When using any type of drip system it is especially important to relate the amount of water applied to the depth of the soil to the crops, root development. There's no sense adding more water than the earth can hold. Calculating the optimum amount of water to apply from a drip system requires applying substantial, practical intelligence to evaluating the following factors: soil water-holding capacity and accessible depth; how deep the root systems have developed; how broadly the water spreads out below each emitter (dispersion); rate of loss due to transpiration. All but one of these factors—dispersion—are adequately discussed elsewhere in Gardening Without Irrigation.
A drip emitter on sandy soil moistens the earth nearly straight down with little lateral dispersion; 1 foot below the surface the wet area might only be 1 foot in diameter. Conversely, when you drip moisture into a clay soil, though the surface may seem dry, 18 inches away from the emitter and just 3 inches down the earth may become saturated with water, while a few inches deeper, significant dispersion may reach out nearly 24 inches. On sandy soil, emitters on 12-inch centers are hardly close enough together, while on clay, 30-or even 36-inch centers are sufficient.
Another important bit of data to enter into your arithmetic: 1 cubic foot of water equals about 5 gallons. A 12-inch-diameter circle equals 0.75 square feet (A = Pi x Radius squared), so 1 cubic foot of water (5 gallons) dispersed from a single emitter will add roughly 16 inches of moisture to sandy soil, greatly overwatering a medium that can hold only an inch or so of available water per foot. On heavy clay, a single emitter may wet a 4-foot-diameter circle, on loams, anywhere in between, 5 gallons will cover a 4-foot-diameter circle about 1 inch deep. So on deep, clay soil, 10 or even 15 gallons per application may be in order. What is the texture of your soil, its water-holding capacity, and the dispersion of a drip into it? Probably, it is somewhere in between sand and clay.
I can't specify what is optimum in any particular situation. Each gardener must consider his own unique factors and make his own estimation. All I can do is stress again that the essence of water-wise gardening is water conservation.
Optimizing Space: Planning the Water-Wise Backyard Garden
Intensive gardening is a strategy holding that yield per square foot is the supreme goal; it succeeds by optimizing as many growth factors as possible. So a raised bed is loosened very deeply without concern for the amount of labor, while fertility and moisture are supplied virtually without limit. Intensive gardening makes sense when land is very costly and the worth of the food grown is judged against organic produce at retail—and when water and nutrients are inexpensive and/or available in unlimited amounts.
When water use is reduced, yield inevitably drops proportionately. The backyard water-wise gardener, then, must logically ask which vegetable species will give him enough food or more economic value with limited space and water. Taking maritime Northwest rainfall patterns into consideration, here's my best estimation:
Water-Wise Efficiency of Vegetable Crops
(in terms of backyard usage of space and moisture)
EFFICIENT ENOUGH