I would appreciate any comments/suggestions on the following question! The Penman-Monteith combination model for estimating evapotranspiration (ET) requires the measurement of vapor pressure, air temperature, and wind speed at one height above the vegetation canopy. Other models for ET estimation require data from sensors at multiple levels in order to measure spatial gradients and calculate vapor flux. The P-M model gets around having to measure the actual gradient as long as the single sensor height is within the part of the boundary layer where wind speed increases in proportion to increasing height above the canopy [ u(z) ~ ln z]. This encompasses about the lower 15% of the boundary layer from various descriptions I have read. My question is: how would I decide at what height to place the sensors above a tree canopy composed of trees of varying height (20-25m, ave.~ 23m) and with variable spacing from each other. Some of my advisors have suggested using 1.5-2m above the average tree height or the same above the tallest tree in the vicinity of the sensors. I have looked through many of the published studies using the P-M model to estimate forest ET. However, the forest under study is often even-aged and less variable in height than in my study area. Thanks in advance for any advice. Dan Redgate Virginia Institute of Marine Science dan@sweethall.wetlan.vims.edu
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