I received a followup email from the caregiver who used umbrellas to drive off the dogs in their area. He said there was a pack of 5 dogs, and they had killed two cats and injured more. What they did was just to go down and wield the umbrellas and wave them at the dogs, while making noise to drive them away. When he and his fellow caregivers did this, the dogs ran across the road and haven't come back since.
I would imagine the watergun/super soaker will also be quite effective - after all Scarecrows also work on cats, and I would imagine a concentrated blast of water will prove quite useful.