Yield reductions due to deficient soil water stresses are computed using procedures described by Shaw (1978).  The relative yield equation may be written as,

 

 

         YRd = 100 1.22 * SDId

 

 

where SDId is the stress day index for drought conditions.  The coefficients of the equation given for corn of 100 and 1.22 are the values for YRIMAX and YSLOPE.  The equation to describe the stress day index is,

 

                     N

         SDId   =  SUM  ( SDdj * CSdj )

 

  j=1

 

where SDdj and CSdj are the stress day index and crop susceptibility factor for growth period j, and N is the number of 5 day periods in the growing season.  For each 5 day period, the stress day index is defined as,

 

                    n

         SDdj =   SUM  (1.0 AETk/PETk)

 

                 k=1

 

where n is the number of days in the growth period j, AETk and PETk are the actual and potential evapotranspiration on the day k of the growth stage.  For Shaw's work, the length of a growth period is 5 days.  The crop susceptibility factors were developed by Shaw (1974) for each growth period relative to the silking stage and are presented in Table 15.  Under severe stress conditions lasting more than one period, Shaw found that an additional adjustment was required.  Thus, if SDdj exceeds 4.5 for two periods, the index for those periods, CSdj, are multiplied by 1.5.

 

Table 15.  Crop susceptibility factors for deficient soil water conditions for corn (Shaw 1974).

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Period*        CSd        Period        CSd

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 -8        0.50        +1        2.00

 -7        0.50        +2        1.30

 -6        1.00        +3        1.30

 -5        1.00        +4        1.30

 -4        1.00        +5        1.30

 -3        1.00        +6        1.30

 -2        1.75        +7        1.20

 -1        2.00        +8        1.00

                 +9        0.50

 

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*5-day periods relative to silking stage, where silking occurs in the 10 day period from -1 to +1; for example, period -8 is the period from 35 to 40 days prior to silking and period +3 would be from 10 to 15 days after silking.  Also, note that CSd is assumed to be zero for any periods prior to -8 and after +9.  These zero values must be entered as inputs to DRAINMOD