I think this has something to do with my question ("Black Equivalent Volatility"). I just realized that the answer might be your question:
Knowing this information what could be the appropriate measure of computing volatility of the economic returns?
In Energy Markets, like oil and electricity, one model we use is the mean reversion in the natural log of the spot prices.
$$d(ln(S)) = a(b-ln(S))de + vdz$$
where:
$S$ = spot price
$t$ = time of observation
$a$ = rate of mean reversion
$v$ = volatility
$b$ = long-term equilibrium
$dz$ = random stochastic variable
Now there are books that would show you how to solve for the volatility in that equation but i think the best one for me is Dragana Pilipovic's book entitled "Energy Risk 2nd Ed" (chapter 5, page 108)
And I think the black-equivalent volatility is a short-form equation that you can use off the bat. So here is my answer to your question:
black-equivalent volatility = volatility x $\sqrt{(1-e^{-2aT})/2aT }$
where:
$T$ = period of time (20 years)
$a$ = rate of mean reversion