(These parameters apply to the Plant Growth Model Method. For the Simplified Coefficient Method, see Simplified Coefficient Method, Irrigation; for the Soil Moisture Method, see Soil Moisture, Irrigation; for the MABIA Method, see MABIA Method, Irrigation)

Irrigation is required when rainfall is insufficient to compensate for the water lost by evapotranspiration. The primary objective of irrigation is to apply water at the right period and in the right amount. By calculating the soil water balance of the root zone on a daily basis, the timing and the depth of future irrigations can be planned.

If you indicate that irrigation is to occur in a Catchment at the time you create the Catchment in the Schematic, the Irrigation tab will appear under the particular Catchment in the Data View.

Irrigation Schedule

Choose the irrigation methods and schedule, using the Irrigation Scheduling Wizard, or choosing a schedule already in use for the same crop. These two options are available on the drop-down menu in the data grid. You may also edit the expression directly in the cell, for example, to change the value of one of the methods (e.g., % of Depletion, from 100% to 90%).  % HU = % of season heat unit accumulation.  % MAD = % of Management Allowed Depletion.  Leave blank if there is no irrigation for this crop or land cover.  

The Irrigation Schedule can be set only once for a scenario.  Therefore, to model a shift in irrigation methods over time (e.g., from furrow irrigation in early years to drip irrigation in later years), you will need to create two branches, one for each irrigation method.  Enter the actual area for the Area variable on the catchment branch, and set the unit to percent share for the two branches.  Change the percent shares for the two branches over time to model the change in irrigation method.  The sum of the shares for the two branches should always equal 100%.  (The easiest way to ensure this is by using the Remainder function on one branch.)

Distribution Uniformity

The distribution uniformity (DU) is a measure of how uniformly irrigation is applied to a field.  It is expressed as a value between 0 and 1.  The larger the value, the more uniform the irrigation.  The distribution uniformity is used in the PGM to increase the amount of irrigation applied so that all portions of the field receive adequate water.  For instance, if 30 mm of water are required to fill the soil moisture store from the current soil moisture level to field capacity and the DU is 0.8, the amount of water applied will be: 30/DU = 37.5 mm.

Irrigation Rate

The irrigation rate is the rate at which the irrigation system applies water to the field.  This parameter is used in calculating infiltration in the PGM.  The maximum infiltration rate is calculated using the Phillip Equation which is a function of the soil physical properties and soil moisture status of the soil.  When the irrigation rate is in excess of the infiltration rate, surface runoff is generated.

Irrigation Use of Runoff

Percent of this catchment's runoff which can be used for irrigation internally within catchment (diverted before it reaches surface water inflow of runoff link).  Note: This will allocate some or all of the runoff for use by the catchment for its irrigation, even if there are higher priority demands downstream, and before all other irrigation supplies the catchment would otherwise use.  However, runoff from irrigation is NOT available to be used again for irrigation -- this would be circular.  Only the runoff from precipitation and from the soil moisture at the beginning of the timestep is available for irrigation within the catchment.  Only runoff links to surface water are considered, not infiltration links to groundwater.  See the Calculation Algorithms for details.

See also: Plant Growth Model Algorithms

Entered on: Data View, Branch: Catchments, Category: Irrigation, Tabs: Irrigation Schedule, Distribution Uniformity, Irrigation Rate.