11 – 1
Chapter 11
11.3.1. T = 20
o
C, 25
o
C, 30
o
C.
From Table 11.3.1,
p
= 0.33, c = 1.0, K
c
= 0.8, C
cu
= 0.5. Using Blaney-Criddle
formula,
ET
o
= c[p(0.46T + 8)]
i) T = 20
o
C:
ET
o
= 1.0[0.33(0.46(20) + 8)] = 5.68 mm/day
E
T
c
= K
c
ET
o
= 0.8(5.68) = 4.54 mm/day
Volume of water consumptively used per acre land = 4.54 ac-mm/day
= 0.015 ac-ft/day
Volume of irrigation water required
= (volume of water consumptively used)/C
cu
= 0.015/0.5
= 0.030 ac-ft/day
ii) T = 25
o
C:
ET
o
= 1.0[0.33(0.46(25) + 8)] = 6.44 mm/day
E
T
c
= 0.8(6.44) = 5.15 mm/day
Volume of water consumptively used per acre land = 5.15 ac-mm/day
= 0.017 ac-ft/day
Volume of irrigation water required
= (volume of water consumptively used)/C
cu
= 0.017/0.5
= 0.034 ac-ft/day
iii) T = 30
o
C:
ET
o
= 1.0[0.33(0.46(30) + 8)] = 7.19 mm/day
E
T
c
= 0.8(7.19) = 5.76 mm/day
Volume of water consumptively used per acre land = 5.76 ac-mm/day
= 0.019 ac-ft/day
Volume of irrigation water required
= (volume of water consumptively used)/C
cu
= 0.019/0.5
= 0.038 ac-ft/day
Using the volume of irrigation water required when T = 20
o
C as a reference value,
the percent increase at the remaining temperature values may be determined for
comparison purposes, as given in the following Table.
T
(
o
C)
Irrigation water
required (ac-ft/day)
%
increase
Remark
20
0.030
--
25
0.034
13.33
30
0.038
26.67
Linear increase
(note the linear
equation used to
determine ET
o
)
11.5.1. Equation (11.5.6) was given as
d
= 0.00006362
P
-0.267
I
1.61
(
DIF
)
-0.123
W
0.0897
The derivative of
d
with respect to price is

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*Sign up*11 – 2
0897
.
0
123
.
0
61
.
1
267
.
1
)
(
)
267
.
0
(
00006362
.
0
d
d
W
DIF
I
P
P
d
±
±
±
0897
.
0
123
.
0
61
.
1
267
.
0
)
(
)
267
.
0
(
00006362
.
0
W
DIF
I
P
P
±
±
±
P
d
267
.
0
±
Thus,
H
±
267
.
0
d
d
P
d
d
P
, that is, the price elasticity of demand is –0.267.
11.5.2. Q = 64.7 + 0.00017(Inc) + 4.76(Ad) + 3.92(Ch) – 0.406(R) + 29.03(Age) –
6.42(P)
P
Q
Q
P
d
d
²
42
.
6
P
Q
±
d
d
(form the given equation).
Thus,
H
= -6.42P/Q
At P = 1.7241 and Q = 75.2106,
H
= -6.42(1.7241)/75.2106 = -0.15.

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