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Dear all, Like soil moisture sensors, evapotranspiration (ET) methods have gained substantially from increases in available technology. This has allowed great improvements in both the Bowen ratio and eddy correlation systems. Bowen ratio techniques rely on being able to determine temperature and vapour pressure gradients between two heights. Over aerodynamically rough surfaces such as forests these gradients can be very small Barr et al. recorded temperature gradients as small as 0.002 oC.m-1 and vapour pressures of 2.5Pa.m-1. Over wet pasture Pasquill (1949) measured temperature gradients of 0.45 oC.m-1 and dew point temperauture gradients of 0.47 oC.m-1. This places very high demands on instrument resolution. Current technology such as the Campbell Scientific Bowen ratio system can resolve temperatures of 0.006 oC and vapour pressures to less than 0.01kPa. This is certainly accurate enough for most studies. This sytem has been detailed by Beringer and Tapper (1996). The use of this system is certainly not restricted to research use as the system has been used to measure ET over dams and reservoirs, tailings dams on mine sites, wetlands, and numerous other studies involving the measurement of ET from surfaces. Similarly eddy correlation systems can now be used for routine ET measurement. Gone are the days of having to stand by the system in case of rain or sensor failure. Many companies now produce weather proof sonic anemometers, some of which can measure during rain. This allows unattended monitoring of ET but at this stage the pricing tends to restrict this method to research applications. Currently a three dimensional sonic anemometer may cost ($13 000 AUD) (CSAT3 - Campbell Scientific). Neutron moisture meteres certainly have been used to measure ET using the water budget method. Grant (1975) compared ET measurements from the Bowen ratio method, aerodynamic method and a lysimeter with a neutron moisture meter and found a good correlation, especially during the growing season when ET was high. Regards, Jason Beringer REFERENCES Beringer, J and Tapper, N (1996) The use of the Campbell Scientific Bowen ratio system for evapotranspiration measurement, IN Proceedings of the 2nd Australian Agricultural and Meteorology Conference, Brisbane, Australia, October. Grant (1975) Comparison of evaporation measurements using different methods, Quart. J. R. Met. Soc., 101:543-550. Barr, A.G., King, K.M, Gillespie, T.J., Hartog, G.D., and Neuman, H.H (1994) A comparison of Bowen ratio and eddy correlation sensible and latent heat flux measurements above a deciduous forest, Boundary-Layer Met., 71:21-41 Pasquill, F. (1949) Eddy diffusion of water vapour and heat near the ground, Proceedings of the Royal Society London [A], 198: 116-140. |==========================================================================| | | | JASON BERINGER | | | | (Environmental Climatology Group) | | (Dept. Geography and Env. Science) Ph: 61-3-99051427 | | (Monash University) Fax: 61-3-99052948 | | (Melbourne Australia) Home: 61-77-216 294 | | Mobile: 0412 103 570 | | email:jasonb@shiva.arts.monash.edu.au | | O O WWW:http://shiva.arts.monash.edu.au/~jasonb/ | |================ooO===O===Ooo=============================================|