The Irristat: key to a new irrigation technology?

Relevant posts on SoWaCS :

• Measuring Evapotranspiration/Lysimeters (Introducing "IRRISTAT" (tm) )
• Re: Measuring Evapotranspiration/Lysimeters

• Measuring Evapotranspiration/Lysimeters

From:	Leonard Ornstein (lenornst)
To:		BRUCE@mailgate.icfrnet.unp.ac.za
Date:	Friday, November 22, 1996  1:38 am
Subject:	Measuring Evapotranspiration

Evapotranspiration, can be very simply, inexpensively and accurately measured 
with the Irristat, a device with which few if any of the SOWACS
Listá’ás 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:

Iá’ám the inventor and manufacturer of the Irristat, a moisture-sensitive,
self-regulating valve, designed to control the delivery of water to a
á“ádrip emitterá”á.  It was introduced in:
 
á“áThe Irristat:  A Moisture-Sensitive, Self-Regulating, Water Valve for
Drip Irrigation Systemsá”á, 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
the . of Washington State Univ.á’ás Irrigated Agriculture Research Center,
Prosser, WA. 


Iá’ám in the midst of preparing a Web Page which will describe the
technology briefly and will contain URLá’ás 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papers as well as four others, including an unpublished paper by Robert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, Prosser.

I had expected to á“áintroduceá”á the Irristat to SOWACS and Trickle-L when
my Web Page is ready. Perhaps I shouldná’át wait. However, if interested
members on the lists caná’át access at least the 2 Acrobat documents, it
may make little sense. On the other hand, if you would be willing to
temporarilly offer them from your Web Page, it might be reasonable to
post part or all of this message now? If youá’áre interested, I can
separately tell you why the Irristat has effectively remained a á“ásecretá”á
this long. )
 HEY, tell us all - its no longer a secret! ;>    

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 á“ácapillaryá”á
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's set up with a gravity-feed water supply from a á“átanká”á, 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 á“ágrowth waterá”á.

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 á“áinstantaneous measurementá”á
usually reflects the evapotranspiration that occurred 2 hours
earlier.)

Leonard Ornstein
Irristat International Inc.