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Minute or Ultra-low microirrigation

All of us on this discussion list know the challenging aspects of sustainable use of water in agriculture (or landscaping for that matter) in the future. Not only will there be other interests for water but it is possible if not probable that prime agricultural land could be gobbled by urban encroachment. We 'might' be farming on foothills instead of valleys, and in intensive greenhouses instead of open gardens. Thus the potential for microirrigation technology will undoubtedly help in the efficient use of water.

Israel has always been on the cutting edge of microirrigation research and implementation. A new aspect of microirrigation has been researched in Israel for the past half decade. Known as "minute or ultra-low rate" irrigation, this new idea involves applying water at a very low rate, even lower than the natural soil infiltration rate. This process is accomplished by using spitters or pulsating drippers. I have never seen this technology with my own eyes, but I envision it as a microspray system which applies water in to a large area with low flow via thousands of pulses per hour (as low as 0.5 ml/hr). Drip emitters could be attached to the pulsator to apply water at a low rate also. As a rule of thumb, flow from minute or ultra-low irrigation is usually 10 times less than common emitters (i.e. 0.2 l/hr).

The advantages of this system include:

1) No run off on heavy soils.

2) No water loss through the root zone on very sandy soils.

3) Water could be applied efficiently on shallow soils in hilly areas.

4) Volume size of containers in greenhouses could be substantially reduced.

I have posted this not only introduce the topic, but also ask those in the industry (especially in Israel) to discuss the pros/cons of this rather new idea or extension to microirrigation. Questions I have concerning this topic are:

1) What happens during very high evaporative demand when using this technology..won't a large percentage of water be loosed through evaporation in mid day?

2) How could this technology be implemented in subsurface drip irrigation (SDI)?

3) If this technology requires the system to be engaged for long periods of time, would this save or increase energy costs?

by Richard Mead

The concept of "Minute" irrigation is not necessarily new but has been impractical until approximately three years ago. The idea is to apply water at a very slow rate. To achieve this we would require a drip emitter with extremely small passages and considerably higher filter requirements. This emitter would be highly susceptible to clogging. To date there is no emitter or tape product that is capable of delivering water at a rate which approaches that considered to be minute irrigation. However, there are a few individual components that when used together can create this minute irrigation. I consider minute irrigation to be in the range of 100 - 400 cc per hour.

The heart of this system is a pulsating device which contains a silicone sleeve seated upon a specially designed piston. As this sleeve or bladder swells with water it reaches a critical point where the stored water is released and then the process repeats itself. This continual action creates the pulsing effect. The rate of flow through the pulser is determined by either a compensated or non-conpensated emission device. It is when this pulser is connected to a secondary emission device that we are able to achieve minute irrigation. In Israel when using the term minute irrigation they are referring only to the use of drip emitters. Pulsated micro-sprinklers or jets is a different concept.

Most applications of this system have been used in green houses. There are two types of systems of minute irrigation. One system connects about 20 individual pot type drippers (stakes with a labyrinth) to one single pulser. If the pulser has a discharge rate of 4 LPH or 4,000 cc/hour we divide this number by the number of outlets and have an individual discharge rate of 200 cc/hour/pot. The second system uses our (Drip In) 1/4" (6mm) soaker dripline with emitters spaced anywhere from 15cm to 30cm connected to the same pulser.

We can not use a dripline with a larger ID because the line will always be partially filled with air. The 1/4" because of its small ID is constantly charged with water. This system is either stretched on top of the pots or laid directly on the bed. The number of emitters varies but is generally not more than 60. I recently installed a system where I used an 8 LPH pulser with 60 emitters or an individual discharge rate per emitter of 133 cc/hour. These emitters normally are 2 LPH.

The beauty here is that we are able to reduce the flow per emitter to minute amounts of water and yet maintain large passageways and relative clog resistance. Like any new technology there are advantages and disadvantages. In fact this technology is considered by some to be revolutionary. Similar to what drip was 20 years ago. Most pots are irrigated by spray stakes or some type of emitter. Water applied at a rate of 2 LPH will form a sausage near the middle of the pot and drainage will begin within a few minutes. Irrigation will continue approximately 7-15 minutes. During this time water will begin to move upwards closer to the sides of the pot pushing the salts further into the root zone. This mandates frequent flushing and an additional waste of water and nutrients. With the pulsated drip system the water will move almost twice as fast laterally until the upper area is completely wetted. Then the movement will be downward as a front until drainage occurs. When the first drops drain the pot is at pot capacity and the irrigation can be shut off. This movement is constantly washing the salts downward. Additional flushing of the salts is only required when the EC of the drainage water exceeds the established limits.

Specific advantages of this system include:

1. Water and fertilizer savings up to 40-50%

2. Optimum growing conditions due to the ability to maintain an optimum balance of air, water and nutrients in the soil.

3. Better utilization of available space; plant density can be increased.

4. Quicker turn around of plant material; reduced growing cycles.

5. Higher yields

6. Better quality

7. Lower system costs; smaller PVC sizes, reduced horsepower requirements....

This system poses significant challenges and requires us to change our way of thinking. For one, the discharge rate of the emitters at the end of the lateral is higher than the rate at the beginning. This is completely opposite from what we expect with conventional drip technology. Second, we are talking about using up to 40-50% less water then existing drip systems. If this is true than we need to reevaluate crop requirements. We applied this technology on a small scale to 40 almond trees in the Sacramento Valley this summer. From mid june through October we applied 1 GPH/tree. The dripline was our 1/4"soaker dripline with emitters spaced at 12". The water was never shut off except for one day at harvest. The surface wetted area was on average 1 foot wide and there was no runoff. These trees received no more than 24 gallons per day. These were mature trees with a full crop. Visual inspection indicated good growth and yields comparable to the rest of the orchard. We intend to expand this system and do a small area of grapes in 1997.

We do not have all the answers to these questions yet. Actually we are not sure what questions we should be asking. While the concept has broad applications the technology to apply this on a large scale to field crops is in its infancy. For the present we will be promoting this minute irrigation technology to the greenhouse industry and evaluating its application in the broader agricultural market. We would like to invite those interested in this technology to explore the possibilities and ramifications along with us. A few papers have been written on the subject in Israel. They have been translated into English and are available in Israel by contacting Jacob Levin at Lego Irrigation or contacting me at Drip In Irrigation.

I hope that I have been able to answer a few of the questions.

by Philip Lubars

>The concept of "Minute" irrigation is not necessarily new but has been impractical until approximately three years ago.<

Well, this gives me an opportunity to introduce Trickle-L list members to a drip technology, the use of moisture-sensitive, self-regulating irrigation valves,Irristats, (typically, but not necessarily, one to a plant, bush or tree) for delivering water at rates which exactly match evapotranspioration, and therefore is as "Minute" as you would want to get! Like other forms of "Minute", it typically uses a water-supply continuously pressurized at up to 15 lbs/in^2, and requires quality filtration. But it has a lot more to offer.

by Leonard Ornstein