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Greetings to all: On Nov. 20, Dean Reynolds initiated a thread on methods that might be useful for updating crop-ET numbers. Because of the expense of lysimeters and lysimetry, he was interested in the pros and cons of using neutron probes and TDR as alternatives for such purposes. Terry Howell quickly responded with a very helpful discussion. He correctly noted that since evapotranspiration modifies the moisture content of the air above soil and foliage, strictly speaking, lysimetry, neutron probe measurements and TDR only provide an indirect measures of ET, whereas eddy correlation can measure ET directly. However he indicated that it probably wasn't a practical alternative at this time. He neatly reviewed the hazards of crop-ET assessment; rain, deep percolation, etc. On Nov. 22, Dean replied with thanks to all for their contributions to the discussion, and revealed just why he had raised the subject; namely the economic factors that governmental bodies, like the State of California face as farmers choose to sell their State-assigned water rights to one another. To oversimplify, it appears the allowed selling price depends upon a State-approved estimate of crop-ET and acreage. If the estimated crop-ET is too high, the State essentially is giving away (and wasting) valuable public resources. What Dean in fact wants to assess is the crop water consumption, which of course results mainly from ET, but THAT is directly measurable by lysimetry. Dean believes, for the reason reviewed above, that updating crop-ET with lysimetry would have substantial desirable economic consequences, but apparently judges that the costs of the equipment and labor for the job will be unacceptable to the officials of the California Department of Water Resources for whom he works. The problems he raises are rather universal. I believe I can offer an inexpensive solution: I am the inventor and manufacturer of the Irristat, a moisture-sensitive, self-regulating valve, designed to control the delivery of water to a "drip emitter". The Irristat is a device with which few if any of the SOWACS List's members will be familiar. Although the Irristat was developed as an automated irrigation-scheduling device, it also works fine as an alternative to a lysimeter to "measure" evapotranspiration: It was introduced in: The Irristat: A Moisture-Sensitive, Self-Regulating, Water Valve for Drip Irrigation Systems: Drip/Trickle Irrigation in Action Vol. 2 ASAE Pub. 10-85, St. Joseph, Mich., pp.623-629; (Proceedings. of the Third International Drip/Trickle Irrigation Congress, Nov. 18-21, 1985, Fresno, CA). The article contains a description of the Irristat and a general discussion of its applications in agriculture and horticulture, including a description of an installation for 60 mature cherry trees at Washington State Uniersity Irrigated Agriculture Research Center, Prosser, WA (WSU). Temporarily, an Abode Acrobat version of that publication, irrst2.pdf, as well as another document, irrist1.pdf, which describes the Irristat technology in greater detail, can be read and/or downloaded from:I am in the midst of preparing a Web Page which will describe the technology briefly and will contain URL's for downloading these two papers as well as four others, including an unpublished paper by Robert G. Evans, documenting the performance of the Irristats that I installed for him and Ed Proebsting for the 60 mature cherry trees at WSU. I had expected to introduce the Irristat to SOWACS and Trickle-L when my Web Page is ready. That should be before the new year. And I will post its address here as soon as possible. Briefly, this is how the Irristat works: The Irristat uses a uniquely formulated, synthetic polyacrylamide gel as its moisture-sensing element. The valve is buried near a plant's roots, in intimate contact with the soil,. Water is conducted by capillary tubing from a water supply, through a thin-walled rubber tube within the body of the Irristat and then through another attached length of capillary tubing to, or near to the soil surface. On its way past the roots, the water spreads by gravity and capillarity through the soil, passes through the Irristat's porous polyester fiber membrane, and reaches the moisture-sensing element, the gel. As the gel becomes more moist, it swells, pushing the Irristat's piston against the rubber tube. When the moisture in the soil surrounding the Irristat reaches a predetermined set-point, (typically -0.15 bars) the swollen gel causes the piston to pinch the rubber tube closed, cutting off the supply of water. As the plant draws moisture from the soil, the gel shrinks, reversing the cycle. As the moisture level falls below the Irristat's set-point, the piston moves back, relieving the pressure on the rubber tube, and water begins to flow. Buried in the soil, Irristats will function reliably for many years, PROVIDING AUTOMATED DRIP/TRICKLE IRRIGATION. The Irristat itself is about 2 cubic centimeters in volume. It, and its connections, are usually encased in a protective polypropylene shell in the form of a spear-tip, which simplifies insertion into the soil. The body parts and piston of the Irristat are molded of polypropylene; the internal water conduit is made of silicone rubber; the semi-permeable membrane, of Dacron-like polyester fibers; and the moisture-sensitive gel, of slightly-cross-linked polyacrylamide. All are chemically, biologically and physically durable. With 0.125-inch inside-diameter capillary tubing, and a water supply at 15 pounds per square inch, an Irristat can deliver up to about 1.7 liters of water per minute. Therefore, one Irristat can service any plant, up to a medium-size tree; a few Irristats, in parallel, can service a large tree. The Irristat delivers to its plant, EXACTLY THE AMOUNT OF WATER NEEDED TO REPLACE LOSSES DUE TO EVAPOTRANSPIRATION, (plus any additional amount incorporated into new growth). If it is set up with a gravity-feed water supply from a tank, after it has been working for a few days, the measured amount of water drawn from the tank per unit time (e.g., per 24 hours) is an accurate measure of evapotranspiration plus "growth water". Alternatively, if the water supply is a typical drip-lateral, a second Irristat can be installed in parallel with, and directly adjacent to the first, with an outlet capillary line of the same inside diameter and length as the first, delivering its output at the same point. After they have been working together for a few days, the outlet of either one can then be placed into a container, and the amount of water delivered per unit time will be an accurate measure of the evapotranspiration associated with that plant, bush or tree. (The Irristat has a time-constant of about 2 hours, so an "instantaneous measurement" usually reflects the evapotranspiration that occurred 2 hours earlier.) Of course, this description of how to use Irristats to measure evapotranspiration is a bit oversimplified; (e.g., if the bottom of the root ball is at the water table, if it has just rained or if the plant is growing in such coarse sand that, at -0.15 bars matric potential, a substantial portion of the delivered water percolates down past the roots), my method works poorly. With two tensiometers; one beside the Irristat in the root ball, and the other inserted to a depth somewhat below the root ball, you can easily check whether the water table is too high or the irrigation water is perciolating too deep. You will of course wonder why I have kept this a "secret" so long? In order to compete with pressure-compensating emitters, we had planned to manufacture the Irristat at a unit cost of under $1.00, with most of the cost coming from the labor of manual assembly. But that depended upon essentially 100% manufacturing-yield of correctly functioning Irristats. And over the years, the best we've been able to do is a bit better than 90%. That means that EACH finished Irristat needs to be checked by cycling it a few times between wet and dry states, and this turns out to be quite expensive. We (and others) judged that the resulting $6.00 unit selling price, for a spear-tip mounted Irristat, would severely restrict the range of its application. With rather limited resources, we have been (very inadequately) trying to enter markets, like landscaping, which might tolerate the higher, cost. I hope the Web will help us change this. But, clearly, for use as an ET-measuring tool, our current manufacturing cost should pose no problem!. I'll be happy to discuss various other details of use and performance, in this forum, or privately. Hope you find this useful. Leonard Ornstein, Ph.D. Irristat International Inc. lenornst@pipeline.com