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21 Nov. 1996 Dear Mike and SOWACS: I don't think either eddy correlation or Bowen ratio measurements can "unequivocally" replace lysimeters (not YET anyway). In principle, water vapor flux can be measured via eddy correlation. But instrumentation problems still linger, and as you noted eddy correlation requires high maintenance. I haven't read your EC papers yet, but typical energy balance closures are seldom less than +-10 % of available incoming energy (Rn - G) [the best weighing lysimeters may be that far off too]. Bowen ratio is less maintenance intensive, but requires dependence on Rn and the assumptions about Kh/Kv. Anyone that assumes a BR-energy balance measurement is better than +-10 % needs to study net radiometers a little longer! We've discovered calibration errors of 5-8 % even in Eppley pyranometers and net radiometers may be considerably worse. Folks that seem the most satisfied with eddy correlation pick THE net radiometer from a group of net radiometers that closes the energy balance the most consistently! You are right that EC (eddy correlation) and BR (Bowen ratio- energy balance) reflect much larger areas (by the footprint analyses). I omitted this point but it should be critical to recognize this fact. The bottom line is that nothing (EC, BR, lysimeters, soil water balance, etc.) is without problems. Good to hear from you. Your points were good! Dean Reynold's original post implied if you had a neutron meter you should be able to measure ET, and that's not the case with anything (EC, BR, TDR, .....) without doing considerable behind the scenes work and making a host of ancillary measurements! One subtle point is that eddy correlation measures ET (if you measure the covariance of water vapor), but the Bowen ratio INFERS ET (does not measure ET; by an energy balance assuming Kh=Kv). As you mentioned, using eddy correlation to measure heat flux and then measuring Rn and G, again INFERS ET (by an energy balance). Terry Howell ********************************************************************* * 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) * *********************************************************************