At the time, I wasn't very sure what he meant. I assumed he meant digging a sizeable reservoir. The nature of the terrain made this an impossible proposition so I was left wondering about his advice, - when he was with me, I didn't like to show my ignorance by asking him to explain !
Gradually I came to realise why storage is valuable, - not to provide a huge volume in the hope of extending generation into the dry months but just a small volume to facilitate day-to-day operation by buying time before action needs to be taken when outflow and inflow don't match up.
How valuable such a buffer proves to be depends to a considerable extent on the behaviour of the flow one is capturing, - and operating a small hydro certainly brings you into an intimate understanding of the behaviour of your particular water source.
Such understanding comes slowly. After three years I am still getting the measure of how, as summer progresses, the flow initially drops off quite quickly but then slows down in its rate of decline. Plotted on a graph, I think it would be an exponential curve.
Another fragment of this understanding is that a bit of rainfall in the summer makes little or no difference to the rate at which flow falls off. Yet another fragment is that when winter rain starts, it takes several weeks before the flow begins to pick up, but when it does pick up, it can pick up very quickly indeed: in my 2014/15 water year, flow went from 1.18 L/s to my maximum of 3 L/s in the space of just one week.
With a source like mine which, when it is diminishing, changes only slowly, my 5 cubic metres of tank storage means I can wait several days, even a week, from the time when the tank stops overflowing to the time when I need to step in and change to a smaller nozzle. Only because of this behaviour am I able to operate as a 'variable head' site, as described in an earlier post.
But for a different site where the flow is more 'flashy', by which I mean the flow rises and falls rather quickly in response to rain falling, even my generous 5 cubic metres of storage would be inadequate to operate in such a way. It wouldn't buy enough time, wouldn't provide enough of a buffer, before a nozzle change were to become necessary.
So the merits of putting in a tank at the top of the penstock, and the size of it, really depend on the site hydrology.
EcoInnovation has recently published a very good document about constructing intakes for their Powerspout turbines. It can be found here. In it, mention is made of the benefits of providing storage and it is well worth reading if you are at the stage of wondering about putting in a turbine.
To illustrate how a storage tank can be sited relative to the take-off point from a water-course, here are some pictures: