proudly sponsored by
|
archiving of SOWACS proudly sponsored by |
|
Dear Terry This is a response to your E-mail on eddy correlation measurements of ET. I have included your E-mail at the base of this one for possible use by other readers. I always wondered when someone would get to eddy correlation (and Bowen ratio) techniques for ET measurements. These systems allow total evaporation to be measured by placing most of the sensors in the atmosphere. In that sense they are more "portable" than buried sensors. Have they replaced lysimeters? I will leave that as a carrot for you or someone else to respond to. The eddy correlation method you described is one way and assumes that one has a reasonably fast-responding sensor. Most users have used the LI-COR LI6262 for fast measurements of "humidity". This works reasonably well. An alternative is to measure the sensible heat flux density from the covariance between the vertical wind speed and air temperature. If the net irradiance and soil heat flux density are also measured, total evaporation can be calculated. This method assumes "closure" - namely that the energy balance would have balanced anyway. Essentially, this eddy correlation method is a point measurement (at one level in the atmosphere). Aspects of the placement height of eddy correlation sensors have recently been published as well as the so-called "footprints" of such measurements: Savage, M.J., K.J. McInnes and J.L. Heilman, 1995. Placement height of eddy correlation sensors above a short grassland surface. Agricultural and Forest Meteorology 74, 195-204. Savage, M.J., K.J. McInnes and J.L. Heilman, 1996. The "footprints" of eddy correlation sensible heat flux density, and other micrometeorological measurements. South African Journal of Science 92, 137-142. The Bowen ratio technique requires measurements of air temperature and water vapor pressure at two vertical points (separated by a distance of about 1 m) above the canopy (typically at 0.5 and 1.5 m above canopy) as well as net irradiance and soil heat flux density measurements. More recently, Campbell Scientific have been marketing a Bowen ratio-CO2 system for total evaporation, sensible heat and carbon dioxide measurement. Some recent Bowen ratio references: Heilman, J.L., K.J. McInnes, M.J. Savage, R.W. Gesch and R.J. Lascano, 1994. Soil and canopy energy balances in a west Texas vineyard. Agricultural and Forest Meteorology 71, 99-114. Heilman, J.L., K.J. McInnes, R.W. Gesch, R.J. Lascano, and M.J. Savage, 1996. Effects of trellising on the energy balance of a vineyard. Agricultural and Forest Meteorology 81: 79-93. The Bowen ratio can run unattended for a week or more whereas the eddy correlation requires almost daily attention. For both eddy correlation and Bowen ratio measurements, fetch is a consideration. Eddy correlation and Bowne ratio evaporation measurements are however representative of larger areas (provided it is the required areas) - at least larger areas than other techniques previously discussed in the SOWACS forum. I am not sure that either of Bowen ratio or eddy correlation techniques are ready for anything else than research technology (- that is, for use by researchers only). That is not to say these techniques cannot be used for problem solving - problems that cannot easily be solved using other techniques. Regards Michael J Savage, Ph.D. Department of Agronomy University of Natal Pietermaritzburg 3201 South Africa Tele: 27 331 2605514 FAX: 27 331 2605426 E-mail: savage@agron.unp.ac.za For WWW junkies: http://www.unp.ac.za For current weather near my office, the Web site is: http://cirrus.sawb.gov.za/observation/current/FAPM.html For wind speed data for the previous one or two days, it is: http://cirrus.sawb.gov.za/observation/graph5/581.wnd.html For air temperature and relative humidity data for the previous one or two days, it is: http://cirrus.sawb.gov.za/observation/graph5/581.thum.html For rainfall data for the previous one or two days, it is: http://cirrus.sawb.gov.za/observation/graph5/581.rain.html PS: The above message is a reply to: 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) * *********************************************************************