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(a)
700
R
n
600
Wm
−
2
500
L
e
E
400
300
H
200
100
G
0
−
100
(b)
7
u
*
6
5
2
4
3
2
1
0
12
18
00
06
12
18
00
06
12
18
24
Time (PST)
Fig. 2.19
E
xample of the daily cycle of (a) the energy balance (in W m
−
2
), with (b) the mean wind speed at 2 m,
u
2
(inms
−
1
, solid line), and the friction velocity
u
∗
(in dm s
−
1
, dashed line), for an irrigated grass
covered surface at Davis, California, on June, 2-4, 1965. The balance equation was assumed to be
R
n
=
L
e
E
+
H
+
G
. The evaporation was measured with a weighing lysimeter (Pruitt and Angus,
1960) and the surface shear stress with a floating drag plate lysimeter (Goddard, 1970). The data are
drawn from Brooks and Pruitt (1966); the roughness of the grassy surface was estimated to be
z
0
=
0
.
97
±
0
.
14 cm (Morgan
et al.
1971).
sometimes even comprise a lake or a vegetational canopy over its entire depth. For many
practical purposes, the energy budget equation can be written as
A
h
=
∂
W
∂
R
n
−
L
e
E
−
H
+
L
p
F
p
−
G
+
(2.72)
t
