Data won't stop aggregating

I've not done any work myself with data at the minute granularity but I have read that the way to handle it is to split it into 2 fields, one for date and one for time and to keep the precision as low as is useful (if you don't need to go to the seconds level then don't).  That way you have a year of dates which is 365 rows and a table of hh/mm which is 1440 rows rather than 31.5 M rows for Date/hh:mm:ss for a single year.

Then you link Flow and Rain into both the Date and Time tables.

You can make a time table in excel just by starting with 1/24/60 in a cell (this is 1 minute in excel terms) then add that to 0:00 and fill down.  Your list will increment by 1 minute on each row.