; Pritchard, 2019). Synthesizing knowledge of the ice
reserves is becoming a fundamental prerequisite for the glacier runoff
project for water resources allocation and socio-economic development
(Cogley, 2012; Radić & Hock, 2011; Vaughan et al., 2013). Most research
was concentrated on glacier change, the precise ice reserves estimates
in the UIB region remain unclear due to limited accessibility (Bolch et
al., 2012; Khan, Naz & Bowling, 2015).
Multiple approaches have been proposed for deducing ice reserves
recently, including the geodetic
surveying, semi-empirical formula, physical models, and other methods
(Farinotti et al., 2019; 2017).
Ice core drilling, gravimeter
instrument, and ground-penetrating radar (GPR) technique can be
classified as geodetic surveying, the high cost makes drilling technique
just used for auxiliary medium to glacier structure although it is the
most intuitive method with the highest precision (Garzonio et al.,
2018); Gravimeter instrument was once tested on Batura glacier in
northern Pakistan (BGIG, 1979); GPR technique is a radio-echo detection
method that using electromagnetic wave to acquire ice thickness, its
high-resolution accuracy and efficiency in noninvasive subsurface makes
it is improving continually. During the past three decades, GPR has been
applied in the inner and eastern TP widely (Gergan, Dobhal & Kaushik,
1999; Ma et al., 2000; Marussi, 1964; Singh et al., 2012).
The second category is the
semi-empirical formula, V-A scaling constructs the correlation
relationship by linear geostatistical regression, considering that the
ice flow and surface landform remain stable, it is suitable for
populations of glaciers (Radić &
Hock, 2013). The physical model can be sorted as the third category,
which consists of shallow ice approximation (SIA), mass conserving,
minimization, and velocity-based model. Glacier Bed Topography version 2
(GlabTop2) has been conducted widely for its operability, efficient, and
theoretical reliability that originated from the classic laminar flow
hypothesis (Linsbauer et al., 2016), it is developed from GlabTop and
avoids the laborious process of drawing branch lines manually. Mass
conserving, minimization, and velocity-based models were applied to the
unique glacier through surface characteristics (Fürst et al., 2017;
Maussion et al., 2019). Even the
valley glacier fill thickness can be estimated using artificial neural
networks that corresponding to the fourth category.
Numerous investigators have applied multiple methods to assess ice
reserves in HKKH region, including V-A scaling law, slope-depth,
HF-model, GlabTop2, GlabTop2_IITB, Open Global Glacier Model (OGGM)
(Bahr, Pfeffer & Kaser, 2015; Chen & Ohmura, 1990; Farinotti et al.,
2019; Huss & Farinotti, 2012; Liu, Shen, Sun & Li, 2002). The
evaluated ice volume of the entire Karakoram-Western Himalaya region
(KH) is 2.2-3.5 × 103 km3 with an
ice area of 16.89 × 103 km2approximately (Frey et al., 2014), it was calibrated by GPR-measured
points on Baltoro and Chhota Shigri glaciers (Azam et al., 2012;
Marussi, 1964). GPR work is the precious verification materials for ice
reserves estimation, rare ice thickness and long-term glacial change
measurements were involved in the South Asia West district (WGI
Partition), in particular of the UIB region (Gärtner et al., 2014;
Grinsted, 2013; Hewitt, 2016). Few mapping and further ground-based
investigations were carried out before 1995 by the Batura Glacier
Investigation Group (BGIG) (BGIG, 1979; Zhang, Chen & Wang, 1996).
In light of the unique geographical and strategic position of the UIB
region, the ancient Silk Road from China to South Asia and the modern
Karakoram highway both pass through this kernel zone, research on ice
reserves can protect against extreme water stress on seasonal and longer
time scales, even to maintain social development and geopolitical
stability. The main objectives of this paper are to (1) present
GPR-surveyed ice thickness of typical glaciers in the UIB sub-basins;
(2) compare the plots and profiles of the GPR-measured ice bed elevation
versus GlabTop2-estimated results, integrated with GlabTop, Volta model,
and the IDW interpolated results, then select the optimized parametric
scheme of GlabTop2; (3) simulate the ice reserves of the entire UIB and
its sub-catchments, and assess the potential hydrological effect of
glacier resources.