One reason it's useful is that data for rainfall go back many years whilst data for generation, especially for a recently installed turbine, go back no time at all. If a relation between rainfall and generation can be established it makes possible a feeling of right perspective when an exceptional year comes along, - you are able to say to yourself "Ah yes, from past rainfall records this is something I know happens once in 'X' number of years".
To give an example: of late, I have been noting with consternation, - consternation bordering on pain, - how generation at the moment is lagging well behind previous years. By the end of February, kWh's generated were more than 700 less than last year and less also than the two years prior to that:
I know, of course, that rainfall varies from year to year but the question in my mind is "What is 'X' in this situation? - how often should I expect to have a bad year such as this?". With only 4 years of my Powerspout being operational, I don't have enough years of generation to feel I can give an answer.
As good fortune would have it, I happen to have a guide who can help. A lady called Ena who used to live on our hillside collected rainfall data for the UK Meteorological Office. She lived at the top of the hill that forms the catchment for the spring which is my turbine's source and so her records are ideal for my purposes. Unfortunately she moved away in 2012, and 2012 was, crucially as I'll explain below, just before I started generating. Before she went, she let me have her data for the previous 23 years, - data which I am sure are as accurate and reliable as is possible.
Re-arranging her 'calendar-year' data into time periods of 'water-years' (Oct 1st to Sep 30th) to make them coincide with the same 12 months over which I measure the generation from my Powerspout, reveals that in those 23 years there were just 2 years when the amount of rainfall fell below the distinct cluster of values which account for 17 of the 23 years:
...so this would suggest that dry years only happen once in ~10 years, with very dry happening once in ~20.
But what I would really like to do is establish a more definite relationship between annual rainfall and annual energy generation, - establish a conversion factor, albeit an approximate one, - so I can convert Ena's 23 years of rainfall data, measured in mm/year, into its electrical equivalent measured in kWh/year. I can't make that correlation immediately because Ena moved away before I had my Powerspout up and running, and to establish the correlation I need at least one year, preferably two, for which there is both rainfall data and generation data.
The direction this train of thought is pushing me is to embark on measuring rainfall; not to measure it for ever, only for two years. The bonus from doing it would be that I would be able to compute surrogate generation data going back to 1988 from Ena's rainfall measurements; the data would become the earliest entries in a reference list of kWh's generated, - a list which would be added to each year with actual data; no need to wait for years of actual data to accumulate, - from the moment it has been compiled the list would go far enough back in time for a reasonable perspective to be gained.
It must have been a labour for Ena to measure rainfall for 23 years. It would be nice if all that labour found a purpose by being applied in this very specific way.