Hi Raoul,

How many irrig. heads are you planning on using in your ten foot rows. Is your spacing 0.5, one, 1.5 or two feet apart. Will you be growing water needy plants? How much water(gallons) do you think you will need in your garden max? If your barrels are sitting on the ground you will get 2 psi of pressure when your barrels are full. With such low pressure, you can increase your flow rate by installing a larger size mainline and laterals. thus increasing the number of heads you can install under very low pressure. You can elevate your barrels to achieve more pressure. Drip systems are designed to operate under 15 to 25 psi of pressure(why drip?). There are some good drip design sites you can find through google on the internet for low pressure systems. Also how much rainwater can you expect in your area during the spring and summer months? Will you need to supplement with filling the barrels with city water during dry spells?
Good luck. JMHO Aloha