Parameters for water stress calculations are input under the Manage Input Data > Crops section. Stress conditions for a crop can be determined without specifying a relative yield input file.  In this case DRAINMOD computes the amount of excess soil water (SEW) and the number of dry days.

 

The first input required for this section is the lower limit water content (usually assumed to be the wilting point), WP, in cm3/cm3 of the soil horizon containing the maximum crop root depth.  This value is generally chosen close to the 15 bar soil water content.  WP may be larger than the 15 bar soil water content for crops that cannot tolerate drought stresses.  This value sets the lower limit for water extraction from the root zone by the crop.

 

The next set of inputs determine how excessive water conditions are evaluated.  The inputs needed for the SEW concept include the period of the year to assess wet conditions and the minimum water table depth for no crop damage (SEWX in cm).  Wet stresses are computed whenever the water table is between the surface and the value of SEWX.  A value of 30 cm for SEWX is generally chosen for corn.  The period to evaluate wet stresses is input as the starting month and day and the ending month and day.

 

The DRAINMOD reference report indicates that corn, which can have root depths to 2 feet, can produce satisfactorily if the water table can be generally kept out of the top 30 cm.  It should be stressed that the top 30 cm should not be considered as a sacred figure.  For instance, shallow rooted crops may have the water table nearer the surface without loss in yield.  Conversely, crops with extensive root systems deeper in the soil should have the water table maintained at a lower level.  All such water table criteria are approximate and dependent on soil properties as well as the crop.  A 30 cm depth may be too shallow for corn on some soils.

 

SEW30 is calculated for the crop growing period from seeding till crop maturity.  SEW30 is the summation of the days times water table encroachment in cm.  If for instance on May 30, the water table has encroached 4 cm into the top 30 cm of the profile and on May 31, 6 cm.  The total SEW30 for these 2 days would be:

 

    1 x 4    =  4

 

    1 x 6    =  6

 

    SEW30  =  10

 

For drought conditions the user specifies the starting month and day and the ending month and day of the period to count drought stresses.  A "dry day" is computed in DRAINMOD whenever ET is less than PET because of limiting soil water conditions.

 

If crop relative yields are being evaluated these periods (for excess and deficient water conditions) should be set large enough to accommodate any planting date delays which may occur because of poor trafficability conditions.  Otherwise, these factors will not reflect the real stresses since these periods are not adjusted when crop relative yields are simulated.