The term drip irrigation is used to describe any low-pressure system that applies water slowly in a small, confined area. It is also called low-volume irrigation or micro-irrigation. Water application is expressed in gallons per hour rather than gallons per minute, as it is for sprinkler systems.
Drip systems leak water at the base of each plant, a drop at a time. Unlike sprinkler systems, which operate on household water pressure of 40 to 60 pounds per square inch (psi), drip systems use 10 to 30 psi. Because of the low pressure, lines and connections are lighter and easier to work with. Drip systems are intermediate in cost between hose-end systems and sprinkler systems. When they are working well they require no attention at all, but they have many parts that come loose and small holes that plug up easily, so they must be monitored regularly to be sure everything is operating properly.
Drip systems are water-thrifty. They applies water very slowly, so that the soil is able to absorb it without sealing up or crusting over. Because only a small surface area is wetted, evaporation loss is minor. When properly adjusted, almost all the water that is applied though them is used by the plants.
Drip Systems and Plant Growth
As water seeps into the soil from a drip emitter, it is pulled downward by the force of gravity, but also spreads sideways due to capillary action. The wet area under an emitter is shaped like a turnip or onion (and is called the “onion”). Since the system is normally left on for hours at a time, and is frequently on, the onion is fairly stable. Instead of the cycle of saturated-to-dry soil experienced with other irrigation systems, the soil in the onion is never saturated and never dry, but constantly moist. Plants thrive on this stability, and generally grow better with drip irrigation than with other irrigation methods.
However, the soil outside the onion never gets watered. Plant roots proliferate in the onion, but decline in the area outside it. This situation has the potential of causing problems if the plant is shifted suddenly from reliance on a drip systems to reliance on a sprinkler or on rainfall.
In saline soil, or when used with hard water or large amounts of fertilizer, salts can accumulate at the edge of the onion. Since there is a steady flow of water from the emitter to the onion edges, the onion is leached adequately, but salt accumulation at the edge of the onion keeps plant roots from growing there, adding to the plant’s dependency on the drip system.
Maintaining Drip Systems
Drip systems require the same attention to water use as sprinkler systems. As days grow warmer and longer in the spring, the system needs to be left on for longer periods. In the fall, the water needs to be reduced again. The water to individual plants is adjusted by adding or removing emitters. In the case of plants that grow a great deal during one season, such as tomatoes, vigilance is required to be sure the plant is getting enough water as it grows larger.
Since, with most systems, the water flow is adequate for hundreds of emitters, there is no need to calculate the flow rate or pressure available, as there is with sprinkler systems. Emitters can be added or removed from the system without affecting the rest of the system. Adding emitters, or even new lines of emitters, is a matter of punching a hole in the line and poking an emitter into the hole. Removing emitters is as simple: the emitter is pulled out of the hole and a plug (called a “goof plug”) is pushed into the hole to keep it from leaking. It takes less than a minute to add or remove an emitter.
The usual way to monitor drip systems is to closely observe the plants dependent on it. When a plant wilts, inspect it to see if the emitter is plugged up or has gotten knocked out of a container. Another way to inspect drip systems is to be sure water is dripping from each emitter. Emitters are usually left visible so they can be inspected while the system is running, even if the lines are buried under mulch.
To lessen plant dependency on the drip system, install enough emitters for each plant that a good portion of its root system is wetted. In addition, if you live in an area with seasonal rainfall, leave the drip system on for a couple of weeks after the rainy season begins to give the plants time to adapt to rainfall again.
Cost
Individual drip components are inexpensive, but a system can grow to the point where a fairly large amount of money is invested in it. Unlike sprinkler systems, which are installed all at once, drip systems can grow as you add plants or bring new parts of the garden into the system. For this reason, the expense is often spread over a long period and not perceived as a single cost.