Water scarcity could become one of the major 21st-century challenges for Bhutan, as more than 60% of the population depends on water-sensitive sectors like agriculture and hydropower. Water scarcity is also associated with a range of other issues like food shortage, poor hygienic practices, and poverty. In the Himalayan region, the common factors attributing to the drying of water sources include climate change, land use, land cover alteration, and changes in water management. Thus, adaptation and mitigation strategies to protect and manage water resources are urgently needed. Unfortunately, policy measures for water resources management in Bhutan are severely impaired by limited observations and poor understanding of hydrological processes in our catchments.
The main sources of potable water in Bhutan include streams, rivers, and springs. These sources are fed by precipitation, seasonal snowmelt, and glacier melt-water. The low elevation valley may also contain a surficial aquifer that is recharged directly by the surface runoff from the surrounding karst mountains, e.g., Rangtse in Haa and Lawa-Lamga in Wangdue. The linkage between surface water and groundwater is often not acknowledged appropriately partly due to a lack of scientific evidence. For example, groundwater is often portrayed as a new source of water in policy documents such as Bhutan Water Vision and Bhutan Water Policy, 2014.
Consequently, afforestation is perceived as the default management intervention to recharge drying water sources across Bhutan. Similarly, there are ‘experimental’ approaches by digging trenches upstream of the catchment to augment the recharge of the spring sources. Such interventions may achieve the expected outcomes when loss of vegetation cover is the underlying cause. However, such interventions without sufficient data on precipitation, infiltration, evapotranspiration, soil properties, and groundwater hydrology are too risky project to achieve the end goals of recharging spring water sources.
According to the latest studies, it is not about “no trees” or “lots of trees” but “intermediate tree coverage” that does a good job of maintaining groundwater reserves. The litter inputs from trees and roots improve soil infiltration and organic matter, thus enhancing soil hydraulic conductivity. These increase the percolation of surface water into the ground and reduce surface runoff. Such a mechanism could relate well to Bhutan, where steep slope topography is dominant. However, this water retention ability of the trees is countered by water loss via evapotranspiration and interception when tree density exceeds the optimum threshold. The infiltration and percolation capacity of soil also depends on factors like rainfall intensity and duration, soil texture and structure, vegetation type, topography, drainage density, and local lithology. Therefore all or majority of these factors are worth considering when coming up with any water source management interventions.
To improve our understanding and develop sustainable water management strategies for forested catchments of mountainous Bhutan, the authors of this article propose the following 4 critical things to do:
• Identifying the groundwater and spring recharge potential zone – Remote sensing and GIS technologies are excellent for first-order mapping of potential groundwater recharge potential zone. The essential data such as soil, rainfall, terrain, remote sensing imagery, and lithological data can be integrated into the GIS environment to map potential groundwater and spring recharge zones. Although it only provides a rough estimation of the larger catchment, such a first-order assessment can be excellent preliminary evidence to further explore interventions with localized finer resolution strategies.
• Determining water balance for the catchment of concern – This exercise should provide scientific evidence on the ‘inflow’ and ‘outflow’ of the water from the catchment. Monitoring of water discharge in surface water and water levels in simple borewells under different seasons can be a minimum requirement.
• Conducting soil and site evaluation investigations to clearly map the role of soil and vegetation profile in catchment hydrology. Probably gathering such hydrological data across the country in ‘sensitive catchments’ could be mainstreamed into extensively monitored national forest inventory plots or similar initiatives for capturing data on hydrological regimes in the catchment of concern are urgently needed.
• Improving our understanding of the linkage between surface and subsurface systems using environmental isotopes – Use of those isotopes to identify compositions of groundwater, river, and canal water to identify the recharge sources, zones of recharge, and groundwater flow is one of the most advanced hydrochemistry techniques. A group of researchers from the Royal University of Bhutan is conducting a pilot study using geochemical and isotopic tracers to determine the sources and mechanism of recharge, groundwater origin and its evolution, and the recharge area of perennial springs and streams used for drinking and irrigation by the rural communities in the Radhi and Phongmey gewogs in Eastern Bhutan. The findings will be helpful in making an informed decision and implementing the site-specific catchment management plans. For the first time in Bhutan, we would be able to track and identify the dominant source of water contributing to streamflow.
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