Hey MB I answered some of this in your other post about your trees. Now as to why your losing so much water? It could be a number of things. Trees maybe being one of them. Lets see what others have to say.
P.S. Here is one other stat a lot of folks don't realize.
A 50 foot Pine or Oak tree and suck up to 50 plus gallons of water a day!!
Ok so times that by 15 trees... That's 750 gallons of water a day sucked up the trees. Plus you lose about 1/4 inch in evaporation a day in the summer and bingo before you know it your down 2 feet!!! Not sure if that's your problem just throwing it out there!
Good Luck
RC