According to Patricia Michalak, in "Rodale's Successful Organic Gardening: Vegetables", water makes up 85 to 95 percent of the weight of living plants. Plants use more water than they do anything else. Without water, plants stop growing, wilt and eventually die. Too much water results in soggy roots and plants that are oxygen starved. Water also carries nutrients from the soil to the plant, so plants that are kept evenly moist will be stronger and healthier.
Most garden plants require 1 inch of water per week. Gardens in hot, dry climates lose moisture at a more rapid rate and may require up to 2 inches of water per week, while gardens in cooler climates may require no watering at all, or minimal watering. For most soils, one good soaking is better than several shallow waterings.
Rainfall is easily monitored with a small rain gauge, available at most hardware stores, that is set in or near the garden. It should be checked immediately after rainstorms, before evaporation occurs. If weekly rains are not adequate in meeting your garden's moisture requirements, supplemental watering should be given.
Check soil for moisture by digging a small hole with a garden trowel to examine the soil in the root zone (the top 6 to 12 inches). Sandy soils with adequate moisture will stick together slightly, and heavy, clay soils with adequate moisture feel slick to the touch. If sandy soils flow freely through your fingers, or clay soils appear hard and crumbly, supplemental watering is needed.
Organic material holds water like a sponge. Maintain soil's organic matter by working in at least 2 inches of compost or 4 inches of other organic materials each season before planting. Insulate the soil surface with a thick layer of organic mulch to decrease moisture evaporation. Minimize weeds, which compete with garden plants for moisture. Provide wind protection for your garden, as wind evaporates moisture more rapidly.
For large garden areas, sprinkler systems or drip irrigation work best, concentrating water where it is needed. However, sprinkler systems loose a large amount of water to evaporation, before it ever reaches your plants. Drip irrigation uses less water, since it is applied directly to the soil where plants need it.