I have a map that I got from Texas A&M that shows exactly what you are looking for. It showed average rainfall and how much watershed area was required for the different areas in Texas. It also had soil information which either added or reduced the watershed potential.

I will look for the book but it could be anywhere. I recommend searching for the books on-line and buying every pond book they have. I think it is actually offered through TEEX (which is part of A&M).

The data is actually pretty accurate when it comes to average rainfall but when you speak to them, tell them that they need to issue a revision to their maps because the drought we have had in the past 18 months has killed their averages!!!!