For a well sealed pond, 32" of annual precipitation will keep it full or reasonably full. Well sealed dug ponds in NW OH will loose 12" -16" maximum of water during our drought summers. Loss of more water than 16"-20" indicates significant leakage. If your contractor spreads and compacts the clay properly that he finds below 15ft, your pond should stay pretty full all year. Plan to have a good over flow pipe that is well compacted into the pond wall for release of excess water from the pretty large water shed. A grassed or rock lined spillway may also be a good idea for heavy rain events. I like to have an elbow on the overflow pipe to capture extra water during the spring rains. A 4"-6" dia overflow pipe should be adequate for 1/2 ac.
See the important discussion and great information about soil compaction in this link. After reading it you may have a different opinion of your pond builder.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=395066#Post395066 The 20ft or more depth will put excessive strain on the windmill diaphragm & bearings due to the greater pressure needed to get air out of the diffuser at deep depths (16+ft). Aeration windmills work best long term in shallow ponds (6ft-10ft) where useful circulating bubbles appear during slower wind speeds 5-10mph. Buying a cheaply or even an average made windmill will likely result in more maintenance of bearings, diaphragms, and mechanics if the windmill has to constantly produce air at high pressure. Do your due diligence well in selecting the proper windmill. Be aware of slick sales pitches and warranties. Talk is cheap and usually empty after they have your money. I sold windmills for 8 years. Extra expense and homework up front will likely pay back dividends years later.
