32 The point of view of FDA

Abstract

As part of a comprehensive study investigating how subsurface drainage systems affect the energy and water balances on land surfaces, a large aperture scintillometer (LAS) was used to determine the sensible heat and momentum fluxes over a corn (Zea mays) field. To keep the flux footprint of the observation within the 22 ha field boundary, the LAS had to be placed at a height no greater than 1.8 m above displacement height. As a result, the surface layer was mostly near-neutral at the LAS path height. Sensible heat fluxes were derived from the LAS measurements (HLAS) using four different Monin–Obukhov Similarity Theory (MOST) functions for temperature (fT). While correlating well with the sensible heat fluxes measured by the Eddy covariance system (HEC), the values of HLAS were systematically higher if the rule of thumb formula for estimating the roughness length (z0) was used. The use of this rule of thumb formula led to higher estimates than the EC measurements of the frictional velocity u*, to which HLAS is particularly sensitive under near-neutral conditions. With a modified formula for z0, a better agreement between HLAS and HEC is achieved for all the fT functions tested. Using the fT function (W71) proposed by Wyngaard et al. (1971) and the improved estimate of z0, HLAS agreed with HEC within 32 W m−2 and with a regression slope of 1.0 ± 0.05.