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NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za with the body of the message containing the line: unsubscribe sowacs Well said, Len. It seems to me that this all goes well towards proclaiming the reality that growers need both; time and amount. A very well versed professional in this field considered these issues in 1976-78 in Arizona. He recognized that the water manager needs to know both: when and how much. He choose the neutron probe. I believe he did this for at least two big reasons: 1) The neutron probe samples (assesses) a relatively huge volume of soil so little soil anomalies have little impact on reliability, and 2) because how much to apply changes through the season, requiring attention soil surface (surface conductance?) conditions, and is really critical information to be providing someone who can be spending several hundred dollars an hour in terms of water and electricity and crop yield. As for water release characteristics, sampling for moisture release curve analysis will provide that. We would pick the predominate soil type, and the side of the field which was re-irrigated first. The implications to crop profitability, as a function of the EXTENTS OF THE DEPLETION* the grower had allowed, could be estimated. Was his timing perfect? Well, arguably, the best way to make such a judgement would be with a water potential measuring device like a tensiometer. But deploying both methods in the field wasn't always cost effective for the grower. In lieu of tensiometers along with neutron probe sites, proper interpretation of the neutron probe readings, in light of the soil moisture retention analysis, proved highly effective. Sure, hysteresis confounds this approach, but from a practical standpoint, I mostly tend to remember the steady, regular readings from the probe before and after irrigations, throughout the season. The experience was profoundly confidence-engendering. *The furrow and border irrigating grower has to struggle to utilize good soil moisture status information because the movement of irrigation heads around the ranch is usually cumbersome, dependent on human activity and ther efor risky. For instance, if the sidedressing rig loses half a day getting around, coupled with litter in the furrows which slows water advancement down the furrow, the water WILL be late. Automated drip irrigation really addresses these concerns. Tom Reynolds, C.I.D. Water Balance Mesa, AZ ----- Original Message ----- From: <owner-sowacs@aqua.ccwr.ac.za> Sent: April 08, 2000 12:07 AM Subject: Re: Volumetric vs Gravimetric > NOTE: To get off this list, send email to majordomo@aqua.ccwr.ac.za > with the body of the message containing the line: > unsubscribe sowacs > > Bill: > > Jason made the CORRECT point that there could be a large change in > water content under conditions where a tensiometer shows a small > change in water potential. For example, a tensiometer will read very > near 0 in uncompacted peat at saturation; drain 1/2 the water from it > and the reading may still be only -0.03 bars. That's a small > tensiometer sensitivity to a large change in water content. > > The point that I was making is that it's only when the change in the > tensiometer reading gets large, that you have to worry that > conditions are approaching a point which might stress the crop. That > is, universally, the tensiometer, INDEPENDENT OF SOIL TEXTURE, tells > you when you ought to irrigate. Any device which only measures soil > water content (neutron probe, TDR, etc.), requires additional > information that "corrects for" the soil texture to translate the > reading into a useful guide for when to irrigate. > > This was an unsolicited endorsement for tensiometers, not a > tensiometer putdown! > > Len Ornstein > > > > > On the other hand, what the crop cares about is water potential, not > > soil water content, so the low sensitivity of a tensiometer to large > > water content changes near field capacity poses no threat to the > > crop. But when the water potential starts to dive, even if the change > > in water content is small, you better start to irrigate! > > > > Len Ornstein Ph. D. >> > > > >I am not sure that I understand this issue of the "low sensitivity of a > >tensiometer to large water content changes near field capacity". I would say > >that it is opposite, with the tensiometer very sensitive. Am I missing > >something in prior messages? > >Regards, Bill Pogue, Irrometer Company, Inc. > > > > >