This paper is written for professionals skilled in above ground drip irrigation who wish to apply their skills to subsurface drip irrigation (SDI) for permanent crops.
The objectives of a SDI system are:
- Yield and quality improvements.
- A dry soil surface to minimize weed growth, moisture loss and disease.
- Broad lateral spread and minimum deep percolation to maximize the wetted root volume and minimize percolation water losses and contamination of the groundwater with agricultural chemicals.
Dry Soil Surface:
To reduce the risk of water erupting to the surface:
1. Pulse the irrigation cycle with set times of one hour or less if practical. To enable pulsing the mains and submains should be designed to stay full of water between irrigations so that the filling time of the system is less than 3 minutes. The use of check valves can assist in this objective.
2. In soil of low infiltration characteristics decrease the emitter discharge rate and increase the number of emitters.
3. Install the system in a position where equipment will not run over the buried line, to avoid compacting the soil.
4. If the soil is compacted lightly till over the buried drip line.
5. Water penetration problems effect about 20% of California farmlands. Key elements which govern the water’s infiltration characteristics are the total salinity of the water (ECw) and the relative sodium content. The relative sodium content is evaluated through a calculated value called the Sodium Adsorption Ratio (SAR). The SAR is calculated from the elemental levels of calcium (Ca), magnesium (Mg), and sodium (Na) in the water. The infiltration rate generally increases when the water is saltier and decreases with high SAR values. When the salt content of the water source is very low there can be water penetration problems irrespective of the SAR (1). Growers have reported improvement of penetration of water into the soil by adding gypsum through the drip system. This should be done under the advice of an expert in this field using special fine grades of gypsum.
6. Disturbing the soil during installation can cause water to rise to the surface. Time will mitigate the problem as the soil settles and the root system of the crop penetrates the region around the emitter..
Broad Lateral Spread and Minimum Deep Percolation:
1. Pulse the system – see 1 above. Claude Phene has shown that a pulsed SDI system can have 46% greater wetted volume and 62% greater wetted interface area than a pulsed surface drip system, each given the same amount of water
2. This is shown in Fig. 1.
FIG1. Differences in the geometrical characteristics of hemispherical and spherical wetting patterns of DI and SDI, respectively.
Problems to Avoid:
Inadequate fertility at the greater root depth, Vacuum ingestion of soil into the drip system, Gophers and Root intrusion.
Fortunately we have secure solutions; fertigation, vacuum relief valves and ROOTGUARD®
Inadequate fertility at the greater root depth – Fertigation:
To achieve the best results it is essential to supply low concentrations of fertilizers almost continuously through the buried drip system. In particular, phosphorus and potassium are often in deficit of the plant needs at the usual 18" to 24" inch depth of burial. Every system must have provision for fertilizer injection both for fertilizers and for injection of chlorine or acids to control pH of the water and bacterial and fungal slimes. Vacuum ingestion of soil into the drip system – Vacuum Relief Valves:
To prevent soil ingestion vacuum relief valves must be positioned at all high points. Conventionally vacuum relief valves are sized to prevent collapse of the pipe - with SDI we need to keep water and soil from being drawn in through the emitters. Use twice the number of vacuum relief valves as are used for an above ground system. Ensure that the vacuum relief valves are in the submain in the irrigation block and not in the main transmission lines. If vacuum relief is needed in the main lines these are additional to those used to prevent soil ingestion. Use extra care when designing with pressure compensating emitters of the type with a rubber diaphragm pressed against a labyrinth (EQUALINE_ Agri+ and RAM). The ingested soil can lodge between the diaphragm and the labyrinth sealing surface and cause an increase in flow. Use slow closing valves.
Gophers have not been a significant problem. To reduce gopher strike risk bury the line more than 18" deep, most gophers do not operate at the deeper levels. Ensure that the wetted circles from the emitters overlap so that all the dripline is in wetted soil. Gophers appear to prefer to dig through dry soil.
Root intrusion – ROOTGUARD®etc.
ROOTGUARD, which is Treflan® fused into the polymer of the emitter, is the only commercially available method guaranteed to protect emitters from root intrusion. Injection of acids and other chemicals which may legally be used for this application is not a long term solution for permanent crops. There is one grade of trifluralin which as of February 1995 may be applied to a buried drip system at the rate of one teaspoon per 15,000 emitters three time per annum. If the grower has 1,500 drippers per acre then the application rate is one teaspoon for ten acres. This registration is new and at this time there is no published evidence that this dose is either practical or effective.
The Three Questions Most Commonly Asked
In addition to questions about root intrusion and gophers, which are answered above, the most common questions are
How do I know if the system is working?
How deep do I bury the dripline?
How to I manage the immediate needs of new plantings with the long term needs of the fully developed tree or vine?
How do I know if the system is working? Monitoring the System:
A flow meter and several pressure gauges are essential parts of any SDI system. To obtain accurate flow readings make sure that the flow meter is positioned in a generous straight section of appropriately sized pipe ; typically 10 pipe diameters in front of the flow meter and 4 pipe diameters behind the meter.
When the system is installed a standard flow rate at standard pressures should be established for each section.
A set monitoring schedule and recording of performance will be useful to determine long term SDI performance. For example, measure and record; flow, input pressure and flushline pressure for each zone weekly.
We cannot over emphasize the importance of the above monitoring procedure.
Due to ‘noisy‘, or ‘bouncing‘ instant flow readings the best way to accurately read flow is to time how long it takes to advance the flow totalizer several hundred gallons, and convert that to GPM. Repeatability is within one percent at same pressure using this method. Instant flow readings are nice for a quick check, but may be too unstable for truly accurate flow readings. Using the 'timed' method of determining flow, one can detect one open hose out of 200 if the break is near the mainline, and can detect 4 or more open hoses if the break is near the hose ends (+/– 600‘ runs, 16mm hose).
Another factor can be how dry the soil is around the hose. Flow may tend to drop by as much as a few percent as the runtime progresses and the soil begins to resist infiltration. ...It is most accurate to take the flow reading at, say, one hour into the run for each check.
Do high accuracy checks every few weeks, as well as right after the injection of some fertilizers (e.g. K2SO4).
Flow may drop a little as summer progresses and the pressure drops as the ground water table drops.
In the event of any variation of flow of more than +5% remedial action should be taken:
If the variation is high flow:
First look for breaks. If the system is not using pressure compensating emitters and the flow is high and there are no breaks look for another mechanical reason. e.g. high pressure, faulty pressure regulator or pressure gauge, valve not opening fully etc. If the dripline is pressure compensating first look for breaks. If that is not the cause then dig up a section of dripline and examine for foreign matter holding open the diaphragm. If this is not the case check the flow of a few individual emitters to check if there is hardening of the diaphragm.
If the variation is low flow:
(a) The usual first step is to check the equipment – flow meters, pressure gauges and regulators, valves opening etc. Then,
(b) clean the system with chlorine or acid or one of the proprietary products.
(c) Flush the lateral lines.
(d) If this does not bring the system back to standard then one must examine the line by digging up a section.
Caution: With pressure compensating emitters equipped with rubber diaphragms even a small increase in flow may signal a serious problem. With turbulent flow emitters an increase in flow indicates a broken lateral, a leaking pipeline or a defective flow meter.
How deep do I bury the dripline?
Like most SDI decisions this is soil and crop dependent. Given deep soil with neither severe changes in structure or stratification with depth nor high water tables then 18" to 24" depth has generally been suitable. Shallower depths may be required for sandier soils. With walnuts we recommend 24" depth to reduce the risk of pinching off of the driplines by the very large walnut tree roots.
How do I manage the immediate needs of new plantings with the long term needs of the fully developed tree or vine?
With well rooted bench–grafts the simplest approach is to bury the drip line at the preferred depth just off to one side and plant the vine in the fall before the winter rains. Or do the same as above but over-irrigate and be prepared to waste water for the first few months. Or, with less well developed cuttings place the dripline about 1" below ground next to the row of vines. At the end of the season pick up the dripline and bury at the preferred depth.
Any trees with a row spacing in excess of 15‘ will usually have two rows of dripline. There are several reports of jets being better than drip for established trees. If one compares one row of jets with one row of drippers this is often correct. Consistently, field data demonstrate that two rows of SDI will consistently outperform a single row of jets. Place the first row of dripline just off to one side of the tree as per vines above. You do not need to connect the second dripline row yet. After one or two years connect the second line on the windward side at the estimated dripline of the fully grown tree.
The above information is passed on as guidelines for the benefit of the industry and the author does not in any way assert that this information will be suitable for any specific application.
1. Ron Brase, 1995. Water Penetration Problems. Central Valley Farmer, Feb. 1 ,1995.
2. Claude Phene, K. R. Davis, R.B. Hutmacher, B. Bar–Yosef, D.W. Meek 1990. Effect of high frequency subsurface and surface drip irrigation on root distribution of sweet corn. Irr. Science, 12:135–140.
3. Craig Thompson, 1995. Re: Design, installation and .... Trickle–L Bulletin Board 29, April 1995.
With thanks to the many people who advised me in writing this guide. In particular I would like to thank Freddie Lamm, Tony Pereira, Claude Phene, Francois van der Spuy and Craig Thompson. All errors and omissions are by the author alone, and all the good stuff comes from the advisors.
©Copyright Rodney Ruskin, April 28, 1995.
Permission is granted to any interested party to reproduce this material provided that the source is correctly credited.