Bhutan has an abundance of freshwater sources such as springs and rivers due to its diverse geographical conditions, pristine environment, and the effect of the Indian monsoon during the summer season. Despite this abundance, the shortage of water for drinking and other purposes is one of the pressing issues in Bhutan, which is even more apparent in urban centres such as Thimphu city. While the Royal Government of Bhutan seeks to provide 24×7 water supply to every household across the country, the lack of enough feasible water sources impedes achieving this goal. This water shortage problem eventually has a cascading effect on promoting health and hygiene amongst the population. This problem stems partly from a inefficient management of the available water resources.

Building reservoir dams for storing water has been used as an efficient way of water resource management in many countries around the world. Through this article we propose exploring options to build a reservoir dam which could serve as a better alternative to managing the available water resources.

A reservoir is generally a large water storage facility that can be designed by the construction of a dam across a river. The dam would then control the amount of water that flows out of the reservoir. Building a reservoir dam is one of the oldest methods adopted by mankind to store water and use it for various purposes including potable use and irrigation.  It especially stands out to be very relevant in places like Bhutan where water flow is seasonal. In Bhutan, the level of water in the rivers and springs increases by more than two-folds in summer due to elevated melting of snow and ice, and a high intensity of precipitation influenced by the monsoon. By contrast, in the winter season water level in some of the springs falls well below the level required to sustain daily requirement of the nearby communities.

Increasing water levels in summer often cause flooding and wash away the structures used for harvesting water at the source, whilst drying up of water at the source during winter significantly reduces the amount of available water. Unfortunately, it is expected that with atmospheric warming, the wetter season become more wetter while the drier season will become more drier over time. Thus the water shortage in Bhutan is an almost perennial phenomenon expected to worsen in the future under continuous warming conditions and increasing demand. Having a properly designed reservoir dam can help even out these differences in the water availability between the dry and wet seasons and ultimately curve the water shortage problem. It can reduce the risk of flood during the summer and ensure a continuous supply of fresh water during the winter dry months.

Building a reservoir dam also looks feasible from the fact that Bhutan has also enormous experience in building massive hydropower dams. Moreover, in recent years, Druk Green Power Cooperation has already started constructing some mini hydropower projects employing their expertise, without any foreign aid. Thus building a reservoir dam for water supply is also feasible both in terms of human and financial resource capacities if the government of the day sets the right priority. For example, in Thimphu City, a water reservoir can be built across the Ola Rongchhu or Ngabi Rongchhu after conducting careful feasibility studies including flood risk management strategies. Having such a reservoir in place would ensure a continuous supply of drinking water throughout the year.

Construction of the storage dam does come with associated risks such as the danger of outburst flooding to the settlement located in the downstream community. |However, there will be a minimal risk if the design is done with proper technical requirements, substantial reconnaissance studies, as well as continuous monitoring. For example, the dam can be designed to incorporate the technical requirements to withstand the maximum probable flood in the catchment. The maximum probable flood can be modelled using available gauged data (both flow and precipitation data), and freely available hydrodynamic and hydrological models like HEC-HMS and HEC-RAS models.  Other essential data like DEM can be also used from freely available sources or can be purchased at a reasonable cost from commercial satellites. Also, to reduce the impact of the probable flooding from the reservoir damn in the downstream settlement, land use zonations can be done using the flood hazard map generated from the flood modelling tools.

Shortage of water for drinking and other purposes has been among the most pressing issues in Bhutan. Unfortunately, it is expected to increase in the future due to increasing climate warming-induced weather extreme phenomena and increasing demand, especially in urban areas like Thimphu. A significant chunk of national budgets has been invested in ensuring a continuous supply of water to every household, but the problems remain far from being resolved. Building a reservoir dam can be an expensive project, but it is also one most unexplored water management strategies relevant to Bhutan.

The article is published based on personal experiences and observations by a group of water researchers from Bhutan