M. Smith Agricultural and Forest Meteorology 103 2000 99–108 105
Fig. 3. Dual crop coefficient curve showing the basal K
cb
thick line, soil evaporation K
e
thin line and the corresponding single crop coefficient curve K
c
= K
cb
+ K
e
curve dashed line.
water management practices are included. Further de- tails on the revised K
c
values and length of growing stages, are included in the new FAO publication Allen
et al., 1998.
4. Computerized crop water use simulations
Computerized procedures greatly facilitate the esti- mation of crop water requirements from climatic data
Table 2 CLIMWAT–Climatic data and calculated ET
o
Monthly Reference Evapotranspiration Penman–Monteith
a
Month Avg. Temp.
◦
C Humid.
Wind kmday Sunshine h
Radiation MJm
2
day ET
o
-Penman–Monteith mmday January
− 9.2
69 156
4.2 6.3
0.4 February
− 5.4
72 181
3.7 8.0
0.7 March
1.8 71
199 3.5
10.5 1.3
April 18.8
64 190
5.5 15.7
3.4 May
16.0 59
190 8.0
20.9 4.0
June 21.9
45 199
9.6 23.8
5.6 July
27.2 35
181 10.7
24.8 6.6
August 22.3
42 164
8.2 19.6
4.8 September
19.4 64
181 9.1
17.7 3.4
October 8.3
57 173
6.6 11.4
1.7 November
3.7 70
164 6.1
8.2 0.8
December −
17.6 72
156 4.5
5.9 0.2
Year 8.9
60 178
6.6 14.4
2.7
a
Meteostation: Bishhket-7; Country: Uzbekistan; Altitude: 730 m; Coordinates: 42.87 N, 74.47 E.
and allow the development of standardized information and criteria for planning and management of rainfed
and irrigated agriculture. The FAO CROPWAT program Smith, 1992 incorporates procedures for ref-
erence crop evapotranspiration and crop water require- ments and allow the simulation of crop water use under
various climate, crop and soil conditions.
The CLIMWAT climatic database Smith, 1993, allows a ready access from CROPWAT to 3262 sta-
tions of 144 countries in Asia, Africa, Near East, South Europe, Central and South America, compiled by the
FAO Agrometeorological Group. An example of the climatic
data and
calculated reference
evapo- transpiration included in CLIMWAT is given in
Table 2. As a decision support system CROPWAT’s main
functions include: 1 the calculation of reference evap- otranspiration according to the FAO Penman–Monteith
method; 2 crop water requirements using revised crop coefficients and crop growth periods; 3 effective rain-
fall and irrigation requirements; 4 scheme irrigation water supply for a given cropping pattern; 5 daily
water balance computations.
The water balance procedures include various management options and allow the development of
irrigation schedules and the evaluation of irrigation practices in terms of water use efficiency and crop
yield-reductions to water-stress. Also the impact of rainfall on crop production can be evaluated and an
assessment made of the effects of drought and dry
106 M. Smith Agricultural and Forest Meteorology 103 2000 99–108
Table 3 :CROPWAT–Calculated crop evapotranspiration and irrigation requirements for spring wheat at Bishkent, Uzbekistan
a
Month Dec
Stage Coefficient
ET
crop
ET
crop
Effective Rain Irrigation require-
Irrigation require- K
c
mmday mmdec
mmdec ment mmday
ment mmdec April
1 Init
0.50 1.39
11.1 23.4
0.00 0.0
April 2
Init 0.50
1.75 17.5
40.0 0.00
0.0 April
3 Init
0.50 1.83
18.3 31.1
0.00 0.0
May 1
InDe 0.59
2.20 22.0
19.0 0.29
2.9 May
2 Deve
0.80 3.09
30.9 11.8
1.91 19.1
May 3
Deve 1.05
4.65 51.2
9.0 3.83
42.1 June
1 DeMi
1.19 6.01
60.1 5.7
5.43 54.3
June 2
Mid 1.20
6.73 67.3
2.0 6.53
65.3 June
3 Mid
1.20 7.12
71.2 2.7
6.85 68.5
July 1
Mid 1.20
7.76 77.6
4.0 7.36
73.6 July
2 MiLt
1.11 7.65
76.5 4.3
7.22 72.2
July 3
Late 0.91
5.62 61.9
4.0 5.26
57.8 August
1 Late
0.70 3.76
37.6 3.4
3.42 34.2
Total 603.1
160.4 490.2
a
Rain climate station: Bishket-7; ET
o
climate station: Bishhket-7; Crop: spring wheat; Planting date: 3 April.
spells on crop production due to water stress, based on the methodologies presented in FAO Irrigation and
Drainage Paper No. 33 ‘Yield response to water’ Doorenbos and Kassam, 1979.
An printout of the standard CROPWAT calculations showing the crop evapotranspiration and irrigation re-
quirements of spring wheat in Central Asia is included in Table 3.
Table 4 provides an example of the water balance calculations, with an assessment of the effects of rain-
fall and irrigation on yield and water use efficiency. The results show that water stress occurring in the matur-
ing phase causes a substantial yield reduction 21.7. The irrigation application has been fully effective, but
rainfall is exceeding actual requirements, resulting in losses.
5. WMOFAO roving seminars
