Small hydro could add up to big damage
A turbine here or there may not affect the river noticeably; but if we are to use the technology extensively and put turbines in every other waterfall in a river, and make small dams on most of its tributaries or feeder streams, the environmental degradation - per kilowatt of power generated - will likely be much higher than that caused by large hydropower systems.
The factors that harm a river habitat with large hydropower projects are also at play with small projects: interrupted water flow, barriers to animal movement, water loss from evaporation and loss of biodiversity from the sacrificed portion of river are some examples.
With smaller dams, storage is an increasingly important problem that may require construction of more low-head systems (hydraulic heads that require a fall of water less than 5 metres) than anticipated. Reservoirs silting up or becoming overloaded with nutrients are common problems with major reservoirs that could be at least as serious where smaller and shallower bodies of water are created - the shallower a water-body, the more easily eutrophic it can become.
Likewise, methane generation occurs largely where water and sediment meet, and this means that a shallower water body is likely to release more methane per unit area than a deeper water body. Shallow reservoirs are not unlike paddy fields which are known to contribute substantially to methane emissions, a greenhouse gas 25 times more potent than carbon dioxide.
Disruption from building roads and power lines is less for small hydropower than for large hydropower projects in absolute terms - but on the basis of disruption per kilowatt of power generated, the impact may be at least as severe, if not more severe.
By using small hydro extensively we could be on course to repeat the environmentally damaging history of large hydropower projects. Countries considering the technology should invest in research into the potential problems, and proceed with caution.
Tasneem Abbasi is assistant professor and S. A. Abbasi is head of the Centre for Pollution Control and Environmental Engineering, Pondicherry University, India. More information about their work is available at www.prof-abbasi.com.