Powering a ram pump with rain water.

I was thinking about using rain water from a gutter to power a ram pump. I don't need to pump the water very far. The fall is about 12' and the area of the roof is 15-40'. Would this be enough water to operate a ram if the gutter just emptied into a pipe instead of a downspout?

interesting thought for sure man, i wonder...

The equation for a ram pump is D=(S x F x E)/L.
D = delivery volume
S = Supply volume
F = fall (or head)
E = efficiency (use 0.30 for a home-made pump)
L = lift height.

Ignoring all time derivatives and stuff, this equation will allow you to figure WHAT PERCENTAGE of the falling water can be pumped. In the best case scenario, where the pumping height is the same as the head height, you can pump 30% of the falling water. (So it is 70% less efficient than simpy using a gutter)

Now say you want to pump your rain water up 30 feet. Now you get D = (S x 10 x 0.30)/30, or D = 0.1S. So, you can pump 10% of your rain volume up to that height.

Of course, with too little flow the pump won't work at all, so you will need a collection tank on the roof to feed the pump.

What exactly is the purpose of this water? Perhaps there is an easier way, like collecting rainwater in an open-top water tower instead?