Thanks for the link to the handbook. If I'm reading it correctly, the 7 acres of watershed is per "acre-foot" which would support 1.1 surface acres (assuming 6' average depth).
Yes, their estimate is per "acre-foot". That makes no sense to me, since the evaporative water losses will only be a function of surface acres. The depth of the pond (hydrostatic pressure) makes a very slight difference on seepage losses. However, I don't think that is enough to use the acre-foot term.
I have posted the question about their units in some Pond Boss threads, but no one has ever explained why their choice is correct. Most of the handbooks published by state agencies give rules of thumb for watershed area per surface acre of pond.
I believe (as a non-expert) that you probably have sufficient watershed for your planned 2 acre pond. Note that it is also a problem to have too much watershed for the size of a pond. If that is the case, then you will constantly fight flooding and a chance of water overtopping your dam during big rain events.
Finally, there is no substitute for empirical data. Is your area still in drought conditions? If so, then go observe the upstream 0.5-acre pond. How much is the water level there below normal full pool? Further, you could pull some satellite views for some nice ponds in your immediate area. Planimeter the area of the pond, and then estimate the contributing watershed from the topo maps. That should get you into a good range for your property.
Have fun on your project. A 2-acre pond in east-central Texas should be a valuable addition to your property!
