Use 36 or 48 oz. plastic juice bottles. Cut the bottoms off and drill 4 or 5 holes in the cap.
Plant bottles in a hole at a 45-degree angle(toward the plant), just above the soil line. Fill them with water and they will keep the soil moist down to about 12 inches or more. Tomato roots will go where there's water.
This is a better system than just watering on top of soil. You want to get water down deep to build a stronger root system. This way there's less evaporation.