Since a drip irrigation system should be designed starting from the plants and moving back to the source of water, start with how many emitters to use and where they should be placed. Two of the most important factors to remember are soil types, and the root structures of the plants.
In sandy soil, where spaces between grains are relatively large, gravitation pulls water down into the soil. In finer soils such as clay, the horizontal water movement is much stronger so water will tend to move laterally before penetrating very deeply. An emitter in sandy soil may suffice for an area of 16 inches in diameter, while the same emitter in clay soil may wet an area 24 inches or more across. When designing your system, take care to see that a sufficient percentage of the root zone is watered. Shallow roots require closer spacing of emitters. Deep roots allow for a much wider spacing. In small systems with mixed plantings, it is best to play safe and design for fuller coverage.
A soil test can be useful in making your decisions. This can be done by observing the effect of slow driping of water on the soil from your garden hose. Be sure to dig down into the soil away from the obvious wet area on the surface to see the extent of coverage. Remember, you may only see a small wet area on top of the soil, but underneath, the water may be moving laterally further than you think.