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.
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 firstname.lastname@example.org