Geoscience Reference
In-Depth Information
Priestley and Taylor (1972); use
α
e
=
1
.
27. (c) Actual evaporation, assumed to be givenbythe
equilibrium evaporation; use
α
e
=
1
.
0. (d) Actual evaporation by the advection-aridity approach of
Equation (4.48).
4.13
Same as Problem 4.12 but with the data of Problem 2.18; assume that the wind speed at 10 m
above the ground was 8.96 km h
−
1
.
4.14
Give an expression for the Bowen ratio, implied by the equilibrium evaporation concept, as
formulated in Equation (4.30)? What is the numerical value of this Bowen ratioat25
◦
C?
4.15
Multiple choice. Indicate which of the following statements are correct. The Penman equation
(4.23):
(a)
has the advantage, in practical applications, that only measurements at one level (instead
of vertical gradients or differences) above the ground are needed;
(b)
is well suited to calculate actual watershed evapotranspiration under drought conditions,
because it takes account of the moisture saturation deficit of the air;
(c)
should, inprinciple, be adjusted for any given surface, as a function of the surface
roughness,
z
0
,
and of the atmospheric stability to yield an accurate result;
(d)
is applicable even in the tropics;
(e)
for calculations over land covered with vegetation for periods of a day or longer, the ground
heat flux,
G
, can often be neglected.
4.16
Consider the same lake as in Problem 2.19, and the following additional data for December and
July: mean heat flux into the lake wat
e
r body,
G
=−
430 and 390 cal cm
−
2
d
−
1
; and mean wind
speed at 10 m above the water surface,
u
10
=
15.3 and 10.1kmh
−
1
. Calculate the mean evaporation,
inmmd
−
1
, from the lake for these typical days in December and July, by means of: (a) the energy
budget method with the Bowen ratio; (b) Penman's method; (c) Priestley and Taylor's method.
4.17
(a) Calculate the mean evaporation from the lake considered in Problems 2.19 and 4.16 for the
same two days, by using the mass-transfer method (inmmd
−
1
). To a first approximation, assume
that conditions are neutral, so you can use transfer coefficients, Ce
10
=
1
.
2
×
10
−
3
. (b) Is thisa
reasonable assumption for these two cases? Why not?
β
e
,asgiven by Equation (4.40), by equating (4.38) and (4.33) (with (4.3)
to represent
E
p
for a wet surface).
4.18
Derive the expression for
4.19
Estimate the time of local (i.e. solar) noon in Central Daylight Savings Time (CDT) for the location
of the measurements shown inFigures 4.9-4.12. The longitude of this location is approximately
96
◦
31
W and CDT is5hbehind Universal Time, or (UTC
−
0500).
4.20
Suppose the following measurements are available in the atmospheric surface layer under unstable
conditions: net radiation,
R
n
; groun
d
heat
flu
x,
G
; mean wind speed at two levels,
u
1
,
u
2
; and
potential temperature at two levels,
θ
1
and
θ
2
. Write down three equations that will allow you
to solve for the three unknowns, viz., the rate of evaporation,
E
; the sensible heat flux,
H
; and
the surface shear stress,
u
∗
. Make sure that the answer contains, beside some constants, only the
variables listed here (or functions thereof).
