I am wondering just what your "goal" is with the partial water changes....?

I think you want the water to not be muddied, and if that is the goal, you need to kill off the bullhead. Bullhead could have swam upstream and into your pond during a rain event, or were stocked as hitch-hikers with other fish.

You could pump the water into your orchard, but I would put netting in the flow back to the pond to catch any size fry, then sterilize the pond with Hydrated Lime and refill/restock. Once you have the problem fish out, you could water your orchard without concern.