Our dug well has just 2 tubs but sits atop a spring. Normally the water rises a foot into the upper tub and since one foot = about 100 gallons, that's a 500-gallon reserve though no more than 450 would be available. In autumn 2001 the level was down 2' from the norm but it's not gone lower than that. As noted above it's groundwater, spring and all, and we just had to update the filtration system.
Two weeks after setting out the Stratus for the season, it finally got wet, 0.03" which didn't even settle the dust. Yesterday's 84/66 not only set a new mark for May minima (by 4°) but the mean of 75 also is May's warmest here. About a 50-50 chance it will be the year's warmest as well, going only by past records. Unless we get a surprise today or tomorrow, this will be the 3rd year of 23 in which we reached June w/o hearing thunder.
why do people say heavy downpour?
Probably for the same reason you posted "light downpour."