BTW, this is classified as 'green' because my wife exclusively uses the water for the two 8x12 ft. gardens we have in our yard. We never water with a hose.
[Update 1 May 2010] We had a storm last night and got 3/8th of an inch of rain. When I looked this morning, the second barrel was full and was slightly overflowing down the sides. I just wanted to make sure that there was no problem with the pipe transferring the water from the first barrel to the second. In fact, despite the fact that the rain had stopped, water was still dripping from the downspout into the first barrel. The same amount of dripping was falling into the second one.
Step 1: Materials needed
-- 2 rain barrels (preferably PVC used by the food industry)
-- Length of 1/2 inch PVC pipe
-- 2 PVC connectors for 1/2 inch PVC pipe (threaded male end on one side and the other side smooth female)
-- 1 extension connector that is female threaded on each end
-- 1 right angle connector that is female smooth on each end
-- small container of PVC cement (optional)
-- Bricks to elevate the first barrel
13/16 inch drill bit
I paid $30 each for the used rain barrels at a local Indianapolis company. They come with with a cover that has both a larger and a smaller filler holes with threaded caps.
The PVC materials cost less than $5.
Step 2: Drill hole in the first barrel
The inside diameter of the PVC will be 1/2 inch but the outside diameter is larger, of course. I guessed that the size of the hole that I would drill was 13/16 inch. This proved to be perfect. It Is just open enough for the connect with male threaded end to grip withing the hole. If you make it any larger, it will wobble around.
Step 3: Attach the connectors to the barrel
From the outside, thread the extension connector to the connector that is threaded through the barrel. Hand tighten until it is snug. The water flowing through the overflow connection will not be under pressure and if it leaks at all, it won't be much.
Step 4: Add the extension pipe and right angle connector
From the 1/2 inch PVC pipe cut a length long enough to reach a position directly over the filler hole of the second barrel. I merely slipped this pipe into the smooth end of the connector we just screwed in.
Finally, slip the right angle connector onto the pipe with it pointed down towards the opening in the second barrel.
I did not cement any of these slip-on connectors because I wanted the option of rotating them or of replacing the pipe with a one of different length. You could also add one final lenght of pipe from the right angle connector down into the barrel but this is not really necessary and it would impede the removal of the lid.
Step 5: Final result
You can optionally insert a spout near the bottom of the first barrel. This barrel should be high enough off the ground the permit a bucket to be placed under the spout. If you also want a spout on the second one, the whole setup is going to have to be raised unacceptably high off the ground. With a previous rain barrel that we had, we found that the spout filled the bucket too slowly. That was true also if we connected a hose to it. With the current setup, my wife said that she would prefer that the barrels be kept lower off the ground and she simple takes off the lids and dips water out of them. This makes it a much faster operation.
Although I haven't done it yet, will somehow fixed some sort of netted material (such as the bag that onions are sold in) over the openings of each barrel. This is to prevent debris from going into the barrels and to prevent mosquitoes from breed there. There is no easy way to do this. I thought of using velcro to hold the material in place.
You will notice that there is no mechanism for letting the second barrel to overflow in a controlled fashion. Note that I have calculated that a tenth of an inch of rain will fill one barrel. If you get a half inch of rain, the second barrel will definitely be overflowing. For my arrangement, the water simply flows over the top and down the sides. Since our house is built on a slab, this is no problem for us. On the other hand, if you feel there is a need to lead the excess water away from the house, just drill another hole in the second barrel and use a similar setup to direct a longer PVC pipe out further into the yard.
Step 6: A variation with three barrels
I decided that, instead of having the first barrel drip into the second one, I would have each barrel flow directly into the side of the next one. The connectors are the same as before. I simply cut a 4" PVC section of pipe and slipped it into the two female smooth receivers on each barrel. If I want to disconnect, I simply pull an outer barrel away from the next one and it slides right out. There is no need to cement the pipe in. You can see the results in the two photos.
Lesson learned: These barrels seem to come with several variations in design and I wasn't aware of this until I got them home. The barrels I used in the main tutorial are squatter but the same capacity of the the two newer ones I bought. The barrel on the left is of this type. Notice that it is raised up on a wood platform. I had to do this because the two newer barrels were taller and the third one had to be raised to meet the pipe coming in from the second one. Another problem is that the two newer ones have an inward slant near the top. Drilling a 13/16th inch hole near the top would have pointed the pipe upwards instead of horizontally. I had to make the hole about 3 inches from the to make the pipe horizontal. That means that I am wasting some of the capacity. When you buy barrels for a project like this, pay attention to the design.
[update 19 June 2010] I just added another instructable involving the rain barrels at this shed: http://www.instructables.com/id/Redirect-water-from-a-raingutter-to-a-rain-barrel/
This shows how I directed water from the front gutter to the barrels in the rear using PVC piping.
This shed is the source of all kinds of experimentation. Yet another instructable shows how we added solar panels to it: http://www.instructables.com/id/How-to-mount-a-solar-panel/.