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and the temperature (known or assumed) at which the original crop coefficient K c
was calibrated. Consequently the remaining need is to specify the value of T pref .
Shuttleworth and Wallace (2010) investigated the sensitivity of the calculated
value of surface resistance to the assumed value of T pref for a range of irrigated
crops in Australia and found a lack of sensitivity for many. On the basis of their
study, they concluded that, pending field studies to better define the crop-specific
values of surface resistance, T pref
C should be used in Equations (23.35) and
(22.36) when estimating ( r s ) c from the tabulated values of K c given by FAO
(Doorenbos and Pruitt, 1977). On this basis, Shuttleworth and Wallace then calcu-
lated the values of R c 50 from Equation (23.32) and ( r s ) c from Equation (23.34) for
the selection of crops given in Table 23.5.
=
20
°
Table 23.5 Values of crop factor, K c , and crop height, h c , for a selection of irrigated crops
together with the equivalent derived values of R c 50 and (rs) c required for in Equation (12.21)
when calculating daily average evaporation using the Matt-Shuttleworth approach.
Irrigated crop
K c (dimensionless)
h c (m)
R c 50 (dimensionless)
(r s ) c (s m −1 )
Reference crop
1.00
0.12
302
70
Alfalfa (average)
0.95
0.70
196
127
Bermuda
1.00
0.35
235
92
Clover (average)
0.90
0.60
204
149
Rye (average)
1.05
0.30
244
66
Pasture (rotation)
0.95
0.23
260
109
Pasture (extensive)
0.75
0.10
314
254
Small vegetables
1.05
0.38
230
72
Solanum family
1.15
0.70
196
50
Cucurbitaceae
1.00
0.34
237
91
Roots and tubers
1.10
0.68
198
66
Legumes
1.15
0.55
209
44
Cereals
1.15
1.00
177
60
Cotton
1.18
1.35
162
60
Maize (grain)
1.20
2.00
143
64
Sorgum (grain)
1.05
1.50
157
100
Rice
1.20
1.00
177
46
Millet
1.00
1.50
157
118
Sugar cane
1.25
3.00
124
63
Cacao
1.05
3.00
124
113
Coffee
0.95
2.50
133
143
Tea
1.00
1.50
157
118
Grape (table)
0.85
2.00
143
184
Grape (wine)
0.70
1.75
149
273
Almonds
0.90
5.00
102
169
Avocado
0.85
3.00
124
186
Citrus (50% canopy)
0.60
3.00
124
345
Kiwi
1.05
3.00
124
113
Walnut
1.10
4.50
107
106
Olives
0.70
4.00
112
265
 
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