Posted this web link on the sizing a spillway thread above but I think it will answer these questions about the basin required for a pond of a given size. For those interested in the runoff calculations it will show the most accurate method. The variables are given for Michigan, so you would have to obtain the corresponding runoff constants for your area.
Meadowlark is right, runnoff is determined by local conditions, especially soil and vegetation types or the presence of paved/impermeable surfaces (clay), also the condition of the vegetation. Runoff is also dependent on the rate of precipitation and slope. Does precipitation in your area tend to fall in light steady rain or big gully washers. Hard rains falling on steep clay soils will have a high runoff factor and produce huge amounts of water. Light rain on flat sandy soils with thick vegetation will produce little or no runoff.
The 10 to 1 rule is a starting point, but to do it right, all aspects of the drainage basin are included e.g., acreage, evaporation rate, annual precipitation (in the north we have to divide precipitation based on the season), transpiration rate, surface water inflows/outflows and groundwater gains or losses. Most of this information is published and available for the design of detention basins, stormwater structures, roads and other engineered structures and can be obtained and used for pond design.
big_pond, you should be able to Google "pan evaporation Georgia" and find the information.
Calculating runoff
