Geoscience Reference
In-Depth Information
30
20
10
0
255
260
265
270
275
280
285
Time,
t
(day of the year)
Fig. 9.27 Evolution of measurements of daily mean values of (
L
e
E
)
−
2
(with
L
e
E
in Wm
−
2
) over a
natural tallgrass prairie surface in eastern Kansas during a prolonged drying period in 1987.
The straight line represents the relationship for the second stage of drying, namely
(
L
e
E
)
−
2
=
2
.
0(
t
−
271)
×
10
−
4
. The data were measured during the FIFE experiment. In the
episode shown here, the last major rainfall had taken place on day 253 and minor rainfalls on
days 258 and 259. (After Brutsaert and Chen, 1995.)
3 0 0
( L
e
E)
2
Σ
2 0 0
( W m
−
2
)
1 0 0
0
0
0 . 5
1
1 . 5
2
t
r
1 / 2
−
t
r 0
1 / 2
( d
1 / 2
)
Fig. 9.28 Cumulative evaporation (with daily values in W m
−
2
) after the onset of the second stage of
drying,
(
L
e
E
)
2
, as a function of the square root of time (
t
1
/
2
−
t
1
/
2
r0
) with the same data as the
previous figure; the straight line represents (9.111) in the form
(
L
e
E
)
2
=
141
.
4(
t
1
/
2
r
−
t
1
/
2
r0
)
in which the summation starts on day 273, that is with
t
r0
=
2 days. (After Brutsaert and Chen,
1995.)
r
use of the desorption formulation in this case. The effective desorptivity derived from the
slopes in Figures 9.27 and 9.28 is approximately
De
0
=
0.495 cm d
−
1
/
2
.
Although this value of
De
0
was obtained for a grassy surface, it is of the same order
as values reported for other studies with bare soil. As mentioned, for a sandy soil Black
et al.
(1969) derived
De
0
=
0.496 cm d
−
1
/
2
from lysimeter measurements. Ritchie (1972)
