What people call drip irrigation can actually be several types of systems. "Drip" should probably be called "low-flow," "low-pressure" or "low-volume" irrigation because that's the one thing the various systems have in common.
The methods all operate on low water pressure with a low rate of application--they deliver the water slowly so it can soak in.
Because they operate at low pressure and low volume, you can hook quite a few devices up to one garden faucet, or one valve. In fact, a big advantage of drip irrigation is that you do not need a lot of valves. One for the front yard and one for the back is usually enough.
Here are the various low-flow systems in broad groups, with some suggestions for starter kits. Pictured are some of the components from the kits, plus a few components available only through irrigation suppliers (see the Yellow Pages), to give you an idea of the great variety available if you are ready to assemble your own system from scratch:
Drip tubing, emitters
This is what most people think of when you say drip--skinny little tubes (one-quarter-inch tubing), often called "spaghetti" with individual "drippers" or emitters, along the way or at the ends. The spaghetti fans out from larger pipes (one-half-inch tubing).
The emitters have very small orifices so that water drips out slowly. As it soaks in it also spreads out so it waters a much larger area than you think.
In the past, these tiny orifices tended to clog, which gave drip systems a bad name, but better filters and "torturous path" or "turbulent flow" emitters have helped tremendously. The filters screen out the inevitable particles found in our water and a zigzag channel inside the emitters keeps the water turbulent so that solids do not have a chance to settle.
Many of the inexpensive starter kits that are commonly available use separate emitters and skinny drip tubing. If you plan to water individual plants--young trees, shrubs or shrubby ground covers--that are spaced several feet apart, this is the easiest system to put together. It is also easy to add onto later.
And if you vary the size of the emitters (which determines how much water comes out, measured in gallons per hour--gph), you can give different plants different amounts of water. It is also easy to figure out how long to leave this kind of system on.
Drip Mist makes inexpensive kits. Raindrip makes several sophisticated starter kits with turbulent flow emitters and pressure compensating devices, as does Drip Irrigation Garden (DIG). Rain Bird makes kits with pressure-compensating emitters with with self-cleaning diaphragms.
Pressure-compensating devices--different than the pressure regulators at the beginning of the system--are contained in each emitter and ensure that the amount of water coming out of emitters at the far end of the system is the same as that close to the faucet.
Such devices are important on long runs of tubing or on hillsides. In fact, it is hard to imagine watering a hillside with anything other than pressure-compensating emitters--there is virtually no runoff and plants at the bottom of the hill get as much as plants at the top.
In some applications, there are problems with spaghetti tubing and individual emitters. The tubes can be unsightly (you may bury the tubing, but not the emitters), they are in the way when weeding or cultivating, dogs make a mess of them and there are a lot of parts to keep track of.
So when plants are spaced close together (as in flower beds or vegetable gardens or even in areas where trees or shrubs are close together), you might consider "in-line emitters."
In-line emitters--almost always turbulent-flow types--are hidden inside the main tubing, are between short lengths of solid tubing. Because the tubing is much larger, it tends to stay put, and while you can't bury the tubing, you can hide it under a mulch.
A particularly nice kit (pictured) is offered through the mail by The Natural Gardening Co. Raindrip also makes a new in-line system.
A variation on this theme uses separate emitters, but they are punched into the sides of larger tubing, with a simple hand punch, so there is not a maze of spaghetti. Again, this works best where plants are fairly close together, although you can vary the placement of the emitters so you are not watering empty ground anywhere. Rain Bird makes a new kit using half-inch tubing. (The pieces, not available as a kit, are available separately through irrigation suppliers. They are made by Olson Irrigation Systems, 10910 Wheatlands Ave., Santee, Calif. 92071, (800) 776-5766.)
Used extensively by agriculture, drip tapes are just beginning to show up as kits. Agricultural tapes are short-lived, designed to be disposable; homeowner versions are sturdier. Some even have tortuous paths molded inside the flattened tube-like tape. They work much like the in-line drip systems, with holes evenly spaced along the lengths of tape.