Shade -
Starting with a disclaimer: I have no way of knowing what will really happen, so all this is conjecture...
Here in Cali, and presumably in many other (though not all) locations that take advantage of hydroelectric power, the strategy is to use reservoirs with dams to capture the water so that it can be gradually fed through the hydro generators. The assumption is that the winters are cold enough to freeze the water in the mountains (e.g. snow & ice) so that there is, in effect, a natural reservoir which gradually replenishes the man-made reservoirs throughout the spring. Any excess amount of water that gets into the man-made reservoirs must be shunted through bypass gates in order to preserve the integrity of the dam. If there is a drought summer or less than average snowfall in the winter, the water release through the hydro generator must be rationed or even shut off completely in order to guarantee a minimum flow through the downstream river throughout the year.
It is possible that global warming / climate change can affect hydro power the following ways:
- As you noted, with more of the earth covered in water and with temperatures a bit higher, the water will evaporate more readily. And therefore it will come down more readily, and possibly (likely) in amounts that the man-made reservoirs were not designed for. Potentially even at such high amounts that the shunt gates cannot keep up with the in-flow. Once the reservoir reaches a max full level, the hydro generators need to be shut down for safety reasons.
- With higher temperatures, it's possible (likely) that the snow pack will be significantly reduced, which means nature's reservoir is reduced, and that there is not enough to gradually fill the man-made reservoir throughout the spring. In turn, there is not enough water to sustain the hydro power throughout the dry summer and early fall seasons.
Most (not all) hydro power systems are dependent on a natural reservoir of frozen water that feeds the man-made reservoir at a known rate (within a range) and a known amount (within a range) year over year. Presumably, the effect of global warming will completely invalidate those assumptions thereby making the existing hydro installations significantly less effective.
All this does assume that the global warming-era typical rainstorms will be more torrential, so that a much greater proportion of water that does fall ends up being unusable for power generation.
On the other hand, it just might work out for the better, with more water available throughout the year such that the man-made reservoir is always full enough, but never too full, for constant hydro power generation... who knows...
