Neal,
Springs can be fickle and the way you develop them can make them productive or dry them up.
Step #1 Be careful how you dig
Step #2 Be VERY careful how you dig /forums/images/graemlins/grin.gif
We've done this, the best one being close to 100 yrs ago by earlier generations- about 8gpm, went dry about 5 or 6 times in the last 50 yrs. due to head waters problems. Never more than a day, after the flows were corrected.
The setup we changed about 25 yrs ago to its current state. We added a holding tank and do all our pumping and demand from there.
If you put too much demand on a spring, you can kill it. Trickles from a draft will last, fast flow will wash out or dry up (divert).
The spring itself was hand dug out about 3'x3' about 3 feet deep, then another 1 foot brick above ground to protect it and add a cover. The reservoir keeps the lower drafts covered with water at all times and when full is just above the upper drafts.
The flow pipe is just a few inches (about 4-5") below the top of the water area and is gravity feed.
Keeping the reservoir full also prevents the drafts from washing out (eroding). Each draft should be no more than about a pinky sized stream without too much pressure- usually beside/under/on top of a rock. Resist digging out these rocks, as they help maintain the integrity of the draft.
About once per year we climb in with a bucket and shovel and dig out silt and mud to keep the drafts clear and reservoir open.
You will want to look for your head sources of water above the stream and protect it from any disturbances and prevent any equipment traffic from crossing between it and your spring.
Spring harnessing is more of an art than a technology. Since you're not using it as a source of water for your house, much of the pressure of potentially ruining your water supply is reduced.
Start by digging a hole about 2 feet below where you believe the draft to be. About 2-3 ft across, about 2 feet back towards the drafts. You'll want to leave the last foot for hand digging (easier to pull soil in to an existing hole). Make the hole about 2-3 ft deep.
This hole should easily fill with water before any hand digging. We have clay in our soil, so water proofing the hole is not an issue here. Follow the same guidelines you would for the breast of a dam in your area and soil conditions.
You'll have to play it by ear, but keep that hole mostly filled with water. If you don't dig it first and just dig at the drafts and let them run to open surface, chances are good you'll stress the spring and may kill it.
Good luck and let us know what you find and how you make out.
-JC