In the recent past, numerous small-scale projects are being implemented for conservation and management of lakes across Bhutan. Major large-scale projects are also are in pipeline for management for water sources in Bhutan. This article highlights numerous uncertainties in the current approach to the sustainable management of these critical ecosystems especially restoration of drying lakes from an ecological perspective.
Revival for drying lakes across Bhutan was mostly aimed at maintaining an adequate water level in these water bodies for cultural and spiritual significance while it also ensures a continuous supply of potable water in some cases. It may to also serve as significant water holes for wildlife in those areas of limited availability of surface water during dry periods. From ecological perspectives, such water bodies can be categorized as wetlands. In general wetland ecosystem is the transition between aquatic and terrestrial system thus they were known for being one of the most diverse and critical ecosystems. International treaties like The Ramsar Convention on Wetlands of International Importance especially advocates for the conservation and sustainable use of wetlands.
In the recent past, wetland conservation projects were implemented in Samtegang Lake, Peling Tsho, Buli Tsho, Tali Tsho, and many others. One of the main project-based interventions in those lakes was the removal of vegetation from the lakes. Drying up of water bodies was also reported in popular sites like Dangling Tsho in Trashigang Dzongkhag and freshwater lakes in Gesarling and Goshi gewogs of Dagana Dzongkhag.
A key question for all such project managers and funding agencies is that do we really understand the main cause of the problem – drying of lakes? What are the best available pieces of evidence that vegetation removal would revive the lake water level? An idea of taking a simple approach but often, the costly approach of removing vegetation from these shallow lakes may not always result in revival or restoration of the water level in the lakes. Or at times vegetation growth in these lakes could be a part of the natural process of change in the structure of ecological community referred to as ecological succession and our interventions of fighting against these natural process could be counterproductive. Unless a proper scientific study is done and proper cost-benefit analysis is carried out, we may not find out the pros and cons of spending millions of public fund in the revival of these water bodies.
Based on literature review and my personal experience of working in the field of Eco-hydrology for more than a decade, main drivers of change to these ecosystems in Bhutan could be attributed to a combination of the following changes:
* Transportation of sediments from watershed during rainy seasons in the long run that would accumulate into the lake, which could be reducing the depth of water body. Once the depth is reduced sunlight can reach the bottom of shallow lake triggering the growth of aquatic plants. This is a critical step that would then result in cascading effects. Such changes in the long run completely alter the structure and function of the lakes. Often turning lakes into a marshy area, which too over the years might see a growth of trees and shrubs. The project interventions in such cases would be to reduce sediment load entering the lakes. Thus, the current approach of just fencing the lake with limited focus on overall watershed may not be enough to revive the lake.
* Surface erosion in the watershed would also significantly contribute to sediment load into the lake. Overgrazing, logging, road construction, ill-managed irrigation channel, forest fires and vegetation removal, in general, are some of the main activities of concern in the watershed. Thus, both natural and manmade causes of surface erosion need to be curtailed to reduce sediment entering the lakes. In some cases, sediment traps could be set around key waterways during the rainy season. Such mitigation measures would reduce sediment and nutrient loads into the lakes.
* Any significant change in the underground aquifer of the lake would also significantly influence water level in the lake. Part of the earth crust holding water in lakes over millennia due to geological changes or heavy mining activities might become more porous making lake bottom leaky. Not much intervention options are available in such cases, but the key question is how sure are we that such incidences are not happening to our lakes? Having a simple piezometer and monitoring the water level in an aquifer for at least a year should be able to give an indication of the reality of aquifer. Thus, lots of knowledge gaps need to be filled in terms of groundwater.
* In some cases increase in nutrient load into a lake can trigger significant growth of vegetation. Leaching of nutrients from upstream agriculture land (if chemical fertilizers used more than required), nutrients washed into a lake during a heavy storm or flash flood or unmanaged stormwater along a road would result in the transfer of all kinds of nutrients into lakes. Increase in nutrient availability in lakes can trigger an explosion of aquatic plant growth transforming the lake into a nutrient-rich eutrophic lake that has just the opposite clear-water appearance of lakes we see in the mountains of Bhutan. A nutrient transfer can also occur through streams draining into the lake. Increase in vegetation growth in lakes is a key indication that the lake is starting to change in its ‘ecological community’ structure. Therefore, maintaining buffer zones along the lakes, monitoring nutrient loads into incoming streams and lakes, and prudent use of chemical fertilizers in the watershed is key to maintain the status quo of the lake water level.
* Vegetation removal around the lakes could reduce shedding effect and expose lake to direct sunlight, increasing loss of water through evaporation. Rise in average temperature due to climate change can also have similar effects. On a larger scale, new growth of vegetation would also increase uptake higher amount of water, with implications on overall water balance. However, we do not yet know how much water is being lost through evaporation and evapotranspiration processes, and does it play a role in the drying of the lakes? It would be wise to have this measured to make an informed decision because the loss of water from each of the lakes via process would be different given the huge variation in the lake surface area as well as the shape and depth of the lake, the surrounding topography and vegetation, and its location.
Other obvious reasons for drying up of lakes could be drying of incoming streams or increase in extraction of water from the lake. There are already numerous claims on drying of spring water sources across Bhutan, which is an indication that next big change may come to larger water bodies like lakes. With so many questions unanswered to the main drivers of change in lakes and associated wetland ecosystems across Bhutan a simple solution of removing wetland vegetation or building temporary ponds in wetlands (e.g. artificial roosting ground for Black-necked cranes in Phobjikha and Gyatsa) may not be a sustainable solution. Overall, a comprehensive watershed management plan needs to be implemented in and around the lake, if the lakes are to maintain their status quo. On the other hand, in some cases, if lakes are undergoing a natural process of ‘change in the structure of ecological community’, is it worth halting a natural process at the expense of millions of public fund?
Kuenzang Tshering (PhD)
Student in Australia