Infrequent deep watering is best. In the height of summer I water for 2 hours twice a week via drip system. This time of year(early spring), because evapotranspiration is low, 15 minutes once a week when it hasn't rained for a week is plenty. Drip systems are best because they deliver water directly to roots and soil so there is less loss from evaporation.
Hand watering isn't exactly as wasteful as sprinkler watering, but it generally isn't effective except on seed beds. You'd need to stand there and hand water for more than two hours to apply the same amount of water to your plants as a drip system. Reality Check: Hand watering for 5 minutes on 20 square feet of soil area only waters the first 2 1/4 inches of soil. That means that all the water you are applying is at the surface where it can evaporate. Also it encourages root growth at the surface where the roots are prone to drying out. Lastly the action of the falling water hitting the surface breaks down the soil particles and causes a crust to form when the soil dries out. This crust prevents the water from penetrating the next time you irrigate unless you break it up with a cultivator in between irrigations. Mulching helps with the surface crusting, but doesn't totally prevent it.
If you must hand water, make sure that your your seedbed is flat, so that water goes into the bed and doesn't run off onto the path. It is also a good idea to make a little bowl around the root zone of each transplant. If you fill a bowl around a plant 2 or 3 times during a hand watering session you can get the water deeper into the soil around that plant.
Overhead watering is so bad I can't even write a coherent paragraph about it. It just sounds like a list of problems, so that's what I wrote.
Problems with overhead watering:
- Wasteful because a lot of water (as much a a third on a summer day) evaporates while it is flying through the air.
- Promotes weed growth, especialy bermuda grass, because you are watering areas, like paths where you don't want weeds to grow.
- Promotes diseases because water sitting on the leaves of plants, especialy in the warm summer months, is great for fungal and bacterial growth.
- Causes surface crusting which impedes water penetration at the next irrigation.
- Annoying because sprinkler spray generally doesn't stay in your plot. Overspray hitting passers by (or the toolshed!) is obnoxious.
If you are going to use sprinklers, 2 hours twice a week in high summer is a good amount of time. Please note that this uses much more than the same amount of time running a drip system because you are watering the whole surface of your plot, not just where the plants are. If you must run a sprinkler system run it just before sunrise. Watering while it is still dark minimizes evaporation, and minimizing the time that water sits on the leaves overnight reduces the risk of disease. Cultivating to break up surface crusts and mulching with rice helps prevent surface crusts.
With all that said, I have to admit that I've overhead watered my plots mostly because it is much cheaper to install sprinklers than drip. They work best on a well weeded and mulched plot where crop plants cover most of the soil surface area. The crop covering the soil shades out weeds and slows down the water drops before they hit the soil and cause surface crusting. DO NOT USE SPRINKLERS IF YOU HAVE ANY BERMUDA GRASS IN YOUR PLOT. It will go crazy if it is watered in the summer. I used sprinklers on my corn last year, and the corn did well, but what started as a tiny area of bermuda grass in the plot got totally out of control in a sunny, well watered area. Never overhead water tomatoes, eggplant, or peppers (cucurbits are pretty suceptible too). They just can't handle the disease that overhead watering causes.