A downspout drain? Damn, never heard of one. I guess I need to get out
more, or maybe I just live in a different part of the country.
{Sorry long post.}

Smitty-Two:
I am sure you have heard about this. I am now living in my third house
where as part of the local municipal property transfer process, there must
be an inspection of the down-spouts not being tied into either the sewer
system, nor the street storm drain system by way of a downspout drain;
direct tie-in, nor drain pipe to street curb. The inspector dumps a
powerful dye in the roof gutter, and sprays water or some such while some
one else watches the water color at a manhole cover or street. Seller of
house must make repairs before sale of home can go through. As far as I
know, this requirement was mandated at a higher government level.

When I lived near Pittsburgh, PA, heavy rains, combined with slope of land,
and downspout runoff could erode a good hunk of yard down the hill to your
neighbor. The home I purchased there had to have a backhoe dig a 6'X6'X6'
deep hole, filled with gravel and covered with 2' of soil. Size and
location of 'dry well' ended up on county land plot document given to new
buyer when house was sold again.

I don't know all the details, but rain surge water in sewage system can
overwhelm a sewage process plant thus forcing raw sewage into water ways.
Plus storm drain systems are more often now required to filter out yard and
leaf debris as best they can before discharging rain water into water ways.
Downspout adds to amount of water to be filtered. Where I live now, the
debris filtering is designed to overflow (raw discharge leaf debris) only
during real heavy rains.

As far as the O.P. of this thread goes, what I as sure of is that his
neighbors will have almost the same problem of rain runoff potential
problem. What is the solution the neighbors have, and also, what does the
municipal building inspector have to say. The O.P. could be correct, the
size of the cistern is too small, but a larger cistern would be more of a
mosquito egg laying and breading pit than helpful for rain run-off.

I did hear on one of the Home-repair PBS shows that downspout rain water was
collected in one region of USA so the garden water was not added to water
bill (very high water and sewage rates,) not for rain runoff reasons.

Phil