Integrated Applications - Transport in the Atmosphere-Vegetation-Soil Continuum - page 283

1.4

1.2

K c

1.0

0.8

0.6

0.4

0.2

0.0

Initial Crop development Mid-season Late season

Time

Figure 8.2 Generalized crop coeficient curve for the single-crop coeficient.

used (see Chapter 2 ). For periods longer than 10 days, an empirical relationship with air
temperature is used (based on the assumption that the soil temperature follows the air
temperature on that time scale).
The net radiation is not based on observed values, but rather on empirical estimates. •

Extraterrestrial radiation is reduced to global radiation using empirical relationships with
sunshine duration (similar to those given in Appendix A ). For the net longwave radia-
tion an empirical approximation is used that incorporates air temperature, humidity and
cloudiness.

Furthermore, Allen et al. ( 1998 ) discuss the problem when the weather data needed
to compute E ref have not been measured in environmental conditions that correspond
to the deinition of reference evapotranspiration. This may happen for instance, if the
crop factor method is to be used to plan an irrigation system in an arid region. The
weather data used will refer to conditions where the irrigation scheme is not yet there,
and therefore will relect hotter and drier conditions than will actually occur once the
irrigation system is in place.

Single-Crop Coeficient
The optimal evapotranspiration of the actual crop E (mm d -1 ) is simply calculated by
Eq. ( 8.1 ).
The crop growing season is divided in four stages: initial, development, mid sea-
son and late season ( Figure 8.2 ). Allen et al. ( 1998 ) provide extensive data on the K c
values as a function of growing stage and for a large number of crops. Because of the
simplifying assumptions this single-crop coeficient approach can be used only for
long-term water balances and basic irrigation scheduling.