Theo is correct about the runoff numbers for different types of vegetation.
The good news is that the runoff numbers are based on the vegetation for the entire watershed.
Obviously you will still get the exact same runoff from the bean field regardless of what you do on your property as regards grass plantings. IMO, putting 10 acres in CRP will only result in a minor diminution in the amount of water making it to your pond.
However, it is quite easy for me to be wrong in theory! That is why I proposed that you try a little experiment first - and determine what best fits your needs.
If your pond still stays full in summer after planting a little grass, then you can probably go the full CRP route if you determine that is in your best interests.
