Several objective functions are calculated in DRAINMOD to quantify the performance of the system that is simulated.  Annual values of these functions are summarized and ranked.  Monthly and daily summaries are also available at the option of the user.

Working Days

The model determines the number of days during two specified intervals when soil water conditions are suitable for field work such as tillage, harvesting, etc.   Normally one of the periods is for seedbed preparation in the spring and the second is for harvest in the fall.  A day is counted as a working day if the drained or water free pore space is greater than a threshold value, if there is less than a given amount of rain that day, and if rainfall sufficient to delay field work has not occurred within a given number of days.

S E W

Stress due to high water tables (excessive soil water conditions) is quantified by the SEW value, which stands for "Sum of Excess Water".  Normally SEW30 is calculated as follows:

    N

SEW30     =SUM (30 - Xi)

   i=1

where Xi is the water table depth in cm on day i and N is the number of days in the growing season.  Negative terms in the summation are ignored; i.e.  no stress is calculated for water table depths greater than 30 cm (12 in.).

While the SEW30 values seem to be appropriate for corn, it should be emphasized that the SEW concept is approximate.  Different depths may be more appropriate for other crops.  For instance, shallow rooted crops may have the water table closer to the surface without damage.  Conversely crops with deeper rooting systems may be stressed by water tables 45 or 60 cm deep.  The depth of the SEW threshold is a DRAINMOD input, thus SEW45 or SEWXX can be determined.  The SEW value is calculated for each growing season from planting to crop maturity.

Dry Days

A dry day is any day in which actual ET is less than PET because of limited or deficient soil water conditions.  The sum of dry days that occur during a specified period, usually the growing season, is a DRAINMOD output and is listed in the summary for each year of the simulation.

Relative Yield

To evaluate the effects of combinations of stresses, a relative yield may be calculated in DRAINMOD at the option of the user.   The relative yield is the ratio (percent) of the actual yield to the potential yield.  The potential yield is the longterm average yield that would occur if soil water conditions are not limiting; i.e., if there are not yield reductions due to deficient soil water conditions, excessive soil water conditions or delays in planting.

Separate models for crop response to excessive and deficient soil water conditions, as well as to delays in the planting date, have been incorporated into DRAINMOD (Skaggs et al, 1982).  The general crop response model may be written as

YR = Y/Yo = YRw x YRd x YRp x YRs

where YR is the relative yield,

YRw = Yw/Yo, YRd = Yd/Yo, YRp = Yp/Yo, and YRs = Ys/Yo

Y is the yield for a given year, and Yo is the base yield or potential yield.  Yw is the yield that would be obtained if only wet stresses occur, Yd is the yield that would be obtained if only drought stresses occur, and Yp is the yield that would be obtained if the only reduction is due to a delay in planting date, and Ys is the yield that would be obtained if only salinity stresses occur.  Individual submodels are used to calculate the relative yields; YRw, YRd, YRp, and YRs.

Irrigation volume

When the irrigation option is used to evaluate the potential for wastewater irrigation, the objective function is the irrigation volume (or depth) that can be applied.  The user specifies the interval between irrigations, the time period for the irrigation, the application rate of irrigation water, and the starting and ending dates for irrigation during the calendar year.

The model simulates irrigation at the scheduled time (according to the irrigation interval specified) if there is a specified minimum amount of drained pore space (or air volume) in the soil profile.  If the drained pore space is less than the specified minimum, the scheduled irrigation is canceled and the soil conditions are evaluated at the next scheduled event.