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20 Nov. 1996 Dear Sowacs and Dean Reynolds: In the strictest sense, ET can only be MEASEURED one way -- by eddy correlation. Other common ET measurements -- lysimeters, water, or energy balance -- must be considered indirect. So neutron probes and TDR are tools useful in measuring a volume water balance, but they can't measure ET! A weather station doesn't measure ET either! I'm not a STRONG fan of eddy correlation either, but theoretically the mean product of short-time perturbations in vertical winds and absolute humidity defines the ET rate (when multiplied times the mean air density). And that's not saying that measuring ET with eddy correlation is perfect nor is it even practical! A weighing lysimeter can "directly measure" ET for time periods when no rain or irrigation occurs (and assuming percolation is measured). A weighing lysimeter measures the mass of the soil water (along with the soil and plant mass); hence, any temporal changes in mass are attributed to water uptake and transpiration by plants or evaporation from the soil (or plant) surface. Of course, the small mass accumulation from photosynthesis also acts against the negative mass change caused by ET. [This is a good exercise for a serious student to determine the weighing error in ET measured by a weighing lysimeter cased by photosynthesis.] Neutron scattering (NS) can be useful in making precise water balance determinations. BUT I have to throw out several precautions: 1. calibration is mandatory and precise methods are critical 2. DEEP (I repeat deep) measurements are required (at least 1 m or more beyond the crop root zone (this may mean going to 2-3 m depths in most cases) 3. deep percolation losses from the profile MUST be avoided (use in high water table areas or where upward flow occurs renders the NS method useless for ET) 4. very accurate water accounting (rain, irrigation) at that spot and for the surrounding area must be accomplished (may involve measuring runoff, etc.). A few comparisons of NS-water balance ET and lysimeter ET have been made. Jim Wright at ARS-Kimberly, ID, Steve Evett at ARS-Bushland, TX, and C.H.M. van Bavel (formerly at ARS-Phioenix, AZ) and others have published brief papers on this topic. I'm sure others may have published papers of which I'm not aware at this moment. Generally, if the above conditions are met the agreement is satisfactory (within +- 0.1-0.2 mm/d for a 3-5 day period). I've been impressed by some of the diurnal traces from the capacitance gauges that appear to offer useful precision in determining daily (with good precision) ET amounts. But I'm not aware of any "rigorous" testing that would validate or "prove" that point. Also, then the "absolute" calibration of the probe becomes somewhat more critical. Terry Howell, Ph.D., P.E. USDA-ARS Bushland, Texas USA tahowell@ag.gov At 11:22 AM 11/20/96 PST, you wrote: > >I recently attended a class where the statement was made that neutron probes can not be used to measure ET. If this is true I would assume the person would say the same about TDR. > >I have become certified to use a neutron probe for two reasons, 1) We have one and 2) I thought I might be able to get some studies going to update crop ET numbers. > >I realize that it would be better to use lysimeters, but.. they are expensive and not readily available and I would guess expensive. It would be great if I could talk DWR into installing its own but not in my lifetime. > >Someone must have done comparison studies with probes of various types and lysimeters. > > >I ask this group to give your opinions on this subject. I am asking the question regarding measurement of real-time ET not as an irrigation scheduling tool. > > ********************************************************************* * Terry A. Howell, Ph.D., P.E. (806) 356-5746 * * USDA-ARS (806) 356-5750 (Fax) * * P.O. Drawer 10 tahowell@ag.gov (E-mail) * * Bushland, TX 79012 http://www.net.usda.gov/cprl/ (Internet) * * * * 1/2 mi. West I-40 South Access Rd. (shipping) * *********************************************************************