I would say we are in the same boat and could go on for days on that's the Thai way. If I could find a good rum here it would ease that somewhat.
I would also lean towards build it and see. I started with 15 feet at the beginning of Feb, 2nd month of our dry season's 5 month span. By the end of November I was overflowing. But, I was half full before the track hoe left the job site, water is usually not more than a meter below the ground surface and usually around a foot.
I was thinking, were I you I would over build the dam, height wise and let nature take it's course, might plant a drain pipe at the bottom or somewhere in the dam, just in case. If you are not getting much runoff and filling only from rain it might work. You will need to consider the wide swing in water level. Mine swings around 3 feet during the year. So plan on how to overcome the sticky clay walk to waters edge in the dry.
I almost for got what is your goal with the pond? Fish? nuclear reactor cooling pond?
