By The Old House Web


Drip irrigation is an approach to watering that is growing in popularity. There are many advantages inherent in a drip irrigation system, including the reduction of water usage by almost 50%. In addition, the lengthy, slow trickle provided by a drip system has increased yields by 84% over other methods of watering, and as high as 30 to 50% in areas with a high amount of summer rain. Drip irrigation also alleviates problems created by improper watering, which can harm the garden's soil.

If a sprinkler throws out more water than the soil can absorb, the soil will become waterlogged and less productive. When soil becomes too wet, it becomes anaerobic (without air), a condition which allows for the growth of harmful fungi and molds, causes rotting of the roots and slows or stops completely the growth of root hairs (responsible for the absorption of nutrients). In addition, an excess of moisture in the soil causes the soil temperature tends to drop. In a colder temperature soil, worms and other soil life die or are rendered effective and nutrients can be leached beyond the plants' root zone. Overwatering also causes the platelike particles of clay to slip closer together, making clay soils stickier and more difficult for roots to penetrate. All of the conditions created by excess moisture are stressful to plants, taking a toll on health and productivity.

A soil that has received too little moisture can cause as much trouble as a waterlogged one. In an overly dry soil, the root hairs will die from a lack of moisture. In the summer months, temperatures rise to harmful levels; worms and other organisms must leave, go dormant or die. Nutrients become "locked up" and unavailable to root hairs in dry soil, and dry particles of clay, if walked on or tilled, fracture and become more claylike when irrigated in soils with a clay-like consistency. Cracks in the soil caused by lack of water allow so much air to enter that small roots can die from exposure.

When soil moisture is kept on a constant level, the soil organisms thrive and create more humus. Humus is the end product of their decomposition, a material which makes the soil loose, fluffy and able to hold moisture while bonding nutrients so they will not leach away, releasing small, steady doses to plants' root hairs. Humus thrives in conditions which provide a thin film of moisture; when conditions are too dry, the nutrients remain locked up in the humus particles. When water is applied, there is a delay before the nutrients are again available to the root hairs. The healthiest growth and greatest yields occur when there is a constant in the moisture level, not cyclical conditions of wet and dry. Drip irrigation delivers very small amounts of water over a long period of time; its method of slow application makes it easy to maintain optimal soil moisture, allowing a garden to achieve greater yields with the least amounts of water.

Drip irrigation methods are similar to those employed when watering with a soaker hose, in the sense that minute streams of water are released through hosing. However, drip irrigation is far more efficient, because the newer hardware used in this method regulates exactly the amount of water released. It can also disperse less water per hour, ensuring that garden soils do not puddle and roots don't drown. New parts for filtration allow a wider range of water supply, and can compensate for changes in water pressure, allowing the water to be distributed evenly on long runs or over hilly ground. Unlike soakers, drip tubing will not be bothered by algae buildup, conditions especially troublesome with unchlorinated water. Drip irrigation can also be set up to combat weeds. However, drip irrigation is also more expensive to employ as an irrigation system.

Because each drip emitter applies water to a localized spot, the water level is easy to check in a soil profile. The more slowly water is applied, the deeper and narrower the area of irrigated soil. The irrigated area usually forms a carrot shape when irrigation is done slowly, a faster application will result in a beet-shaped wet area. However, this is also dependent upon the consistency of the soil; with the same rate of application, a sandy soil will absorb water in a carrot pattern, and the clay soil in the beet shape. If the emitters are spaced far enough apart, there will be an area of dry soil between their moist circles. In areas where rains are absent or infrequent, this dry zone means that weeds will not have enough water to germinate or grow well. With the drip irrigation system, there is no water loss due to the evaporation of windblown spray or from puddles - the bulk of the water is able to directly reach to the plant roots.

There are several components which comprise the drip irrigation system. Every drip system should have a valve between the main water supply and all the other components, known as a shut-off valve. In a simple system, this will be a standard hose bib. An anti-siphon valve is required by some building codes, as it is responsible for the prevention of water in the drip system from siphoning back into drinking water. Without such a valve, if any portion of the drip system is above the level of the shutoff valve it may allow for dirt, fertilizer and other contaminants to be sucked backwards into the pipes and drinking water of nearby houses.

The filter is necessary, as small orifices in many drip emitters can easily clog with dirt or other suspended particles. A filter with a 150-to 200-mesh screen must be plumbed in before the pressure regulator is applied to the system. The pressure regulator consists of drip hoses and parts which are designed to operate at pressures much lower than the usual domestic water pressure. A pressure reducer should be used to keep the drip system at or below 25 pounds per sq. inch. (Most city water and deep-well pressure tanks operate at 40 psi or greater). If pressure is not reduced with a regulator, there is a risk of drip fittings blowing apart.

Either a manual or an automatic valve is needed for each separate drip line; although an inexpensive ball valve plumbed into the drip-hose line can be substituted in most cases. An automatic timer/controller is a means to regulate the watering time. A convenient method it the use of a spring-run timer, similar to those that are used in the kitchen. Once turned to a specific time, these timers always shut off automatically. Timers that run on water pressure and shut off after a certain amount of water passes through are not recommended, because they will not shut off due to the low water pressure and small volume of water used in a drip system. Electric valves allow control of the entire system with the addition of a computerized timer, which are relatively inexpensive and easy to program and use. Electric valves free the gardener from daily watering, and provides a manual override which allows for hand watering.

A special-diameter hosing is needed for each line of piping in the system; its diameter only 0.7 inches and constructed out of linear low-density polyethylene. Drip hose is very flexible and can bend in a tight arc with-out crimping; a 90-degree fitting or elbow is available but is rarely needed. The cost of hosing is surprisingly low - about 4 to 12 cents per running foot.

Emitters are those odd-shaped little parts that control how much water comes out of the drip-hose. They are usually inserted or punched directly into the drip hose, although some are not separate pieces, but are merely holes or built into special tubing. Emitter tubing is a smaller tubing, which is strung along the ground to an emitter that needs to be located away from the main drill hose. Drip irrigation kits bought at a store or gardening shop usually don't have the optimal garden design - parts from one kit will not fit other kits, and spare parts are usually sold two to four a packet. When purchasing a drip irrigation system, it is better to buy from a standardized dealer who stocks the standard parts.

When designing a system, it is important to know and keep in mind the evaporation rate (ET), a rate which tells how much water is lost by evaporation and transpiration of the plant's foliage. The ET is based on a formula that takes into account such factos as: the air temperature, humidity, the dew point, wind velocity, and the amount of sunlight. This figure is available through a local university extension service or the US Office of Soil Conservation and is expressed as the number of inches per day, month or season or a fraction thereof.

To survive, plants need at least the same amount of water as the ET. In order to be kept consistently moist, the soil should be irrigated several times a week. Figures needed to calculate the amount of water needed on the garden or landscape include the knowledge that 1 inch of water over 100 sq feet amounts to 62.5 gallons. The amount of rainfall should be subtracted from the amount of water the landscape requires.

An assessment of the landscape includes a scale drawing of it, including all structures and emphasizing permanent planning. The presence of of rodents should be considered, especially gophers; as rodents are able to eat through buried hose. If the rodent population is high, consider leaving the hose on the surface, not buried or covered by mulch. However, surface drip hose becomes brittle in cold climates, and should be stored for the winter where it will not be stepped on or broken.

Some water contaminants are particularly troublesome for drip irrigation: these include sand, sediment, silt, dissolved calcium, and soluble or dissolved iron. For soils containing sand, sediment and silt, it is best to use a filter with a mesh of at least 200. For calcium problems, the largest orifice emitters (4 GPH) or spray stakes are best. For iron problems, a water softener or water treatment system should be purchased. The controls should be placed near a spot that is frequented in the yard, garage or toolshed.

If the elevation changes 20 feet or more, pressure- compensating emitters are required. Pressure-compensating emitters are constructed so as to deliver the same amount of water regardless of the change in elevation or length of the line. Without the more costly pressure-compensating emitters, there can be a lot of water at the bottom of hill and at the beginning of a line, and little water at the top of hills and at the end of the line. In planning the implementation of the drip irrigation system, a grower should estimate current and future needs of the landscape. The length of each line of drip hose and the number of lines should be able to accommodate the emitters that may be added each year as the plants grow. The types of plants to be irrigated should be considered, as each plant has different water and fertilizer requirements. The best drip systems have a separate drip line for each group of plants with similar needs. There are four basic irrigation groups necessary to design for: fruit trees, herbs, vegetables, and perennial shrubs and flowers.

Keeping in mind the needs of different plants, decide the flow rate (measured in gallons per hour-GPH) of the emitters to be used. The greater the GPH, the faster the water comes out and the broader the area soaked below the ground The GPH should be determined by the different needs of the soils. Sandy soils require a high-flow emitter, 2 to 4 GPH, in order to disperse water sideways. In clay soils, low-flow emitters, 1/2 to 2 GPH 18 to 24 inches apart, or higher flow (2 to 4 GPH) spaced farther apart.

The wetting pattern per hour must be determined by experimentation. In dry soil, start one emitter dripping and dig a trench every hour or two to check the width of the wet spot; with well water, use 1/2 or 1 GPH emitters in order to decrease the chance of the well running dry.

The number of drip-hose lines must be estimated, keeping in mind that a 1/2 inch drip hose can pass no more than 240 GPH. The water needs of each area divided by the size of the emitter reveals the number of emitters needed; there should be a valve assembly for every 240 gallons' worth of emitters.

Vegetable beds require special attention to design. Suggested dimensions for raised vegetable beds are 3 to 4 feet wide and never more than 25 feet long. Each three- foot-wide bed should get two lengths of in-line drip hose, dividing the beds into thirds, while four foot wide beds should have three drip lines.

This table documents daily water needs as determined by the plant cover (or type of plant) to be irrigated. Read horizontally until you are aligned under the evapotranspiration rate for your area in a given month. (The U.S. Soil Conservation Service or you county extension agent can give you the ET for your area.) The figure in the column will tell you how many gallons per day your planting requires. For example, if you have 100 square feet fully covered by vegetables and the ET is 7.5, your little 10-by-10-foot bed needs 15.6 gallons of water a day. (Keep in mind that as vegetables grow they will transpire more water through their lush foliage than they did as seedlings planted in the same space.) A single mature apple tree on a hot dry summer day requires some 56 gallons of water.

ET*: Inches/month 3 6 7.5 9

Sq. Ft. of Plant Cover Daily Water Use in Gallons Per Day

1 sq. ft. 0.062 0.125 0.156 0.187

4 sq. ft. (1-yr old fruit tree) 0.250 0.500 0.620 0.750

10 sq. ft. (2-yr.-old fruit tree) 0.620 1.250 1.560 1.870

75 sq. ft. 4.650 9.400 11.700 14.000 (mature grapevine)

100 sq. ft. (semidwarf mature or 4 yr.-old) 6.200 12.500 15.600 18.700

200 sq. ft. (2-ft.-wide, 100-ft. raspberry row) 2.400 25.000 31.200 37.400

300 sq. ft. (large standard mature tree) 8.600 37.500 46.800 56.100

1 acre solid cover 2715 5431 6788 8145

*ET (evapotranspiration): amount of water that evaporates and amount that transpires through leaves in inches per month.

Installation involves proper preparation. Before picking out the hardware, review the plan and parts with the staff of the store. Always buy more fittings than called for, and make sure that all unused parts can be returned. There are filters and pressure regulators that have garden-hose threads; these are different from and will not attach to pipe threads. All of the hose thread fittings that screw into a rubber gasket fitting need no additional pipe-thread sealer. All of the iron pipe thread should be coated with pipe sealer or Teflon tape to make a watertight seal.

It is essential to flush the drip system before assembly. Plumb the hose bib or control valve, filter and pressure regulator into place, then turn on the hose bib or valve to flush the pressure. It is necessary to flush the drip system at several specific times during installation in order to keep the lines clean and free from emitter- clogging dirt.

A stake should be pounded into the ground on each side of the valve assembly to prevent hoses and people from knocking into the assembly and breaking or loosening a fitting. To join the threads at the end of the straight filter to the drip hose, an adaptor is needed. This adaptor is called a 3/4-inch female hose swivel by 1/2 drip compression fitting. One end has garden-hose threads and the other has a drip-hose compression fitting; the drip hose should be laid out and the solid hose strung along the path according to plan. Each vegetable bed should have a drip-hose assembly with a valve controlling it. When the main line and beds are laid out, keep the end of each line open and turn on all of the valves, flushing the lines. The lines should be closed off one at a time, working down the slope. Any necessary trenching should be done at this time, testing the lines for leaks before burying them. After all of the emitters are punched into the line, all end closures should be opened and the lines flushed for three to five minutes. The line ends should be reclosed in sequence from the highest to the lowest, during which the system should be allowed to run and each emitter should be inspected carefully.

When using an automatic controller, the lines must be double-checked to see if they are turning on and off as planned. When implementing its use, set the timer for a short period and watch all lines go through the on/off cycle.

There are several requirements in the maintenance of the irrigation system. The filter should be flushed out every two to four weeks, and any green algae build-up on the filter's screen should be removed as soon as spotted with a a strong solution of bleach and water and a used toothbrush. At least once a month, each drip line should be flushed out for a minute or two. The emitters should be visually inspected on a monthly basis. If an emitter is not dripping, firmly tap it to dislodge any dirt; if this is unsuccessful, replace it with an emitter endowed with a bigger orifice, as this has a greater GPH rating.

In severe cases of silting or calcium buildup, it is possible to clean the emitters by pumping a solution of hydrochloric or sulphuric acid through the entire drip system. With this method, there is no damage to the trees, the system is flushed and the acid pumped through while the plants are dormant. All hose-thread fittings should be closely watched, to ensure that the rubber gaskets aren't leaking.

In areas known for cold winters, the unburied drip emitter lines should be rolled up and stored inside in the fall. The lines must be drained before they are rolled up, and caps or figure-8 end closures placed on all drip hose to keep hibernating bugs. Emitter lines under a heavy mulch and solid drip hose that is several inches below ground can be left if all lines are drained and there won't be any traffic on the frozen soil over the drip line. To prevent damage due to compressed air, shut off the main valve to the entire irrigation system and slightly open all in-line valves. Each spring, reinstall the drip hose following the same steps of sequential flushing as used in the initial installation. Even if the drip hose stayed outdoors all winter, flush all components prior to use.

There are many advantages inherent in the use of drip or trickle irrigation systems. The reduction of the amount of water necessary, up to 30-70% less water than overhead sprinklers is one factor in its adoption. The ability to water only the soil immediately around the crop allows fewer weeds to germinate and allows leaves to stay dry, inhibiting the spread of fungal diseases. There is also a reduction in fertilizer and pesticides needed, as pesticides are not washed from the foliage, smaller quantities may be effective and for longer periods of time. However, the advantages must be weighed against the disadvantages of implementing the irrigation system. The initial cost may be several times that of a sprinkler and hose. In addition, designing and installation take time and planning. Maintenance increases the growers involvement with the system; for example emission holes through which water flows becomes clogged and must be periodically opened.


WEED 'EM AND REAP FEBRUARY-MARCH 1987 'Trickle Irrigation for the Home Garden' by Nancy J. Butler

"Drip Irrigation" By Robert Kourik Harrowsmith Magazine

Go To Top of File               Main Page for this Data Base

Search Improvement Project