by Water Research Bhutan, 28 June 2022, waterresearchbhutan@gmail.com
The river basins in Bhutan mostly originate from glacierized headwater
catchments. The hydropower and agriculture (crops) in Bhutan contributed
almost 11 and 18% respectively to Bhutan's total gross domestic product in
2020. However, the cryospheric water sources of major river basins in Bhutan
are under increasing threat from rising global temperature. Impacts of climate
change could potentially reduce cryospheric contribution to streams and rivers
affecting water availability and food security in Bhutan and beyond.
Picture - Injecting isotopes into natural water bodies in Chamkhar Chhu headwaters for tracking of their flow paths
Research in the Himalayan regions by Nepal et al. (2014) has already
reported reduced glacier melt runoff and changes in the precipitation phase
have increased variability of the flow regime affecting economic activities.
Similarly, a study by Rupper et al. (2012) demonstrated that an increase of
one-degree celsius in glacierized Bhutan would reduce annual meltwater flux
by almost 65%. The impact of increasing global temperature on the
cryospheric resources was also evident from the recent study by the National
Centre for Hydrology and Meteorology (NCHM) which reported that Bhutan
lost about 17 gigatons of glacier ice since 2004 to date. This calls for an
urgent investigation into the influence of annual meltwater flux on the streams,
rivers, springs, and groundwater in low-lying areas.
To address the knowledge gap, the researchers from Sherubtse College and
the University of Colorado, USA studied the Chamkhar Chhu in central Bhutan
using a combination of in-situ hydrochemistry and isotope datasets that drive
mixing models from 2014 to 2017. The research was aimed at analyzing
changes in the role of meltwater during different seasons and with distance
from the glaciers. The research was critical in providing a piece of first-hand
information on the role of climate-sensitive snow and ice-melt contributions to
our water sources.
The researchers used a tracer-based mixing model that offers alternative
options to remote sensing and other traditional techniques for understanding
hydrology. Naturally occurring tracers such as stable isotopes of water and
geochemistry were used for estimating the proportions of river discharge from
various sources and flow paths. The use of the tracer method in remote and
data-scarce regions like Bhutan is particularly strategic as it does not require
discharge data to estimate relative flow contributions from ice, snow, and rain.
In addition, the mixing-model results also provide insights into the timing and
volume of water discharge in response to changes in the climate.
Hydrologic processes such as source waters and flow paths that control river
flow in the Chamkhar Chhu basin were studied using tracer isotopes. Samples
including surface water, groundwater, glacier meltwater, and precipitation
were collected in pre-monsoon, monsoon, and post-monsoon seasons along
an elevation transect from 2,538 to 5,158 metres above sea level.
This study for the first time showed that pre-monsoon (March) baseflow
consists of mostly rain and snow (38 and 39%, respectively) while ice melt
contributed 23%. With the onset of the monsoon, the river changed to a rain-
dominated system, with rain making up the majority of June (52%) and August
(71%) flow. In the post-monsoon (October) the river transforms into essentially
a 2-part system with ice and rain each sourcing nearly half the flow. Because,
in the post-monsoon period, glacier ice, no longer protected by seasonal snow
cover, is more susceptible to melting and contributes a majority of river flow
during this period.
Overall, the study demonstrated that the Chamkhar Chhu is a rainfall-
dominated basin, with seasonally varying snow and ice melt contributions
which conceptually agrees with expectations of the annual hydrological cycle
typical of the eastern Himalayas. Further, the researchers observed that
monsoon rain plays the dominant role in sourcing river flow below an altitude
of 3,500 m during monsoon (contributing up to 48%) and post-monsoon
(contributing up to 88%) periods (July to November). Even during the drier
periods such as the pre-monsoon season, much of the river flow may
indirectly be rain-fed.
The high dependence of our water sources on rainfall could be highly
vulnerable to temporal and spatial variability of precipitation patterns
associated with rising temperatures in the Himalayas. For example, a recent
study on springshed in southwestern Bhutan by Jambay & Uden (2022)
reported a change in rainfall pattern, which impacted the recharging of local
aquifers and caused springs to dry up. It also means that our future
infrastructure development, disaster management, and management of
drinking water sources need to consider the influence of changing rainfall
patterns.
The research also demonstrated an increasing contribution of groundwater to
river flow in the Chamkhar Chhu basin with decreasing elevation. However,
our current knowledge of groundwater hydrology, e.g. residence time and lag
time between recharge and streamflow are almost non-existent. Thus,
understanding the groundwater dynamics of our major river basins is also a
critical area of future research. The same group of researchers is currently
engaged in piloting a study on groundwater dynamics using a tracer-based
method in Eastern Bhutan, which is expected to gather crucial data on the
sources and recharge mechanisms of perennial springs and streams.
Currently, Bhutan’s Himalayas are losing snow and glacier resources due to
increasing temperatures. The discharge in melt-sourced rivers like the
Chamkhar Chhu would lose contribution to its flow from snow and glacier
resources. Snow melt was found to contribute significantly to river flow during
early monsoon, whereas ice melt is important in the post-monsoon period,
when much of the lower elevation seasonal snow has melted and the glacier
ice is exposed and no longer has the protection of seasonal snow cover.
However, on an annual basis, the rainfall may dictate the river discharge at
the non-alpine elevations where people, hydropower, and agriculture utilise
the water.
The article is published based on personal experiences and observations by a group of
water researchers from Bhutan. The group can be contacted at waterresearchbhutan@gmail.com
The article was also published in National Newspaper of Bhutan (Kuensel). The link https://kuenselonline.com/understanding-the-linkage-between-snow-capped-peaks-to-flow-patterns-of-streams-and-rivers/
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