Appendix IV
Establishing the sowing date
Start of the growing season can be either the sowing or emergence date.
If sowing date is not provided, it will be determined by the model
(Subroutine FindSowingDate) .
First soil workability before sowing is checked on the basis of soil and crop
specific data. Soil workability dependends on the excess water amount in the
plow layer, assuming that capillary rise to the surface is negligible in case the
excess water amount during the previous time step is more than 0.5 cm, and
soil evaporation equals evaporation of a wet surface calculated with
PENMAN (see eq. 4.20).
If the excess amount in the plow layer is less than 0.5 cm, the maximum
capillary rise has to be considered, which is calculated with an
AFGEN table using the excess amount of water in the plow layer
as independent variable. This AFGEN table is soil dependent,
however, in WOFOST/CGMS only one fixed table is used. Soil evaporation
soil is calculated as:
where |
Es |
: | Evaporation rate from a bare soil
| [cm.d-1]
|
|
EOs |
: |
Potential evaporation rate from a bare soil surface (see §4.1.3) |
[cm] |
|
CRmax |
: |
Maximum capillary rise |
[cm d-1] |
|
P |
: |
Precipitation intensity |
[cm d-1] |
The excess amount of water in the plow layer at time step t can
now be established according to:
where |
Wexc,t |
: |
Excess amount of water in the plow layer at time step t |
[cm] |
|
Es |
: |
Evaporation rate from a bare soil | [cm d-1] |
|
P |
: |
Precipitation intensity |
[cm d-1] |
Wexc,t has to be corrected for seepage from the plow zone into
deper soil layers during present time step in case the excess water amount
is more than zero. Seepage amount should be subtracted from Wexc,t.
where |
Seep |
: |
Water amount infiltrating in the plow layer |
[cm] |
|
Wexc,t |
: |
Excess amount of water in the plow layer at time step t |
[cm] |
|
Sp1 |
: |
First top soil seepage parameter |
[-] |
|
Sp2 |
: |
Second top soil seepage parameter |
[-] |
Soil is considered workable when the excess water amount
in the plow layer is below a preset value, which is soil
specific and should be provided by the user. The number of workable days is
established, however, in case a certain day cannot be considered as a
workable day, the sum of workable days is set back to zero. Note
that two types of seepage parameters exist. One for shallow seedbeds
and another for deep seedbeds. The user should indicate which one should be
used. Sowing can take place when more then three consecutive workable
days occur and provided the sowing date falls within a predefined period (the
earliest sowing date and the latest sowing date). This period is also crop
specific and should be provided by the user.