Two periods are specified in DRAINMOD, a spring and fall period, for calculating trafficable conditions in the field.  The screens for editing these data are accessed by selecting 1st and 2nd work period tab in the Crop input screen. The inputs for each period are the month and day to start counting work days (MOBWx and IDABWx), the month and day to stop counting work days (MOEWx and IDAEWx) and the starting and ending hours for working (SWKHRx and EWKHRx).  The "x" is either 1 for the spring period or 2 for the fall period.  The final inputs are the drained volume (or water free pore space) in cm needed to work the soil (AMINx), the amount of rainfall in cm to postpone field operations (ROUTAx) and the number of days following postponement to recommence field operations (ROUTTx).  Typical values for some North Carolina soils are shown in Table 9.

 

If DRAINMOD is computing relative yields for a design, the user should use a long spring trafficability window since work days are accounted for in the planting date delay calculations.  In this case the model starts simulation of crop response to deficit and excess soil water stresses only AFTER a preset number of working days have been accumulated.  Working days are counted only within the trafficability window.  Therefore if the window is too short, the required number of working days may not occur in some years.  In this case the crop will not be planted and the yield will be zero.

 

Table 9.  Trafficability parameters for plowing and seed bed preparation for some North Carolina soils.

 

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Soil        Water Content*        Corresponding

 in plow layer        pressure head        AMIN        ROUTA         ROUTT

 (cm3/cm3)        (cm)        (cm)        (cm)        (days)

 

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Cape Fear l.        0.395        -65        3.3        1.2        2

Lumbee s.l.        0.265        -70        2.8        1.5        1

Coxville -

  Ogeechee l.        0.39        -80        3.4        1.2        2

Goldsboro s.l.        0.23        -80        3.2        1.5        1

Rains s.l.        0.25        -70        3.9        1.2        2

Wagram l.s.        0.15        -65        3.5        1.5        1

Bladen s.l.        0.40        -60        3.0        1.0        2

Portsmouth s.l.        0.32        -75        3.0        1.2        2

 

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* Water content in plow layer when soil is just dry enough for plowing and seed bed preparation.