Irrigation

(These parameters apply to the MABIA Method. For the Simplified Coefficient Method, see Simplified Coefficient Method, Irrigation; for the Soil Moisture Method, see Soil Moisture, Irrigation; for the Plant Growth Method, see Plant Growth Model 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. To avoid crop water stress, irrigations should be applied before or at the moment when the readily available soil water (RAW) is depleted (Depletion <= RAW).

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%).  Leave the expression 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.)

Fraction Wetted

Many types of irrigation systems wet only a fraction of the soil surface. For example, for a trickle irrigation system, the fraction of the surface wetted, Fw, may be only 0.4. For furrow irrigation systems, the fraction of the surface wetted may range from 0.3 to 0.8.

Irrigation Efficiency

Irrigation efficiency is the percentage of the supplied water available for evapotranspiration.  The remainder goes to groundwater, runoff or evaporation.  If 100% is available, leave blank.

Loss to Groundwater

Of the supplied water NOT available for evapotranspiration (100% - Irrigation Efficiency), Loss to Groundwater is the percent that infiltrates to groundwater, unless constrained by Maximum Percolation Rate. That which does not infiltrate or run off is assumed to evaporate.  NOTE: MABIA already calculates evaporation and infiltration, so this is in addition to that.

Loss to Runoff

Of the supplied water NOT available for evapotranspiration (100% - Irrigation Efficiency), Loss to Runoff is the percent that runs off to surface water. That which does not percolate or run off is assumed to evaporate.  NOTE: MABIA already calculates evaporation and infiltration, so this is in addition to that.  Additional runoff can be generated if the maximum infiltration rate limits precipitation or irrigation from infiltrating.

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: MABIA Calculation Algorithms

Entered on: Data View, Branch: Catchments, Category: Irrigation, Tabs: Irrigation Schedule, Fraction Wetted, Irrigation Efficiency, Loss to Groundwater, Loss to Runoff.