Hate to see you problem there Big Pond. Sounds like to me your are in the drought zone that runs all the way back into my area. I only have 1 pond that works off water shed and that is my 3 AC pond (est 22' max depth) that also had covered a couple small sprihgs and no upland water to direct into the pond. Like yours it was full and running out in 2009 into 2010. When we went into a drought faze in Mid 2010 I chose to drain it not long there after. I reworked the bottom again in 2011 / 2012 to make sure I stopped any new leaks as best I could. I had run into this very same problem in the past when drought times would kick in and I put in a high volume well first. I live in an area that is less than 200' above sea level and have alot of underground water. Many years ago when I had your same problem I dug an 8" well 220' deep. I have a 10 HP pump that was originally measured to pump right at 438 GPM (well over 600,000 gallons per day). I have only the well hooked to this electric meter so I can get a serveral cents per KWH break as agricultural useage only. Back in the day I was losing about 1"+ water per day during summer drought. Now I am losing alot less and when I get rain I do not have to even run the well. The same well today would cost you about 15K. All that takes me to the long road of what I am saying. Basically you have 2 things to do to get out of this problem with a full pond. That is dig a well or rework the pond and hope mother nature provides enough water to cover any loss (in my case I have done both even though the well worked fine by itself). Both in my opinion require getting someone that can help you figure out where you are at and what you need to do. A good pond man (some have mentioned Mr grimes for example) can come out and look at your situation and tell you are really even leaking that bad. Dropping 12' fot your situation in drought times may even possibly be normal and all you would need to do is get a well / water specialist in to tell you what you can do on that end to keep the pond full. Me personally I chose to go with the high flow well and came back a few years later and reworked the pond at my convienance.
Also to give you an idea on what the well cost to operate to pump to keep your pond full I can tell you this. 1 acre foot of water (326,000 us gal) cost me between $12 to $13 in electric bill. If your whole pond was down 12' over 9.5 acres (over estimation since your leakage is probably more at full pull and you pond should have a slope) and if that is how much water you would have had to pump to keep it full. That would come out around 114 acre water foot. It would take me probably between $1400 to $1500 to pump that much water. That is one of the reasons why I chose the well route first and to rework later at my convienance.
As a side note Along the way I discovered that I can constantly pump 68+ degree water into my pond and extend my fish growing time when my other ponds are cold. Also I can jump start that pond early in the spring when I see the cold weather is coming to an end by pumping in a bunch of warm water. The high volume well gives me alot of advantages others do not have. Several times over the years I have just opened a valve to drain the 3 ac pond. It is something easy to do when you know for about $500 to 700 (depending on the rain you get while filling up).
In the end you are getting sound advice to have someone come out and look to help you make the proper decisions. Also do not be suprised it you get an answer telling you to maybe do both.
