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Savage Terry Howell and others replied to my eddy correlation and Bowen ratio E-mail: A. Terry said: "I don't think either eddy correlation or Bowen ratio measurements can "unequivocally" replace lysimeters (not YET anyway)." My opinion: I do however think that these eddy correlation/Bowen ratio techniques are getting there fast, particularly for forest canopies, and then with CO2 measurements allow real-time water use efficiency to be determined. I would not like to build a lysimeter for such canopies - eddy correlation/Bowen ratio for tree canopies is bad enough and that is probably why many researchers have gone the sap flow/heat pulse route. Not much has been said about these techniques in SOWACS. B. Lindsay Hutley wrote: >>As a number of people have pointed out eddy correlation >>(increasingly referred to as eddy covariance) and/or Bowen Ratio is the >> most 'direct' method of ET measurement. These techniques are slowly >>coming out of the realm of research only applications. Campbell >>Scientific market commercially available Bowen Ratio and Eddy >>Covariance systems. The Institute of Ecology and Resource >>Management at the University of Edinburgh also market a complete >>eddy covariance system. >>Lindsay Hutley >>CRC for the Sustainable Development of Tropical Savannas >>School of Biological Sciences >>Northern Territory University >>Darwin NT 0909 >>AUSTRALIA I agree. I deliberately tried to downplay EC/BR measurement of evaporation. I also however agree with Jason Beringer (Environmental Climatology Group, Dept. Geography and Env. Science, Monash University, Melbourne Australia email:jasonb@shiva.arts.monash.edu.au) - I know that EC/BR techniques are not restricted to research use - sometimes however, I have a hard job convincing funders about these technologies. The commercially available sensors are usually water-proof now. However, the sonic anemometer does take a while to dry after a rain or an irrigation. Water on the sensor may not damage it but the water does make sure that the data is useless or at least until all the water has evaporated! The same applies to net radiometers. The net radiometer problem that Terry Howell mentioned is real (he said that net radiometer error may be larger than 5 to 8 %). In a micrometeorological investigation (Kanemasu et al. 1987), the five groups that participated measured a midday net irradiance of between 300 and 530 W m^-2 - more than a 75 % variation!! We have worked on a convenient calibration method for routinely calibrating net radiometers (and infrared thermometers) to minimise the error in net irradiance measurement. The net radiometer height above surface is also crucial, particularly in a row crop situation. One needs to know the sensing area of the net radiometer for the given (but user chosen) height above surface. E-mail if you need these details. However, do not forget the soil heat flux density - and its spatial and temporal variation. This can be a major headache and is not an insignificant energy balance term. With correct placement of sensors and using sap flow, one can separate soil evaporation and transpiration using such technologies (see 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). C. Trevor Finch (Research Services New England, 8/16 Nicholson St, Balmain NSW 2041 Australia email: rsne@mpx.com.au tel: +61 (2) 9810 3563 fax: +61 (2) 9810 3323) had a question: >>In order to calculate actual daily ET rates (which is presumably what is >>wanted), do the Eddy correlation methods have to integrate over a 24 >>hour period ? Answer: Eddy correlation (and Bowen ratio) methods are usually used to measure ET at half-hourly time intervals - the daily total may easily be calculated. The sensors are placed in the atmosphere so they do reflect much larger areas than soil based measurements. The time scale, as already mentioned, is also much less - usually half-hourly. 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