In article ,
Alastair McDonald k
writes:

"Dave.C" wrote in message
.uk...

Also, the forecaster on News 24 (Jay?)said that today's rainfall would be
beneficial to gardens in the SE and also help the reservoirs. I wouldn't
have
thought it would make much difference to either if there is only the
predicted 5-10mm.

It will help, but only a little.


If you compare the area of land not covered by reservoirs with the area
covered by them, and assume all the land drains into the reservoirs, then
multiplying 10mm by that ratio, I reckon all the reservoirs would overflow :-)



If it worked like that, then we'd never have a problem. But once you
consider the limited catchment areas of the reservoirs, and what is lost
to evaporation, transpiration by plants, and percolation, then I'd guess
that you'd have to multiply in another factor of at least 0.00001. (Or
pedantically "at most" rather than "at least", but you know what I
mean.) No doubt the water engineers will have done the calculation. Of
course, the factor will vary a lot depending on the time of year and the
weather.
--
John Hall

"I am not young enough to know everything."
Oscar Wilde (1854-1900)