I did something similar - used this pond pump. It has an auto-turn off is the water level is too low, then turns on when the water reaches the right depth.
Amazon has them (as usual)!
I bought this for my run off, works great! Has a water sensor on it so once the water level drops the pump cuts off.
https://www.amazon.com/dp/B004SZV4WQ?psc=1&ref=ppx\_yo2ov\_dt\_b\_product\_details
I use this pump in my swamp cooler, as it has a built in auto shut off when dry. That way the fan still blows for ventilation but the pump won't kill itself.