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]

W_exc,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 W_exc,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.