Just did a search and found this site. Hope it helps. http://www.wateruseitwisely.com

Techniq,

interesting question and I believe there is not one source to answer your needs. A plants has growth stages and needs differing amounts of water through its life cycle. A good illustration of this is a tomatoe plant I have grown in an Earthtainer that feeds water to the plant from a reservoir below the soil. When I first planted the small seedling I had to replenish the water very little but by the time the plant was 5 feet tall I was adding 2 gallons of water a day. The root ball was very large. The soil type, conditioning, Ph and make a big difference as well as the type of plant. What I have read is that you need to check what the water is doing in your specific soil and is it getting to your plant roots. Understand how the roots will grow for your specific plants. Some grow deep and other grow wide and shallow. There are many methods for doing this checking. The other forums for your plants will give rules of thumb concerning average water for types of plants. For example mature lawns take an average of 1" of water a week. A good place to read info on root grow and water habits of specific plants are university agricultural websites. I believe gardening is an art that utilizes science as a tool. The growing conditions never will be the same twice; just ask the tomatoe forum. Aloha

Might want to check with your local extension office. Ours has several papers/publications about vegetable gardens.

http://www.csrees.usda.gov/Extension/

In most cases, vegetable gardens usually need about one inch of water per week in the form of rain or irrigation during the growing season. Gardens in sandy soil may need as much as 2 inches of water per week in midsummer.

You also might want to buy a moisture meter.

Mike1059, I looked at the link but didn't find an answer there. Maybe I missed something.
lehua13, How can I convert inches/wk to gallons/wk?
tn_gardening. I've been doing that, but it's extremely tedious.
I can't see why this information hasn't been tabulated. If you include only the plants that are commonly found in nurseries (which is what most people have), it should not be an overwhelming task to compile that data. Then a correction factor for climate zones should take get you pretty close. Another correction factor for plant maturity should pin it down to a great starting point. Then you could make slight corrections from there for your soil type. It would save a lot of guesswork.
Oh well, I will probably find my answers the hard way like everyone else. Someday, this reference will exist. I would bet on it.
Thanks, Guys. I'm still digging for an answer, but I appreciate your advice.

techniq. What zone are you in?

I like the Texas A&M site.

http://aggie-horticulture.tamu.edu/earthkind/drought/water/

Maybe I'm over simplifying things, but I sorta subscribe to something similar: Generally speaking, if you keep your tomatoes happy, the rest of the vegetables will receive enough water

1 inch of rain equals 0.62 Gallons of rainwater falls on one square foot of land.

Rule of thumb is vegetable gardens need 1 inch of rain a week. That's just over 1.5 gallons a week. If you have a 1 gph emitter, then you might want to run the emitters for 1.5 hours a week, adjusting for rainfall, and zone, temperature, and humidity, and soil, and plant size, root size, and type of plant, yada, yada, yada, etc.

Way too many variables to absolutely sum it up. That's why most of us try to have good draining soil, use mulch, and simply look at our plants and make a judgement call or stick our finger in the dirt, or simply say: about a gallon of water a week (seems most folks get some rain every week or so).

If you want actual data, I would call the local extension office and/or university. Talk to somebody about research papers and data.

t,

Here is the math behind Tn g's statement. 1" = 0.0833 ft, 1 ft x 1 ft x 0.0833 = 0.833 cubic ft. There are 7.481 gallons per cubic foot , so 0.833 cf x 7.481 g/cf = 0.623 gallons. This amount fills pore space in the soil. Different soils have difference amount of pore space, therefore the same amount of water will spread out or spread down for different types soil. Each plants have different shaped root ball at different stages of growth. Therefore most gardeners use a trial and error method of finding the right amount of water for their plants by looking at the plant rather than a table. You will have to check at some point check on what is happening in your garden rather than relying on what a chart is telling you. That is why there are no charts out there. Although you may find one, you would still have to adapt it to your specific condition and you would still be doing trial and error checking to make sure the chart is not off or your calculation are inaccurate. JMHO Aloha

Too much depends on the plant, the soil, the weather.

If your soil drains readily, then over watering is less risky.

Some tricks:

1. Mulch heavily. When the bottom of the mulch is dry, water.

2. Plant a stick next to the plant. Any smooth stick will do. It should be larger in diameter than your finger. It should go into the soil at least 4 inches, and it should have enough sticking out to grab. Now: Pull the stick out, and put your finger in to test the soil moisture level. and water accordingly.