Does anyone know approximately how much water per square foot a mulched vegetable garden would need per day in the Phoenix area? I am planning to install rain gutters and we have a nice sized roof to collect rainwater to supplement the garden. We are going to start with 550 gallons of storage capacity for now and keep the garden bed small.
I just hooked up a drip system for my sqaure foot garden, I thought each square foot would only need about a gallon a day which would be about right if I was using the hose or watering can.
It seems with a drip emitter I need much more because it drips so slowly that it doesn't spread around. Tomato plants need more water than most of my other veggies. For your system I would guess about a gallon per day or every other day per square, but if you are hand watering its probably less than that.
Janet, how are you distributing the water onto your garden soil? Are you using a garden hose attached to your rain barrel?
The problem with using gravity-fed systems for irrigation is that you never get the required pressure required for landscape irrigation equipment - even drip components. Rainbird drip emitters are meant to operate at 15-30 psi. One psi is equal to 2.31 feet of head. So to get 15 psi out of your rain barrel, it would have to be almost 35 feet above ground. Beth - that's why your drip emitters don't work as well as you'd like (Unless I misunderstood and you're using metered water - in that case you need larger flow emitters - like 4 GPH). Even the Rainbird Landscape Dripline (which is what I'd recommend for row gardening) requires 8 psi to operate. So hand watering is really your only option, unless you install a pump. Soaker hoses are an option, but they clog easily and can rot in a few years; and you'll never really know how much water you're applying.
If you're using a hose, you can just count the time it takes to fill a gallon or 5-gallon bucket, and use that time to determine the flow rate of your garden hose at your low pressure.
As far as how much water to use per square foot - that depends on what you're growing, the sun and wind conditions where your garden is located, and what time of year it is.
PS - Be prepared to want to add to your water tank. You say you have a 'nice size' roof. A 2,000 sf roof will collect over 550 gallons of rainwater from just a half-inch of rain. The great thing about water tanks is that they can be added to by linking them with hoses or pipe at the bottom. I haven't met anyone yet who installed a rainwater harvesting tank who hasn't wished they'd gone bigger.
Congratulations to you for conserving a precious and scarce natural resource!