I feel your pain...if your pond is losing significant quantities of water, anything short of a complete renovation or relining the pond will not likely meet your long term needs. We are currently in the midst of a complete irrigation pond renovation after nine years of a progressively worsening problem with a degrading pvc liner that was installed in the early 90's.
If I understood you correctly, the pond doesn't appear to leak much in the winter time, but increases noticeably during the warmer months when you're irrigating....likely due to a lower groundwater level in-season? The severity of pond leaks can fluctuate wIth groundwater levels, and can be very erratic depending on recent rainfall amounts.
The tricky part about irrigation ponds is there is never a good time to fix them. It's almost impossible to do pond work in the off season due to the cold wet weather, and the water is needed when conditions are favorable for pond work....talk about a catch 22! It took a change in ownership here, and a window of opportunity this spring ahead of a no-till greens renovation, to finally get the support needed to address our leaking pond problem.
I searched high and low over the years trying to find cost effective alternatives to a complete renovation. If you are not losing a tremendous amount of water, here are a couple of "patches" you may want to consider...
http://www.ess13.com/full.htmhttp://www.aquablok.com/index.php?src=&submenu=Good Luck!