Geoscience Reference
In-Depth Information
2 0 0
L
e
E
1 5 0
W m
−
2
1 0 0
5 0
0
2 7 3
2 7 8
2 8 3
T i m e , t ( d a y o f t h e y e a r )
Fig. 9.29 Comparison of diurnal cycles (shown as hourly values during the daytime) of the calculated (by
means of Equation (9.114)) and measured latent heat flux from natural tallgrass prairie in the
FIFE experimental area in eastern Kansas during the later stages of a major drying episode in
1987; the time shift was taken as
t
0
=
271. The solid lines represent the measurements, and the
dashed lines represent the calculated values. (From Brutsaert and Chen, 1996.)
inferred the effective desorptivity for bare soil from several other studies; for a clay loam
soil he reported 0.508 cm d
−
1
/
2
, for loam 0.404 cm d
−
1
/
2
, and for a black clay soil
De
0
=
0.350 cm d
−
1
/
2
. In the study by Parlange
et al.
(1992) the reported value was
De
0
=
0.58 cm d
−
1
/
2
. This similarity suggests that by the time evaporation from a grassy
surface comes down to this stage, the vegetation becomes quite inactive, and most of the
drying takes place from the surface, as if it is bare. The similarity for the different soil types
also suggests, as already indicated by the results of Jackson
et al.
(1976) shown in Figure
9.25, that vapor diffusion plays an important role in the second stage of drying, in addition
to the capillary rise of liquid water. Clearly, this phenomenon will require further study.
Diurnal variation by self-preservation approximation
In catchment hydrology the daily times scale is a common one; nevertheless, in many
applications a daily time resolution is too coarse, and time steps of 30 min to 1 h are required.
Further analysis of the same data observed over natural prairie, discussed in the previous
section, also indicated (Brutsaert and Chen, 1996) that, while the total daily evaporation
could be described with a
t
−
1
/
2
dependency, this day-to-day evolution is modulated during
the day by the available energy at the surface, that is by the hourly radiation input. Moreover,
during the daytime hours the surface energy budget often displays self-similarity or self-
preservation, in the sense discussed in Section 4.3.4. This dual structure of the evaporative
evolution during very dry conditions suggests that it can be described, by combining the
desorption parameterization (9.108) for the total daily evaporation, or for any dimensionless
counterpart (such as the evaporative fraction EF, the Priestley-Taylor
α
e
and possibly others),
with the assumption of self-preservation as expressed in Equation (4.51). The combination
of these two concepts yields the following evaporation rate at time
t
=
t
i
of day
t
d
,
1
2
De
0d
(
t
d
−
t
0
)
−
1
/
2
F
−
1
E
i
=
F
i
(9.114)
d
