Yes, using a trench is a great way of keeping your sunflowers watered. I do something similar, but I like to call it mounding. Basically, I order a truck load of mulch and composted manure, and every week, or two weeks, I put the manure and mulch around the sunflower with a diameter about as large as distance across the sunflower; so it's the same as the plant's drip line. As the sunflower grows larger, the mound gets larger as well, and if spaced correctly, it forms trenches around all of the sunflowers. The only thing you have to be careful of is making too much of an incline on your mound. If your incline is too steep then the soil can wash away leaving roots exposed. This is a lot of work, but there are some benefits to doing it this way. You generally don't have to stake your plants, because they have such firm base. It promotes increased and deeper rooting. Constant supply of nutrients through the additional adding of composted manure each week.

I definitely recommend not overwatering your plants for at least a month after first emerging. Too much watering will create a shallow root system, which you will pay for in the hottest days of summer. Usually I give my sunflowers one heavy watering a week for the first month. After the first month I give my sunflowers shallow watering nearly every day, and sometimes more, depending on the weather. Bernard Lavery, in his book, Bernard Lavery's Secrets of Giant Sunflowers, mentions experiments with a polythene capillary growing system that he could use to measure sunflower water intake. He found that a sunflower 6 feet tall would consume 8 gallons of water a week, and a sunflower 10 feet tall would consume 10 gallons of water a week. He also found that much of the water poured around the roots was quickly lost when it passed beyond the reach of the roots; so watering should be light and frequent.

Here is a link that might be useful: watering your sunflowers

I cover all of my garden row in betweens with sawdust that seems to keep everything moist and the last 2 years haven't had to water! Mulch! I believe! I also let my ducks and geese have range of my fenced in garden areas over the winter and then go about 2 months bird free before tilling the soil. It seems to work well. Good luck!