India is a dammed country. According to the National Registry for Large Dams (NRLD), in 2016 a total of 4877 dams were built in India and 313 dams were still under construction. These include the Bhakra Nangal, Nagarjunasagar, Kosi, Chambal, Hirakud, Kakrapar and Tungabhadra that were built to harness water for hydropower, irrigation, and domestic water supply and have been seen as a sign of development and economic growth, informs this paper titled 'Dam-induced hydrological alterations in the upper Cauvery river basin, India' published in the Journal of Hydrology: Regional studies.
The dammed Cauvery
Cauvery is one of the important peninsular interstate rivers in India that has been intensely altered by reservoirs, barrages, canals, and anicuts (masonry check dams constructed across streams to divert water) to meet the rapidly growing water demands for irrigation, household consumption, and power generation.
While the reservoirs have helped to expand the irrigated areas in the basin and securing water availability during water stress conditions, it has also given rise to water conflicts between upstream and downstream states over water allocation.
There are around 96 dams constructed in the Cauvery basin during the last 1000 years. Of these, 70.30 percent are being used for irrigation, 19.80 percent for hydro-power generation, 6.93 percent for both irrigation and hydropower generation, and the remaining 2.97 percent for drinking water supply. The Cauvery delta, Hemavathi, Mettur, Krishna raja Sagara, and Harangi irrigation projects are among the major irrigation projects in the Cauvery basin.
Extensive damming has led to degradation in water quality and sediment transport has also been adversely affected, which has impacted freshwater ecosystems leading to changes in aquatic species composition in the river waters. The population of migratory fish species such as Tor spp., Lates calcarifer, Bagarius bagarius, and Anguilla spp has declined due to reduced flow rates in the river.
Hydrological alterations in the Cauvery
These changes call for the need to assess the degree to which hydrological flows have been altered, i.e., deviated from the natural flows, by the construction of dams in the river. The paper describes the findings of a study that conducts an assessment of the hydrological alterations triggered by the construction of major dams in the upper Cauvery basin.
The study thus aims at i) developing and implementing a robust human influenced hydrological model that can reliably simulate pre reservoir hydrological regimes and ii) deploying a systematic assessment of pre and post reservoir flow regime changes in the basin.
The study finds that:
There has been a decrease in monthly flows of the river
The average monthly flow in the Upper Cauvery basin has been greatly influenced by reservoir operations and subsequent water abstractions in the basin. The reduction of streamflow in most of the months has been leading to increased scarcity of water in different seasons.
There has been a decrease in monthly flows across all the sub-basins throughout the year due to reservoir operations when compared to its natural flow regimes. The different operation rules in different sub-basins have varying degrees of influence on downstream flow timing, pulse behaviour, change rate, and frequencies of flow.
The decrease in summer flows can have negative effects on the aquatic habitats and migratory and reproductive biology of fish species in the downstream areas. The frequency and duration of high and low pulses are critical for supporting the migratory behaviour of fish during the spawning season.
The construction of reservoirs has led to reduction in the natural flows of the river and changes in natural flow pulses have threatened the survival of migratory fish species Tor pitutora in the river basin. In addition, variations in fish assemblage structures have been affected since the structures are strongly associated with mean daily flows, base flows, number of zero-flow days and high-flow pulses.
Frequency and duration of low pulses affected
The frequency and duration of low pulses have also been impacted across all the studied sub-basins in the river.
Decreases in low pulse durations was observed in the Kudige, M.H. Halli and Kollegal sub-basins which can worsen the eco-hydrological environment of the river and surrounding floodplains. In contrast, an increase in low pulse duration was observed in the T. Narasipur sub-basin where the hydro-power reservoir is located since the reservoir releases excess water to preserve flood control capacity from June till August, after which it releases flow to meet the power generation requirements, which then increases the low flow duration in the T. Narasipur sub-basin.
Hydrological connectivity between the river channels and floodplain is dependent on the intensities and durations of high and low pulses and determines the habitat for aquatic species in the dry and wet seasons. Flow pulses also provide essential carbon inputs to the riverine ecosystem and strongly support the aquatic food web. Thus, floodplain ecosystems are dependent on naturally dynamic river-flow patterns. Changes in flow patterns directly affect the floodplain habitats and biodiversity.
Dam impoundments and desilting affect water quality and water availability
Dam impoundments cause salinisation and waterlogging which impact the water quality while desilting of reservoirs and canals has been found to reduce irrigation availability as the tail-end areas do not get adequate irrigation water for the second crop thereby reducing the area for agricultural production.
The reported geomorphic consequences of dams include bed armouring, changes in bedform morphology, and sediment depositions downstream that directly affect the channel morphology by narrowing widths, deepening channels and arresting flow within the channel.
Inclusion of environmental flows to assess impacts of dams is crucial
Evidence increasingly suggest that dams significantly affect river flow regimes. The costs of dam removals are huge and have both economic and social implications. However, the ill effects of the dams can be minimised by incorporating environmental flows as an integral part of dam development programs.
Since irrigation reservoirs have a distinct hydrological influence over hydropower reservoirs, there may be a need to differentiate the e-flow setting based on the purpose of the reservoirs. There is thus a need for more research to compare the flow regime changes made by reservoirs serving various purposes to establish e-flow standards that specifically target the impact of that type of reservoir operation.
In rivers where dams are already operational, there is a need to recalculate the reservoir operation rules to take environmental flow requirements into account, thereby reducing their negative effects. However, environmental flows are still not widely acknowledged in India. Even though the Supreme Court of India has mandated a minimum flow of 10 per cent for rivers like Yamuna and Cauvery to improve the water quality, the water released from various dams is not well aligned with such environmental flow requirements.
There is also a lack of data on the relationships between flows and ecosystem functioning, which impedes the implementation of environmental flow assessment. Further, the existing Environmental Impact Assessment (EIA) system in India has failed to examine and mitigate the broader consequences of widespread dam-building.
There is a need to improve the EIA by including the timing and duration of low/high flow pulses during the impact assessment of dams in relation to the environmental flows. It can be more effective if the flow requirements for dry and wet years are assessed separately for different reservoir storage levels and reservoir purposes. In addition, the tradeoffs between water security of different stakeholders need to be considered during the design and construction of the dams.
The study thus presents a way forward to understand the impacts of dams at the basin scale under data-scarce conditions and can help basin managers in formulating strategies to allocate water for both human and environmental needs, argues the paper.