This paper deals with the parameter kB(-1), the logarithm of the ratio
between momentum and heat roughness length, of sparsely vegetated sur
faces and hare soil. The bare soil surface is included as a reference,
since it is fairly homogenous and smooth, having no distinguishable r
oughness elements. The mean value of kB(-1) is about 8 for the vineyar
d and 12 for the savannah. These values are significantly greater than
kB(-1) = 2, which is usually assumed to hold for vegetation. The mean
value of kB(-1) for bare soil is small and negative, which agrees wit
h the literature. A large variation of kB(-1) during the day is measur
ed for all three surfaces. This behavior has been observed for sparse
vegetation in previous studies. Some authors explained the phenomenon
with a vertical movement of the source of heat through the day as sola
r angle varies, or with the use of an inappropriate value of effective
surface temperature to calculate kB(-1). For the first time. this diu
rnal variation is measured for a smooth surface, the hare soil, for wh
ich neither explanation is valid. A sensitivity study reveals that the
calculated kB(-1) is very sensitive to measuring errors in the microm
eteorological variables and errors in the roughness length for momentu
m. This explains the large range in observed kB(-1) values for one par
ticular surface type. In addition, several semiempirical expressions f
or kB(-1) from the literature are tested. Two well-established formula
s, both based on a simple combination of Reynolds and Prandtl numbers.
appear to produce the best estimates of daily averaged kB(-1) values.
None of the formulas are able to describe the diurnal variation. The
authors conclude that the concept of kB(-1) is questionable as it is b
ased upon extrapolating a theoretical profile through a region where t
his profile does not hold. reward a ''surface temperature'' that is di
fficult to define and to measure. It should therefore be avoided in me
teorological models, for example, by applying canopy boundary layer re
sistances. Unfortunately, In remote sensing, the bulk transfer equatio
ns are up to now the only option, which therefore requires the use of
kB(-1).