“When an abundant natural fall of water is at hand, nothing can be cheaper or better than water power. But everything depends upon local circumstances. The occasional mountain torrent is simply destructive. Many streams and rivers only contain sufficient water half the year round and costly reservoirs alone could keep up the summer supply.”
-W. S. Jevons (1865)
Serious students of energy policy should read the blogs at the Institute for Energy Research (IER), not only those at this site. The current blog at IER, “Renewables Generated 103 Percent of Portugal’s Electricity Consumption in March [2018],” explains that country’s unique situation of being hydro-dependent and wind-tied. And so it is that abnormally high rainfall has blessed Portugal this year–quite the opposite from a year ago.
Enter the wisdom of the ages, which in this case gets to W. S. Jevons’s The Coal Question (1865), the first treatise on energy economics (see here).
Jevons identified several problems with falling water as a major energy source, including location, seasonality, “failure by drought,” and flat terrain. States Jevons (p. 129):
When an abundant natural fall of water is at hand, nothing can be cheaper or better than water power. But everything depends upon local circumstances. The occasional mountain torrent is simply destructive. Many streams and rivers only contain sufficient water half the year round and costly reservoirs alone could keep up the summer supply. In flat countries no engineering art could procure any considerable supply of natural water power, and in very few places do we find water power free from occasional failure by drought.
Another problem: water (like other renewables) is a flow of potential energy, not a store that can be transported to where it is needed. It must be used where it originates. Jevons explains (ibid.):
The necessity … of carrying the work to the power, not the power to the work, is a disadvantage in water power, and wholly prevents that concentration of works in one neighbourhood which is highly advantageous to the perfection of our mechanical system. Even the cost of conveying materials often overbalances the cheapness of water power
Conclusion
Falling water, whether to power a simple mill (as in the 19th century) or a steam turbines today, cannot reliably or continuously power industrial society. A good water year will lower energy bills in hydro country. But a bad water year will shift the burden to fossil-fuels to fill the gap–short of a price spike to lower demand to available supply.
Hydropower is a combination of reliable power and source of opportunity power. The trick is determining how much of the power capacity of the hydropower facility is reliable, as California discovered 20 years ago. The remainder of the power capacity must be treated like solar and wind, useful when available.