How much to water depends greatly on your plants. It depends on a lot of things. The kind of soil (e.g. clay needs less frequent waterings, sand needs more frequent waterings), plant density, how hot it is and the kind of plants involved. As a general rule, most things I have read suggest 1–2 inches of water per week for a tomato plant. There is an old saying that if you keep your tomatoes happy, the rest of your garden will be doing fine.
How to Water
Using drip irrigation will reduce the possibility of disease and wasted water. It will also help keep the weeds down by providing water only where needed.
How much to water will also depend on your plant density. If you do raised beds and cram the lettuce in there, you will have to have drip emitters to cover the whole bed. If you use a micro sprinkler, you can just have one or two.
If you use a garden hose you will have to pay a little more attention to the existing conditions before you start. More on that later...
So how do you provide 1–2 inches of water per week with drip irrigation or micro sprinklers? I found a bunch of calculators around the Internet. I did not get it. From my experience, you will need to have enough emitters and run them long enough to penetrate 6 inches into the soil. A micro-sprinkler or hose will take much less time. I suggest a trial and error.
General Watering Tips for perfecting your trials and reducing your Errors
I found a fantastic guide to watering at:
To summarize what they said:
Check your soil. When plants are young, keep the surface moist. If it is drying out, water. As plants mature, you want the surface soil to be dry and moist a couple inches from the surface. If it rains, turn off your water.
When it gets hot, water in the morning to give your plants a chance to suck up the water before they get hit with the heat of the day. You do not want to water at night as you do not want water sitting on the foliage for extended periods of time. This will increase chance of disease.
Change Your Watering Patterns as the Plants Mature
Seedlings need more frequent waterings and less deep waterings. Mature plants should have access to moisture at least 6 inches below the surface. Soil should be moist – not wet. Wet or waterlogged soil will deprive your plants of oxygen and the ability to uptake nutrients. Signs of overwatering can include yellowing or curling of leaves, and stunted growth.
Add organic material to retain moisture
Organic material helps the soil to retain moisture. Clay soil will hold on to the water and deprive it from your plants. Sandy soils drain too quickly. Adding the compost or peat or whatever will give the water a place to stay until the plants need it.
Mulching with grass clippings, straw, et cetera, will help reduce evaporation and allow you to use less water. Keep the mulch away from the trunks or plants themselves, however. You do not want moist material sitting right up against the stems as it can encourage disease or cause them to rot. Note that mulching with wood chips will significantly deplete the nitrogen from your soil as they break down.
What you do NOT want
Soggy, wet dirt or water running off and eroding the surface. If you need to, break up your waterings so the water has a chance to soak in.