Glacier surface mass balance observations in the Tien Shan and Pamir are
relatively sparse and often discontinuous. Nevertheless, glaciers are one of
the most important components of the high-mountain cryosphere in the region
as they strongly influence water availability in the arid, continental and
intensely populated downstream areas. This study provides reliable and
continuous surface mass balance series for selected glaciers located in the
Tien Shan and Pamir-Alay. By cross-validating the results of three
independent methods, we reconstructed the mass balance of the three benchmark
glaciers, Abramov, Golubin and Glacier no. 354 for the past 2 decades. By
applying different approaches, it was possible to compensate for the
limitations and shortcomings of each individual method. This study proposes
the use of transient snow line observations throughout the melt season
obtained from satellite optical imagery and terrestrial automatic cameras. By
combining modelling with remotely acquired information on summer snow
depletion, it was possible to infer glacier mass changes for unmeasured
years. The model is initialized with daily temperature and precipitation data
collected at automatic weather stations in the vicinity of the glacier or
with adjusted data from climate reanalysis products. Multi-annual mass
changes based on high-resolution digital elevation models and in situ
glaciological surveys were used to validate the results for the investigated
glaciers. Substantial surface mass loss was confirmed for the three studied
glaciers by all three methods, ranging from
Overview map of central Asia showing the location of glaciers (blue)
with available long-term SMB measurements. Glaciers investigated in this
study are marked in red. The insets show the position of the automatic
weather station (AWS) for
Glaciers are important components of the hydrological cycle in
central Asia. In this arid continental region, the intensely populated and
irrigated downstream areas strongly depend on a supply of water from the
cryosphere such as glaciers and snow
During the Soviet era, in the 1950s and 1960s, an extensive system of
cryospheric monitoring was launched in the Tien Shan and Pamir-Alay. Most
programmes stopped abruptly after the breakdown of the USSR in the mid-1990s.
Monitoring activities were maintained only on Tuyuksu Glacier, Kazakhstan and
Urumqi Glacier (no. 1), China. In recent years, different initiatives have
aimed at the re-establishment of glacier monitoring in central Asia
Available data on glacier monitoring for the three glaciers used in this study. The dates marked with an asterisk indicate the DEMs used as a topographic base for the modelling.
Different studies derived continuous mass balance series for selected
glaciers based on modelling
The snow line is recognized as a valuable proxy for glacier mass balance
In this study, three pillars of a multi-level strategy for glacier observation are combined, covering the period of the past 2 decades, to improve the understanding of mass change evolution of Abramov, Golubin and Glacier no. 354, three benchmark glaciers in the Tien Shan and Pamir-Alay. (1) We integrate in situ glaciological measurements, when available, to compute annual SMB using a model-based extrapolation of the measurement points to reach glacier-wide coverage. (2) We calculate geodetic mass changes based on high-resolution digital elevation models (DEMs) on decadal to semi-decadal timescales. (3) We infer daily SMB series using a model approach supported by TSL observations, as a proxy for glacier mass balance. In this way, a temperature-index model is calibrated with the snow-covered area fraction (SCAF) of the glacier observed on satellite optical imagery and time-lapse photographs throughout the ablation season. This approach represents a new tool for glacier observation at high temporal and spatial resolution. The remote snow line observations provide valuable information, especially for periods for which no direct measurements are available. By combining different independent approaches, we aim to overcome the limitations and shortcomings of each individual method and to deliver a robust mass balance estimate for the three selected glaciers at annual resolution for a period for which only limited data have been available so far.
Glacier monitoring network at
In this section we present a brief overview of the study sites. A detailed
description of the three selected glaciers and their geographic and
climatological settings is given in
Abramov Glacier (39
Mean daily air temperature and total daily precipitation sums were measured
at a glaciological station located at 3837 m a.s.l. from 1967 to 1998
(Fig.
The SMB was measured intensively from 1967 to 1998
Golubin Glacier (42
We used meteorological data from the Alplager station located in the Ala
Archa Valley, situated at an elevation of 2145 m a.s.l. at a distance of
about 10 km from the glacier (Fig.
Intense glacier monitoring started in 1958 and continued until 1994
Image availability and distribution for snow line mapping. Numbers indicate the total available scenes per year and glacier. Prior to 1998, image coverage is sparse for all three glaciers. For Golubin and Glacier no. 354, the first summer seasons for which enough TSL observations could be collected were in 2000 and 2004, respectively. Snow-covered high-resolution images have not been used to delineate the snow line and are not shown here.
Glacier no. 354 (41
Since 2010, in situ SMB has been obtained annually in late summer
(Fig.
To compute geodetic mass balances for Abramov, high-resolution DEMs were used
based on Pléiades stereo images acquired in 2015 and on stereo images
from 2003 and 2011 from Satellite Pour l'Observation de la Terre (SPOT) 5.
For Glacier no. 354, DEMs from 2003 (QuickBird) and 2012 (GeoEye) were
available from
We used freely accessible, orthorectified and georeferenced Landsat TM/ETM+ and OLI, Terra ASTER-L1B and Sentinel-2A scenes to repeatedly observe the glacier outlines and the TSL throughout the melt season for all three glaciers. In addition, we used the snow-free high-resolution optical satellite images as described above for TSL and glacier outline mapping.
Two terrestrial cameras (Mobotix M25) overlooking Abramov were installed in
August 2011. One camera was located next to the AWS
Glacier extents were mapped manually based on satellite images for all three
glaciers and for each year of the corresponding study period. Only cloud- and
snow-free images were selected. The surfaces of Glacier no. 354 and Golubin
are mostly debris-free. We excluded a debris-covered part with strongly
reduced melt rates at the western margin of Abramov
For Abramov, the air temperature data measured at the AWS were adjusted to the
elevation of the former glaciological station by applying a constant lapse
rate of
Mean daily air temperature data measured at the Ala Archa AWS and Tien Shan
(Kumtor) AWS were extrapolated to the median elevation of the corresponding
glacier with monthly temperature lapse rates for the northern and central
Tien Shan provided in
A visual preselection of suitable camera and satellite images was taken in
order to preclude problems associated with image quality such as fresh
snowfall and extensive cloud cover. Oblique ground-based photographs were
first corrected automatically for lens distortion, then projected and
orthorectified following
Errors occurred due to the pixel size of the images, slope of the terrain,
the accuracy of the georeferencing and the quality of the DEM
We assumed the spatial depletion pattern to be approximately constant in time
so that camera and satellite images with minor invisible sections of the snow
line due to shading, cloud cover, Landsat 7 SLC-off void-stripes or the
terrestrial camera viewing angle could be included. To fill in those data
gaps, we extrapolated the snow line based on information from repeated TSL observations of images with
good quality over a
Ablation stakes are distributed over the entire ablation zone in order to
provide an optimal representation of melt patterns (Fig.
For Abramov, the 4 m Pléiades DEM from 1 September 2015 was used as
a reference. It was created using the AMES stereo-pipeline
We created DEMs with a spatial resolution of 5 m for Golubin and Glacier
no. 354 from the available stereo and
tri-stereo pairs of high-resolution satellite imagery using standard
procedures and the software PCI Geomatica
An accumulation and temperature-index melt model closely constrained by
TSL observations
was implemented in order to infer glacier-wide SMBs. The applied methodology
is a further stage in the approach presented by
A surface mass balance model with a spatial resolution of 20 m was driven
with daily mean air temperature and precipitation sums measured at nearby
meteorological stations or inferred from reanalysis data (see
Sect.
Daily air temperatures are extrapolated to each grid cell using a constant
temperature lapse rate based on literature values
(Table
Snow accumulation
Calibration procedure to obtain an ideal combination of
DDF
We calibrated
As
First, we defined a plausible range for DDF
Second, the performance of each DDF
A minimum of two images was needed to enable application of our calibration
approach. The influence of the image frequency and distribution was assessed
in detail with sensitivity experiments described in
Sect.
Survey period of glaciological measurement for each glacier and each year.
Geodetic surveys provide an estimate of the total mass change of a glacier
To compare the results of the different methods, the time periods covered by
the data sets also needed to be homogenized. We thus adjusted the observation
period of the modelled mass balance constrained by TSL observations to exactly match
the respective periods of geodetic and glaciological mass balance. However,
the final results
Uncertainty
Constant model parameters. The temperature lapse rate for Abramov
was adopted from
The total uncertainty of the geodetic mass balance includes a random and
systematic error. We followed
The uncertainty introduced by the mass balance model constrained by TSL observations
The accuracy of the mapped SCAF is dependent on the positioning and
the transect of the snow line, the georeferencing of the images and the
extrapolation of the snow line to invisible areas To estimate the effect of varying image availability, we repeated the
modelling using different snow line observation frequencies and temporal
distributions throughout the summer for calibration. Due to limited image
availability, this could only be conducted for the few years in which many
images were available (Fig.
Overall average uncertainties related to the three methods used,
namely the glaciological
Examples of the
To estimate the uncertainty caused by the DEM used for the modelling,
we compared our results to those obtained from model runs that used
lower-resolution DEMs. For this experiment, we replaced the high-resolution
DEM with the SRTM (Shuttle Radar Topographic Mission) DEM. This enabled us to both investigate the sensitivity of
the results to DEM quality and to assess our assumption of unchanged
topography during the entire study period. The effects of a reduced DEM
quality for all three glaciers were found to be small
( We investigated the uncertainty related to the meteorological input
data, To test the uncertainty introduced by the constant (i.e. uncalibrated)
model parameters,
Comparison between the annual SMB obtained from the TSL-constrained model when using meteorological and climatological average daily data for Abramov, Golubin and Glacier no. 354.
Model sensitivity to the different constant input parameters for
each glacier in m w.e. yr
Components 1 to 5 are assumed to be independent of each other and are
combined as the RSS to represent the total error of the annual SMB
We found that the mass balance model constrained by TSL observations is capable of
representing the observed SCAFs on satellite and terrestrial camera images
within
Comparison between observed and modelled SCAF for Abramov, Golubin and Glacier no. 354.
Annual glacier-wide modelled surface mass balances constrained by TSL observations, calculated for
Abramov (1998–2016), for Golubin (2000–2016) and for Glacier no. 354
(2004–2016), are predominantly negative (Fig.
Modelled annual SMB
Annual, glacier-wide SMB for the three glaciers for the hydrological
year
Annual SMB
The glaciological and geodetic surveys delivered two extensive and
independent data sets for validation of the modelled mass balance series
constrained by TSL
observations. Joint analysis of the data sets permitted robust conclusions to
be drawn about the mass change and its temporal dynamics over the past 2
decades. The glaciological SMB measurements showed good agreement with the
SMB inferred using TSL
observations for the same time periods (Table
Comparison of modelled annual SMB constrained by TSL observations and glaciological SMB for Abramov, Golubin and Glacier no. 354. Uncertainties in the annual SMB are indicated.
Geodetic mass balance for
Table
Geodetic mass change
Cumulative TSL-constrained modelled mass change
In order to demonstrate the advantage of using TSL observations on repeated
remote sensing data throughout the melt season to increase the confidence in
mass balance modelling, we ran the same accumulation and temperature-index
model without the use of snow lines or any other direct observations to
calibrate for all three glaciers from 2004 to 2016 (see
Sect.
Comparison of the cumulative SMB derived from unconstrained mass balance modelling to the results obtained from TSL-constrained modelling from 2004 to 2016.
A satisfying agreement was found between all three independent methods used to compute glacier-wide mass balance for the three benchmark glaciers in the central Tien Shan and Pamir-Alay for the past 2 decades. In the following, we discuss the shortcomings and advantages of each method and point out the limitations of the individual approaches.
The snow line model reproduces the direct measurements well and shows a
satisfactory performance for all three glaciers
(Tables
A significant problem is related to the varying measurement periods of the
glaciological SMBs for the selected glaciers (Table
Cumulative daily glacier-wide SMBs inferred from the snow line
approach for
An important factor limiting the applicability of mass balance modelling
constrained by TSL
observations is the dependence on good satellite imagery to map the snow line
throughout the ablation season. However, the sensitivity analysis
(Sect.
SMBs inferred from the snow line approach are closely tied to the representativeness of TSL observations. The method might be able to yield reliable SMB estimates for many glaciers in different climatic regimes in which the TSL is an indicator of the surface mass balance. The relationship between the snow line and the SMB can, however, be challenged when the position of the TSL is blurred by fresh snow or superimposed ice. The applicability of the snow line approach presented here can thus be critical when the TSL on remote sensing data cannot unambiguously be identified. This is mainly a problem for glaciers with a summer accumulation regime due to frequent fresh snowfall and glaciers with a high relevance of superimposed ice.
The geodetic mass balance and the results obtained from the snow line approach
agree well, in particular for recent years (Fig.
Long-term average mass balances from various studies in comparison
with the TSL-constrained modelled mass balance for
The glaciological and the modelled results constrained by TSL observations
refer to SMB components only. The geodetic mass balance, on the other hand,
takes into account the total glacier mass change, thus including internal and
basal ablation and accumulation. This is a limiting factor for direct
comparison. Evidence of refreezing meltwater in cold firn is reported for all
three glaciers
We performed a comprehensive comparison of long-term averages of mass balance
derived from the snow line approach to independent studies based on geodetic
surveys using different sensors and modelling, both for the investigated
glaciers as well as for the regional mass budget (Fig.
For Abramov, we find mass balances in between the results derived by
For Golubin, the inferred mass balance is in close agreement with the
geodetic mass change reported by
We also compared our results for the investigated glaciers to region-wide
assessments in order to investigate their regional representativeness
(Fig.
In this study we used three independent methods to reconstruct robust mass balance series at high temporal resolution for Abramov, Golubin and Glacier no. 354 located in the Pamir-Alay and Tien Shan mountains over the past 2 decades – a period for which very little is known about glacier behaviour. We proposed a methodology to derive glacier SMB series for unmeasured glaciers based on mass balance modelling constrained by repeated TSL observations, relying either on in situ temperature and precipitation data or climate reanalysis data sets. We recommend including TSL observations in the glacier monitoring strategy to reduce uncertainty and to increase the robustness of the data. We used extensive geodetic and glaciological surveys to validate the results and found satisfying agreement between the independent methods. Our snow line approach reproduced observed annual to decadal SMB for all three glaciers and enabled the calculation of daily SMBs for arbitrary periods and is hence capable of covering the entire hydrological year based on minimal data input. Some of the shortcomings of the glaciological and geodetic surveys could thus be overcome.
The results of all three methods confirm a continuous mass loss of the three
glaciers Abramov, Golubin and Glacier no. 354 since
At present, mass balance observations in the Pamir and Tien Shan are relatively sparse but crucially needed to improve our understanding of glacier behaviour in the region and its effect on future water availability. Direct measurements are important but costly and laborious and require an immense logistic effort. For remote and unmonitored regions and countries, lacking in financial resources and infrastructure necessary to support such monitoring programmes, our proposed approach delivers a tool for investigating and reconstructing the SMBs of unmeasured and remote glaciers with minimal effort. The integration of TSL observations into conventional modelling is shown to be highly beneficial for filling the gaps in long-term SMB series for periods for which direct glaciological measurements were discontinued or are missing completely.
Measurements and detailed model results are available in the Supplement. Model code and raw data are available upon request from the first author (Martina Barandun) or can be downloaded through official, open-access databases and online portals (see the Supplement for more information).
The authors declare that they have no conflict of interest.
This study is supported by the Swiss National Science Foundation (SNSF),
grant 200021_155903. Additional support by the German Federal Foreign
Office in the frame of the CAWa project (