Models in Irrigation andWater Management - Practices of Irrigation and On-farm Water Management - page 396

and

ω s = arccos( − tan ψ tan δ ) (10.17)

0.033 cos 2

365 J

π

d r = 1 + (10.18)

0.409 sin 2 1.39

π
365 J

δ = − (10.19)

J = number of the day in the year (Julian day)

10.3.1.2 CropET 0 Model
The Excel template “CropET 0 ” was developed by Ali ( 2005 a ) following the FAO
Penman-Monteith approach as described by Smith et al. (1992) and Allen et al.
( 1998 ) . The basic inputs of the model are the monthly average values of the weather
variables (i) maximum temperature, (ii) minimum temperature, (iii) humidity, (iv)
wind speed, (v) sunshine hour, (vi) latitude, and (vii) altitude of the location. For
north latitude, the value would be positive, and for south latitude, the value would
be negative.
The user can adjust the parameter “Angstrom coefficients” (used in solar energy
calculation) if the local calibrated values are available. Average values ( a s = 0.25,

b s 0.50) as suggested by FAO can be used if local calibrated values are not

available. Input data should be provided through “Model Interface,” a data input
platform.

=

Model Output
There is dynamic interaction between input and output in “CropET 0 .” The

“CropET 0 ” produces the following:

(i) The monthly average reference evapotranspiration (mm/d) throughout the

months of the year;
(ii) Display graph. As the model output ET 0 values are linked to a graphical display,
the display graph respond instantaneously with the input value.

In addition, there are a “Print Output” and a “Print Input” window, from where the
output values and input data can be printed. The graph can be printed from “Display
Graph” window.