**2. Sustainable water flows**

Changes in the flow of river due to channel straightening, disconnect with flood plains, dam construction and loss of river ecology has a great impact on the overall ecology of the river, climate change is added problem. Fresh water ecosystems were studied through PRINCE (preparing for climate change impacts on freshwater ecosystems) project in UK which has shown that changes in climate have directly

affected the aquatic ecosystems [14]. The changes in climate including changes in duration, frequency and magnitude of precipitation, especially extreme events, influences the aquatic ecosystems through episodic pulse effects. Combined with the changes in water velocity and dissolved oxygen content of the water bodies, the effect of temperature becomes more pronounced for fishes, amphibians, as well as birds. The more conspicuous effect would be on migratory or dispersal patterns between ecosystems such as between marine and freshwater, long-distance migrants or across watersheds. Hot summers with limited rainfall eventually decrease the amount of dissolved oxygen, particularly in standing waters, and rivers in middle and lower courses, where re-aeration in less. In addition, such water bodies also have high amount of nutrients resulting in profuse plant growth but leading to low oxygen levels which can be threat to fishes and invertebrates of water bodies. On the other hand, the high flow of storm water improves oxygen levels, but increases the contaminants such as pesticides and fertilisers. Such conditions take a toll on vulnerable sections such as ephemeral ponds, ditches and small water bodies which are most affected due to changing conditions. The sensitivity and resilience of the surface water ecosystem gets effected due to magnitude, variability, frequency and mistiming of the water flow and quality.

Surface water resources like lakes, rivers and reservoirs, as well as frozen water in glaciers and snow covers, the internal renewable water resources, receive water from total precipitation and upstream streamflow produced. The aquifer recharge is also dependent on the same. Ratio between total precipitation and total streamflow, runoff ratio, vary from country to country and is a factor of terrain, the mountainous region and steep terrain produce more run-off [15]. Hence total renewable water resources are the sum of internal renewable resources and surface run-off water. The upstream and island countries like Bhutan and Sri Lanka respectively have minimum run-off component whereas Bangladesh receives more than 90% [1]. The risks with water bodies gradually get worse with the climate change. Studies in tropical and sub-tropical regions have predicted increase in extreme flows due to climate change [16]. Land use induced by climate change may increase the erosion rates. Use and managing water resources which are differentially affected by modified rainfall patterns, temperature increase, and sea-level rise are very crucial in the times of climate change.

Hydrological impacts on river systems are aggravated due to climate change. The prime cause of reduced water flows is the increased water extractions, also a consequence of global warming. Overexploitation of both surface and ground water has affected the surface water flows more predominantly. The health of the rivers has been severely impacted due to consumption being overpowering. The per capita water availability has sharpy declined in many countries in the recent past [17]. With growing economy and ever-growing population, this is expected to become worse [18, 19]. The microscale studies on effect of climate change on different rivers or studies at global scale showing anthropogenic stresses on river systems, have not been thorough in understanding the situation in most countries, the meso-scale analysis of cross-continental basin-scale comparisons have revealed that water extractions from the rivers of past and present, have exceeded the projected impacts of climate change to much larger extent [20]. This finding realises the importance of effective water governance to reduce present water stresses and further deterioration because of decline in water flows and consequences of climate change. The current water-use practices consume water at the expense of natural water flows [21, 22]. With the threat of climate change, the policies to reduce consumptive use of water and maintain sustainable in-flow water uses for a longer time need to be encouraged. The water allocation management plan should share the variability equally between the users and the environment.
