Data View, Branch: Catchments,
Category: Crop, Irrigation Dates, Irrigation Trigger, Irrigation Amount.
This screen is used in conjunction with the Plant Growth Model Method for catchment hydrology and crop water requirements. It is accessible via the drop-down menu on the data grid for the "Irrigation Schedule" variable under Irrigation.
Once you have entered the appropriate data into the Irrigation Scheduling Wizard, click the Save button and WEAP will create the corresponding IrrigationSchedule expression that embodies this data. The following will describe how to enter this data into the wizard.
At the top of the wizard, select the number of irrigation schedules per year (for all crops combined). For example, if you had corn followed by winter wheat, and each had 1 irrigation schedule, you would enter 2. If the irrigation schedule expression was initially blank, when the Irrigation Scheduling Wizard starts WEAP will automatically add all crops defined for this branch, and set the default irrigation schedule for each: triggered at 50% of MAD, with an application of 100 % of Depletion. This will ensure that soil moisture depletion never goes below 50% of the management allowable depletion so that no crop water stress will occur. Of course, you can make changes to these defaults.
Each row of the table defines one irrigation schedule, with the following columns of information:
Choose the crop for the irrigation schedule. Only the crops already chosen for this branch are available here for selection.
This parameter specifies when this irrigation schedule takes effect. This can be specified using one of three methods. The first, “% of Season Heat Units,” starts the schedule as a function of the percent of crop heat units that have accumulated. For instance, if a value of 10% is used for this parameter, the schedule will become active once 10% of the heat units required to reach maturity have accumulated. The second option, “Days After Planting,” starts the schedule a fixed number of days after planting. The third option, “Specify Exact Date,” allows the user to set the start date for the schedule using a calendar day.
Note: if an irrigation event is to be scheduled for a specific day, use the “Days After Planting” or “Specify Exact Date” and specify the same day for both the Irrigation Start Date and Irrigation End Date.
This parameter specifies when this irrigation schedule is no longer in effect. Similar to the Irrigation Start Date, there are three options. The first, “% of Season Heat Units,” ends the schedule as a function of the percent of crop heat units that have accumulated. For instance, if a value of 90% is used for this parameter, the schedule will cease being active once 90% of the heat units required to reach maturity have accumulated. The second option, “Days After Planting,” ends the schedule a fixed number of days after planting. The third option, “Specify Exact Date,” allows the user to set the end date for the schedule using a calendar day.
Note: If an irrigation event is to be scheduled for a specific day, use the “Days After Planting” or “Specify Exact Date” and specify the same day for both the Irrigation Start Date and Irrigation End Date.
If a particular irrigation schedule is active per the settings in the Irrigation Start and End Dates, then on each day the PGM code determines if an irrigation event should occur. The event is triggered using one of the three irrigation trigger methods: “%MAD”, “Fixed Interval”, or “Fixed Depletion”.
Irrigation is triggered by the “%MAD” method when soil moisture in the root zone is less than the product of %MAD and MAD, SM < %MAD x MAD. Note: MAD is the management allowable depletion specified in the crop database. The %MAD parameter allows the user to vary the management allowable depletion. For instance the user may want to use a lower management allowable depletion during growth periods, such as flowering, that have a large impact of yield.
Irrigation is triggered in the “Fixed Interval” method every x days, where x is the value entered for the fixed interval. For instance, if irrigation will occur every 2 weeks, a value of 14 days should be entered for the interval.
Irrigation is triggered by the “Fixed Depletion” method when the soil moisture depletion is greater than that specified. The soil moisture depletion is calculated as the difference between field capacity and the current soil moisture content multiplied by the root zone depth, (FC – SWC) x RZ.
Note: If an irrigation event is to be scheduled for a specific day, use the “Fixed Interval” method and enter a value of 1.
If an irrigation schedule is active and an event has been triggered then the PGM code calculates the amount if irrigation to apply using the parameters specified in the Irrigation Amount Method.
If “% Depletion” or “% MAD” are selected then the amount of irrigation is calculated using the following:
Irrigation = (FC – SWC) x RZ x % Depletion / DU
Or
Irrigation = (FC – SWC) x RZ x % MAD / DU
where:
FC = soil water content at field capacity
SWC = current soil water content in root zone
RZ = root zone depth
DU = distribution uniformity
If the “Fixed Depth” method is used then the irrigation amount is simply the depth specified. In this case the distribution uniformity (DU) is not used in calculating the applied water depth.
See also: Plant Growth Model Algorithms
Entered on: Data View, Branch: Catchments,
Category: Crop, Irrigation Dates, Irrigation Trigger, Irrigation Amount.